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git://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext
Pull sched_ext updates from Tejun Heo:
- Improve the default select_cpu() implementation making it topology
aware and handle WAKE_SYNC better.
- set_arg_maybe_null() was used to inform the verifier which ops args
could be NULL in a rather hackish way. Use the new __nullable CFI
stub tags instead.
- On Sapphire Rapids multi-socket systems, a BPF scheduler, by
hammering on the same queue across sockets, could live-lock the
system to the point where the system couldn't make reasonable forward
progress.
This could lead to soft-lockup triggered resets or stalling out
bypass mode switch and thus BPF scheduler ejection for tens of
minutes if not hours. After trying a number of mitigations, the
following set worked reliably:
- Injecting artificial cpu_relax() loops in two places while
sched_ext is trying to turn on the bypass mode.
- Triggering scheduler ejection when soft-lockup detection is
imminent (a quarter of threshold left).
While not the prettiest, the impact both in terms of code complexity
and overhead is minimal.
- A common complaint on the API is the overuse of the word "dispatch"
and the confusion around "consume". This is due to how the dispatch
queues became more generic over time. Rename the affected kfuncs for
clarity. Thanks to BPF's compatibility features, this change can be
made in a way that's both forward and backward compatible. The
compatibility code will be dropped in a few releases.
- Other misc changes
* tag 'sched_ext-for-6.13' of git://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext: (21 commits)
sched_ext: Replace scx_next_task_picked() with switch_class() in comment
sched_ext: Rename scx_bpf_dispatch[_vtime]_from_dsq*() -> scx_bpf_dsq_move[_vtime]*()
sched_ext: Rename scx_bpf_consume() to scx_bpf_dsq_move_to_local()
sched_ext: Rename scx_bpf_dispatch[_vtime]() to scx_bpf_dsq_insert[_vtime]()
sched_ext: scx_bpf_dispatch_from_dsq_set_*() are allowed from unlocked context
sched_ext: add a missing rcu_read_lock/unlock pair at scx_select_cpu_dfl()
sched_ext: Clarify sched_ext_ops table for userland scheduler
sched_ext: Enable the ops breather and eject BPF scheduler on softlockup
sched_ext: Avoid live-locking bypass mode switching
sched_ext: Fix incorrect use of bitwise AND
sched_ext: Do not enable LLC/NUMA optimizations when domains overlap
sched_ext: Introduce NUMA awareness to the default idle selection policy
sched_ext: Replace set_arg_maybe_null() with __nullable CFI stub tags
sched_ext: Rename CFI stubs to names that are recognized by BPF
sched_ext: Introduce LLC awareness to the default idle selection policy
sched_ext: Clarify ops.select_cpu() for single-CPU tasks
sched_ext: improve WAKE_SYNC behavior for default idle CPU selection
sched_ext: Use btf_ids to resolve task_struct
sched/ext: Use tg_cgroup() to elieminate duplicate code
sched/ext: Fix unmatch trailing comment of CONFIG_EXT_GROUP_SCHED
...
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On 2 x Intel Sapphire Rapids machines with 224 logical CPUs, a poorly
behaving BPF scheduler can live-lock the system by making multiple CPUs bang
on the same DSQ to the point where soft-lockup detection triggers before
SCX's own watchdog can take action. It also seems possible that the machine
can be live-locked enough to prevent scx_ops_helper, which is an RT task,
from running in a timely manner.
Implement scx_softlockup() which is called when three quarters of
soft-lockup threshold has passed. The function immediately enables the ops
breather and triggers an ops error to initiate ejection of the BPF
scheduler.
The previous and this patch combined enable the kernel to reliably recover
the system from live-lock conditions that can be triggered by a poorly
behaving BPF scheduler on Intel dual socket systems.
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Douglas Anderson <dianders@chromium.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
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Instead of solving the underlying problem of the double invocation of
__sched_fork() for idle tasks, sched-ext decided to hack around the issue
by partially clearing out the entity struct to preserve the already
enqueued node. A provided analysis and solution has been ignored for four
months.
Now that someone else has taken care of cleaning it up, remove the
disgusting hack and clear out the full structure. Remove the comment in the
structure declaration as well, as there is no requirement for @node being
the last element anymore.
Fixes: f0e1a0643a59 ("sched_ext: Implement BPF extensible scheduler class")
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Acked-by: Tejun Heo <tj@kernel.org>
Link: https://lore.kernel.org/r/87ldy82wkc.ffs@tglx
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struct scx_iter_scx_dsq is defined as 6 u64's and scx_dsq_iter_kern was
using 5 of them. We want to add two more u64 fields but it's better if we do
so while staying within scx_iter_scx_dsq to maintain binary compatibility.
The way scx_iter_scx_dsq_kern is laid out is rather inefficient - the node
field takes up three u64's but only one bit of the last u64 is used. Turn
the bool into u32 flags and only use the lower 16 bits freeing up 48 bits -
16 bits for flags, 32 bits for a u32 - for use by struct
bpf_iter_scx_dsq_kern.
This allows moving the dsq_seq and flags fields of bpf_iter_scx_dsq_kern
into the cursor field reducing the struct size by a full u64.
No behavior changes intended.
Signed-off-by: Tejun Heo <tj@kernel.org>
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Add sched_ext_ops operations to init/exit cgroups, and track task migrations
and config changes. A BPF scheduler may not implement or implement only
subset of cgroup features. The implemented features can be indicated using
%SCX_OPS_HAS_CGOUP_* flags. If cgroup configuration makes use of features
that are not implemented, a warning is triggered.
While a BPF scheduler is being enabled and disabled, relevant cgroup
operations are locked out using scx_cgroup_rwsem. This avoids situations
like task prep taking place while the task is being moved across cgroups,
making things easier for BPF schedulers.
v7: - cgroup interface file visibility toggling is dropped in favor just
warning messages. Dynamically changing interface visiblity caused more
confusion than helping.
v6: - Updated to reflect the removal of SCX_KF_SLEEPABLE.
- Updated to use CONFIG_GROUP_SCHED_WEIGHT and fixes for
!CONFIG_FAIR_GROUP_SCHED && CONFIG_EXT_GROUP_SCHED.
v5: - Flipped the locking order between scx_cgroup_rwsem and
cpus_read_lock() to avoid locking order conflict w/ cpuset. Better
documentation around locking.
- sched_move_task() takes an early exit if the source and destination
are identical. This triggered the warning in scx_cgroup_can_attach()
as it left p->scx.cgrp_moving_from uncleared. Updated the cgroup
migration path so that ops.cgroup_prep_move() is skipped for identity
migrations so that its invocations always match ops.cgroup_move()
one-to-one.
v4: - Example schedulers moved into their own patches.
- Fix build failure when !CONFIG_CGROUP_SCHED, reported by Andrea Righi.
v3: - Make scx_example_pair switch all tasks by default.
- Convert to BPF inline iterators.
- scx_bpf_task_cgroup() is added to determine the current cgroup from
CPU controller's POV. This allows BPF schedulers to accurately track
CPU cgroup membership.
- scx_example_flatcg added. This demonstrates flattened hierarchy
implementation of CPU cgroup control and shows significant performance
improvement when cgroups which are nested multiple levels are under
competition.
v2: - Build fixes for different CONFIG combinations.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: David Vernet <dvernet@meta.com>
Acked-by: Josh Don <joshdon@google.com>
Acked-by: Hao Luo <haoluo@google.com>
Acked-by: Barret Rhoden <brho@google.com>
Reported-by: kernel test robot <lkp@intel.com>
Cc: Andrea Righi <andrea.righi@canonical.com>
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SCX_TASK_BAL_KEEP is used by balance_one() to tell pick_next_task_scx() to
keep running the current task. It's not really a task property. Replace it
with SCX_RQ_BAL_KEEP which resides in rq->scx.flags and is a better fit for
the usage. Also, the existing clearing rule is unnecessarily strict and
makes it difficult to use with core-sched. Just clear it on entry to
balance_one().
Signed-off-by: Tejun Heo <tj@kernel.org>
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From 1232da7eced620537a78f19c8cf3d4a3508e2419 Mon Sep 17 00:00:00 2001
From: Tejun Heo <tj@kernel.org>
Date: Wed, 31 Jul 2024 09:14:52 -1000
p->scx.disallow provides a way for the BPF scheduler to reject certain tasks
from attaching. It's currently allowed for both the load and fork paths;
however, the latter doesn't actually work as p->sched_class is already set
by the time scx_ops_init_task() is called during fork.
This is a convenience feature which is mostly useful from the load path
anyway. Allow it only from the load path.
v2: Trigger scx_ops_error() iff @p->policy == SCHED_EXT to make it a bit
easier for the BPF scheduler (David).
Signed-off-by: Tejun Heo <tj@kernel.org>
Reported-by: "Zhangqiao (2012 lab)" <zhangqiao22@huawei.com>
Link: http://lkml.kernel.org/r/20240711110720.1285-1-zhangqiao22@huawei.com
Fixes: 7bb6f0810ecf ("sched_ext: Allow BPF schedulers to disallow specific tasks from joining SCHED_EXT")
Acked-by: David Vernet <void@manifault.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
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We currently only allow calling sleepable scx kfuncs (i.e.
scx_bpf_create_dsq()) from BPF_PROG_TYPE_STRUCT_OPS progs. The idea here
was that we'd never have to call scx_bpf_create_dsq() outside of a
sched_ext struct_ops callback, but that might not actually be true. For
example, a scheduler could do something like the following:
1. Open and load (not yet attach) a scheduler skel
2. Synchronously call into a BPF_PROG_TYPE_SYSCALL prog from user space.
For example, to initialize an LLC domain, or some other global,
read-only state.
3. Attach the skel, which actually enables the scheduler
The advantage of doing this is that it can preclude having to do pretty
ugly boilerplate like initializing a read-only, statically sized array of
u64[]'s which the kernel consumes literally once at init time to then
create struct bpf_cpumask objects which are actually queried at runtime.
Doing the above is already possible given that we can invoke core BPF
kfuncs, such as bpf_cpumask_create(), from BPF_PROG_TYPE_SYSCALL progs. We
already allow many scx kfuncs to be called from BPF_PROG_TYPE_SYSCALL progs
(e.g. scx_bpf_kick_cpu()). Let's allow the sleepable kfuncs as well.
Signed-off-by: David Vernet <void@manifault.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
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DSQs are very opaque in the consumption path. The BPF scheduler has no way
of knowing which tasks are being considered and which is picked. This patch
adds BPF DSQ iterator.
- Allows iterating tasks queued on a DSQ in the dispatch order or reverse
from anywhere using bpf_for_each(scx_dsq) or calling the iterator kfuncs
directly.
- Has ordering guarantee where only tasks which were already queued when the
iteration started are visible and consumable during the iteration.
v5: - Add a comment to the naked list_empty(&dsq->list) test in
consume_dispatch_q() to explain the reasoning behind the lockless test
and by extension why nldsq_next_task() isn't used there.
- scx_qmap changes separated into its own patch.
v4: - bpf_iter_scx_dsq_new() declaration in common.bpf.h was using the wrong
type for the last argument (bool rev instead of u64 flags). Fix it.
v3: - Alexei pointed out that the iterator is too big to allocate on stack.
Added a prep patch to reduce the size of the cursor. Now
bpf_iter_scx_dsq is 48 bytes and bpf_iter_scx_dsq_kern is 40 bytes on
64bit.
- u32_before() comparison factored out.
v2: - scx_bpf_consume_task() is separated out into a separate patch.
- DSQ seq and iter flags don't need to be u64. Use u32.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: David Vernet <dvernet@meta.com>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Cc: bpf@vger.kernel.org
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struct scx_dsq_node contains two data structure nodes to link the containing
task to a DSQ and a flags field that is protected by the lock of the
associated DSQ. One reason why they are grouped into a struct is to use the
type independently as a cursor node when iterating tasks on a DSQ. However,
when iterating, the cursor only needs to be linked on the FIFO list and the
rb_node part ends up inflating the size of the iterator data structure
unnecessarily making it potentially too expensive to place it on stack.
Take ->priq and ->flags out of scx_dsq_node and put them in sched_ext_entity
as ->dsq_priq and ->dsq_flags, respectively. scx_dsq_node is renamed to
scx_dsq_list_node and the field names are renamed accordingly. This will
help implementing DSQ task iterator that can be allocated on stack.
No functional change intended.
Signed-off-by: Tejun Heo <tj@kernel.org>
Suggested-by: Alexei Starovoitov <ast@kernel.org>
Acked-by: Alexei Starovoitov <ast@kernel.org>
Cc: David Vernet <void@manifault.com>
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Add Documentation/scheduler/sched-ext.rst which gives a high-level overview
and pointers to the examples.
v6: - Add paragraph explaining debug dump.
v5: - Updated to reflect /sys/kernel interface change. Kconfig options
added.
v4: - README improved, reformatted in markdown and renamed to README.md.
v3: - Added tools/sched_ext/README.
- Dropped _example prefix from scheduler names.
v2: - Apply minor edits suggested by Bagas. Caveats section dropped as all
of them are addressed.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: David Vernet <dvernet@meta.com>
Acked-by: Josh Don <joshdon@google.com>
Acked-by: Hao Luo <haoluo@google.com>
Acked-by: Barret Rhoden <brho@google.com>
Cc: Bagas Sanjaya <bagasdotme@gmail.com>
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Currently, a dsq is always a FIFO. A task which is dispatched earlier gets
consumed or executed earlier. While this is sufficient when dsq's are used
for simple staging areas for tasks which are ready to execute, it'd make
dsq's a lot more useful if they can implement custom ordering.
This patch adds a vtime-ordered priority queue to dsq's. When the BPF
scheduler dispatches a task with the new scx_bpf_dispatch_vtime() helper, it
can specify the vtime tha the task should be inserted at and the task is
inserted into the priority queue in the dsq which is ordered according to
time_before64() comparison of the vtime values.
A DSQ can either be a FIFO or priority queue and automatically switches
between the two depending on whether scx_bpf_dispatch() or
scx_bpf_dispatch_vtime() is used. Using the wrong variant while the DSQ
already has the other type queued is not allowed and triggers an ops error.
Built-in DSQs must always be FIFOs.
This makes it very easy for the BPF schedulers to implement proper vtime
based scheduling within each dsq very easy and efficient at a negligible
cost in terms of code complexity and overhead.
scx_simple and scx_example_flatcg are updated to default to weighted
vtime scheduling (the latter within each cgroup). FIFO scheduling can be
selected with -f option.
v4: - As allowing mixing priority queue and FIFO on the same DSQ sometimes
led to unexpected starvations, DSQs now error out if both modes are
used at the same time and the built-in DSQs are no longer allowed to
be priority queues.
- Explicit type struct scx_dsq_node added to contain fields needed to be
linked on DSQs. This will be used to implement stateful iterator.
- Tasks are now always linked on dsq->list whether the DSQ is in FIFO or
PRIQ mode. This confines PRIQ related complexities to the enqueue and
dequeue paths. Other paths only need to look at dsq->list. This will
also ease implementing BPF iterator.
- Print p->scx.dsq_flags in debug dump.
v3: - SCX_TASK_DSQ_ON_PRIQ flag is moved from p->scx.flags into its own
p->scx.dsq_flags. The flag is protected with the dsq lock unlike other
flags in p->scx.flags. This led to flag corruption in some cases.
- Add comments explaining the interaction between using consumption of
p->scx.slice to determine vtime progress and yielding.
v2: - p->scx.dsq_vtime was not initialized on load or across cgroup
migrations leading to some tasks being stalled for extended period of
time depending on how saturated the machine is. Fixed.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: David Vernet <dvernet@meta.com>
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The core-sched support is composed of the following parts:
- task_struct->scx.core_sched_at is added. This is a timestamp which can be
used to order tasks. Depending on whether the BPF scheduler implements
custom ordering, it tracks either global FIFO ordering of all tasks or
local-DSQ ordering within the dispatched tasks on a CPU.
- prio_less() is updated to call scx_prio_less() when comparing SCX tasks.
scx_prio_less() calls ops.core_sched_before() if available or uses the
core_sched_at timestamp. For global FIFO ordering, the BPF scheduler
doesn't need to do anything. Otherwise, it should implement
ops.core_sched_before() which reflects the ordering.
- When core-sched is enabled, balance_scx() balances all SMT siblings so
that they all have tasks dispatched if necessary before pick_task_scx() is
called. pick_task_scx() picks between the current task and the first
dispatched task on the local DSQ based on availability and the
core_sched_at timestamps. Note that FIFO ordering is expected among the
already dispatched tasks whether running or on the local DSQ, so this path
always compares core_sched_at instead of calling into
ops.core_sched_before().
qmap_core_sched_before() is added to scx_qmap. It scales the
distances from the heads of the queues to compare the tasks across different
priority queues and seems to behave as expected.
v3: Fixed build error when !CONFIG_SCHED_SMT reported by Andrea Righi.
v2: Sched core added the const qualifiers to prio_less task arguments.
Explicitly drop them for ops.core_sched_before() task arguments. BPF
enforces access control through the verifier, so the qualifier isn't
actually operative and only gets in the way when interacting with
various helpers.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: David Vernet <dvernet@meta.com>
Reviewed-by: Josh Don <joshdon@google.com>
Cc: Andrea Righi <andrea.righi@canonical.com>
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Scheduler classes are strictly ordered and when a higher priority class has
tasks to run, the lower priority ones lose access to the CPU. Being able to
monitor and act on these events are necessary for use cases includling
strict core-scheduling and latency management.
This patch adds two operations ops.cpu_acquire() and .cpu_release(). The
former is invoked when a CPU becomes available to the BPF scheduler and the
opposite for the latter. This patch also implements
scx_bpf_reenqueue_local() which can be called from .cpu_release() to trigger
requeueing of all tasks in the local dsq of the CPU so that the tasks can be
reassigned to other available CPUs.
scx_pair is updated to use .cpu_acquire/release() along with
%SCX_KICK_WAIT to make the pair scheduling guarantee strict even when a CPU
is preempted by a higher priority scheduler class.
scx_qmap is updated to use .cpu_acquire/release() to empty the local
dsq of a preempted CPU. A similar approach can be adopted by BPF schedulers
that want to have a tight control over latency.
v4: Use the new SCX_KICK_IDLE to wake up a CPU after re-enqueueing.
v3: Drop the const qualifier from scx_cpu_release_args.task. BPF enforces
access control through the verifier, so the qualifier isn't actually
operative and only gets in the way when interacting with various
helpers.
v2: Add p->scx.kf_mask annotation to allow calling scx_bpf_reenqueue_local()
from ops.cpu_release() nested inside ops.init() and other sleepable
operations.
Signed-off-by: David Vernet <dvernet@meta.com>
Reviewed-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Josh Don <joshdon@google.com>
Acked-by: Hao Luo <haoluo@google.com>
Acked-by: Barret Rhoden <brho@google.com>
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When some SCX operations are in flight, it is known that the subject task's
rq lock is held throughout which makes it safe to access certain fields of
the task - e.g. its current task_group. We want to add SCX kfunc helpers
that can make use of this guarantee - e.g. to help determining the currently
associated CPU cgroup from the task's current task_group.
As it'd be dangerous call such a helper on a task which isn't rq lock
protected, the helper should be able to verify the input task and reject
accordingly. This patch adds sched_ext_entity.kf_tasks[] that track the
tasks which are currently being operated on by a terminal SCX operation. The
new SCX_CALL_OP_[2]TASK[_RET]() can be used when invoking SCX operations
which take tasks as arguments and the scx_kf_allowed_on_arg_tasks() can be
used by kfunc helpers to verify the input task status.
Note that as sched_ext_entity.kf_tasks[] can't handle nesting, the tracking
is currently only limited to terminal SCX operations. If needed in the
future, this restriction can be removed by moving the tracking to the task
side with a couple per-task counters.
v2: Updated to reflect the addition of SCX_KF_SELECT_CPU.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: David Vernet <dvernet@meta.com>
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Allow BPF schedulers to indicate tickless operation by setting p->scx.slice
to SCX_SLICE_INF. A CPU whose current task has infinte slice goes into
tickless operation.
scx_central is updated to use tickless operations for all tasks and
instead use a BPF timer to expire slices. This also uses the SCX_ENQ_PREEMPT
and task state tracking added by the previous patches.
Currently, there is no way to pin the timer on the central CPU, so it may
end up on one of the worker CPUs; however, outside of that, the worker CPUs
can go tickless both while running sched_ext tasks and idling.
With schbench running, scx_central shows:
root@test ~# grep ^LOC /proc/interrupts; sleep 10; grep ^LOC /proc/interrupts
LOC: 142024 656 664 449 Local timer interrupts
LOC: 161663 663 665 449 Local timer interrupts
Without it:
root@test ~ [SIGINT]# grep ^LOC /proc/interrupts; sleep 10; grep ^LOC /proc/interrupts
LOC: 188778 3142 3793 3993 Local timer interrupts
LOC: 198993 5314 6323 6438 Local timer interrupts
While scx_central itself is too barebone to be useful as a
production scheduler, a more featureful central scheduler can be built using
the same approach. Google's experience shows that such an approach can have
significant benefits for certain applications such as VM hosting.
v4: Allow operation even if BPF_F_TIMER_CPU_PIN is not available.
v3: Pin the central scheduler's timer on the central_cpu using
BPF_F_TIMER_CPU_PIN.
v2: Convert to BPF inline iterators.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: David Vernet <dvernet@meta.com>
Acked-by: Josh Don <joshdon@google.com>
Acked-by: Hao Luo <haoluo@google.com>
Acked-by: Barret Rhoden <brho@google.com>
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It's often useful to wake up and/or trigger reschedule on other CPUs. This
patch adds scx_bpf_kick_cpu() kfunc helper that BPF scheduler can call to
kick the target CPU into the scheduling path.
As a sched_ext task relinquishes its CPU only after its slice is depleted,
this patch also adds SCX_KICK_PREEMPT and SCX_ENQ_PREEMPT which clears the
slice of the target CPU's current task to guarantee that sched_ext's
scheduling path runs on the CPU.
If SCX_KICK_IDLE is specified, the target CPU is kicked iff the CPU is idle
to guarantee that the target CPU will go through at least one full sched_ext
scheduling cycle after the kicking. This can be used to wake up idle CPUs
without incurring unnecessary overhead if it isn't currently idle.
As a demonstration of how backward compatibility can be supported using BPF
CO-RE, tools/sched_ext/include/scx/compat.bpf.h is added. It provides
__COMPAT_scx_bpf_kick_cpu_IDLE() which uses SCX_KICK_IDLE if available or
becomes a regular kicking otherwise. This allows schedulers to use the new
SCX_KICK_IDLE while maintaining support for older kernels. The plan is to
temporarily use compat helpers to ease API updates and drop them after a few
kernel releases.
v5: - SCX_KICK_IDLE added. Note that this also adds a compat mechanism for
schedulers so that they can support kernels without SCX_KICK_IDLE.
This is useful as a demonstration of how new feature flags can be
added in a backward compatible way.
- kick_cpus_irq_workfn() reimplemented so that it touches the pending
cpumasks only as necessary to reduce kicking overhead on machines with
a lot of CPUs.
- tools/sched_ext/include/scx/compat.bpf.h added.
v4: - Move example scheduler to its own patch.
v3: - Make scx_example_central switch all tasks by default.
- Convert to BPF inline iterators.
v2: - Julia Lawall reported that scx_example_central can overflow the
dispatch buffer and malfunction. As scheduling for other CPUs can't be
handled by the automatic retry mechanism, fix by implementing an
explicit overflow and retry handling.
- Updated to use generic BPF cpumask helpers.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: David Vernet <dvernet@meta.com>
Acked-by: Josh Don <joshdon@google.com>
Acked-by: Hao Luo <haoluo@google.com>
Acked-by: Barret Rhoden <brho@google.com>
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It would be useful to see what the sched_ext scheduler state is, and what
scheduler is running, when we're dumping a task's stack. This patch
therefore adds a new print_scx_info() function that's called in the same
context as print_worker_info() and print_stop_info(). An example dump
follows.
BUG: kernel NULL pointer dereference, address: 0000000000000999
#PF: supervisor write access in kernel mode
#PF: error_code(0x0002) - not-present page
PGD 0 P4D 0
Oops: 0002 [#1] PREEMPT SMP
CPU: 13 PID: 2047 Comm: insmod Tainted: G O 6.6.0-work-10323-gb58d4cae8e99-dirty #34
Hardware name: QEMU Standard PC (i440FX + PIIX, 1996), BIOS unknown 2/2/2022
Sched_ext: qmap (enabled+all), task: runnable_at=-17ms
RIP: 0010:init_module+0x9/0x1000 [test_module]
...
v3: - scx_ops_enable_state_str[] definition moved to an earlier patch as
it's now used by core implementation.
- Convert jiffy delta to msecs using jiffies_to_msecs() instead of
multiplying by (HZ / MSEC_PER_SEC). The conversion is implemented in
jiffies_delta_msecs().
v2: - We are now using scx_ops_enable_state_str[] outside
CONFIG_SCHED_DEBUG. Move it outside of CONFIG_SCHED_DEBUG and to the
top. This was reported by Changwoo and Andrea.
Signed-off-by: David Vernet <void@manifault.com>
Reported-by: Changwoo Min <changwoo@igalia.com>
Reported-by: Andrea Righi <andrea.righi@canonical.com>
Signed-off-by: Tejun Heo <tj@kernel.org>
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SCHED_EXT
BPF schedulers might not want to schedule certain tasks - e.g. kernel
threads. This patch adds p->scx.disallow which can be set by BPF schedulers
in such cases. The field can be changed anytime and setting it in
ops.prep_enable() guarantees that the task can never be scheduled by
sched_ext.
scx_qmap is updated with the -d option to disallow a specific PID:
# echo $$
1092
# grep -E '(policy)|(ext\.enabled)' /proc/self/sched
policy : 0
ext.enabled : 0
# ./set-scx 1092
# grep -E '(policy)|(ext\.enabled)' /proc/self/sched
policy : 7
ext.enabled : 0
Run "scx_qmap -p -d 1092" in another terminal.
# cat /sys/kernel/sched_ext/nr_rejected
1
# grep -E '(policy)|(ext\.enabled)' /proc/self/sched
policy : 0
ext.enabled : 0
# ./set-scx 1092
setparam failed for 1092 (Permission denied)
- v4: Refreshed on top of tip:sched/core.
- v3: Update description to reflect /sys/kernel/sched_ext interface change.
- v2: Use atomic_long_t instead of atomic64_t for scx_kick_cpus_pnt_seqs to
accommodate 32bit archs.
Signed-off-by: Tejun Heo <tj@kernel.org>
Suggested-by: Barret Rhoden <brho@google.com>
Reviewed-by: David Vernet <dvernet@meta.com>
Acked-by: Josh Don <joshdon@google.com>
Acked-by: Hao Luo <haoluo@google.com>
Acked-by: Barret Rhoden <brho@google.com>
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The most common and critical way that a BPF scheduler can misbehave is by
failing to run runnable tasks for too long. This patch implements a
watchdog.
* All tasks record when they become runnable.
* A watchdog work periodically scans all runnable tasks. If any task has
stayed runnable for too long, the BPF scheduler is aborted.
* scheduler_tick() monitors whether the watchdog itself is stuck. If so, the
BPF scheduler is aborted.
Because the watchdog only scans the tasks which are currently runnable and
usually very infrequently, the overhead should be negligible.
scx_qmap is updated so that it can be told to stall user and/or
kernel tasks.
A detected task stall looks like the following:
sched_ext: BPF scheduler "qmap" errored, disabling
sched_ext: runnable task stall (dbus-daemon[953] failed to run for 6.478s)
scx_check_timeout_workfn+0x10e/0x1b0
process_one_work+0x287/0x560
worker_thread+0x234/0x420
kthread+0xe9/0x100
ret_from_fork+0x1f/0x30
A detected watchdog stall:
sched_ext: BPF scheduler "qmap" errored, disabling
sched_ext: runnable task stall (watchdog failed to check in for 5.001s)
scheduler_tick+0x2eb/0x340
update_process_times+0x7a/0x90
tick_sched_timer+0xd8/0x130
__hrtimer_run_queues+0x178/0x3b0
hrtimer_interrupt+0xfc/0x390
__sysvec_apic_timer_interrupt+0xb7/0x2b0
sysvec_apic_timer_interrupt+0x90/0xb0
asm_sysvec_apic_timer_interrupt+0x1b/0x20
default_idle+0x14/0x20
arch_cpu_idle+0xf/0x20
default_idle_call+0x50/0x90
do_idle+0xe8/0x240
cpu_startup_entry+0x1d/0x20
kernel_init+0x0/0x190
start_kernel+0x0/0x392
start_kernel+0x324/0x392
x86_64_start_reservations+0x2a/0x2c
x86_64_start_kernel+0x104/0x109
secondary_startup_64_no_verify+0xce/0xdb
Note that this patch exposes scx_ops_error[_type]() in kernel/sched/ext.h to
inline scx_notify_sched_tick().
v4: - While disabling, cancel_delayed_work_sync(&scx_watchdog_work) was
being called before forward progress was guaranteed and thus could
lead to system lockup. Relocated.
- While enabling, it was comparing msecs against jiffies without
conversion leading to spurious load failures on lower HZ kernels.
Fixed.
- runnable list management is now used by core bypass logic and moved to
the patch implementing sched_ext core.
v3: - bpf_scx_init_member() was incorrectly comparing ops->timeout_ms
against SCX_WATCHDOG_MAX_TIMEOUT which is in jiffies without
conversion leading to spurious load failures in lower HZ kernels.
Fixed.
v2: - Julia Lawall noticed that the watchdog code was mixing msecs and
jiffies. Fix by using jiffies for everything.
Signed-off-by: David Vernet <dvernet@meta.com>
Reviewed-by: Tejun Heo <tj@kernel.org>
Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Josh Don <joshdon@google.com>
Acked-by: Hao Luo <haoluo@google.com>
Acked-by: Barret Rhoden <brho@google.com>
Cc: Julia Lawall <julia.lawall@inria.fr>
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Implement a new scheduler class sched_ext (SCX), which allows scheduling
policies to be implemented as BPF programs to achieve the following:
1. Ease of experimentation and exploration: Enabling rapid iteration of new
scheduling policies.
2. Customization: Building application-specific schedulers which implement
policies that are not applicable to general-purpose schedulers.
3. Rapid scheduler deployments: Non-disruptive swap outs of scheduling
policies in production environments.
sched_ext leverages BPF’s struct_ops feature to define a structure which
exports function callbacks and flags to BPF programs that wish to implement
scheduling policies. The struct_ops structure exported by sched_ext is
struct sched_ext_ops, and is conceptually similar to struct sched_class. The
role of sched_ext is to map the complex sched_class callbacks to the more
simple and ergonomic struct sched_ext_ops callbacks.
For more detailed discussion on the motivations and overview, please refer
to the cover letter.
Later patches will also add several example schedulers and documentation.
This patch implements the minimum core framework to enable implementation of
BPF schedulers. Subsequent patches will gradually add functionalities
including safety guarantee mechanisms, nohz and cgroup support.
include/linux/sched/ext.h defines struct sched_ext_ops. With the comment on
top, each operation should be self-explanatory. The followings are worth
noting:
- Both "sched_ext" and its shorthand "scx" are used. If the identifier
already has "sched" in it, "ext" is used; otherwise, "scx".
- In sched_ext_ops, only .name is mandatory. Every operation is optional and
if omitted a simple but functional default behavior is provided.
- A new policy constant SCHED_EXT is added and a task can select sched_ext
by invoking sched_setscheduler(2) with the new policy constant. However,
if the BPF scheduler is not loaded, SCHED_EXT is the same as SCHED_NORMAL
and the task is scheduled by CFS. When the BPF scheduler is loaded, all
tasks which have the SCHED_EXT policy are switched to sched_ext.
- To bridge the workflow imbalance between the scheduler core and
sched_ext_ops callbacks, sched_ext uses simple FIFOs called dispatch
queues (dsq's). By default, there is one global dsq (SCX_DSQ_GLOBAL), and
one local per-CPU dsq (SCX_DSQ_LOCAL). SCX_DSQ_GLOBAL is provided for
convenience and need not be used by a scheduler that doesn't require it.
SCX_DSQ_LOCAL is the per-CPU FIFO that sched_ext pulls from when putting
the next task on the CPU. The BPF scheduler can manage an arbitrary number
of dsq's using scx_bpf_create_dsq() and scx_bpf_destroy_dsq().
- sched_ext guarantees system integrity no matter what the BPF scheduler
does. To enable this, each task's ownership is tracked through
p->scx.ops_state and all tasks are put on scx_tasks list. The disable path
can always recover and revert all tasks back to CFS. See p->scx.ops_state
and scx_tasks.
- A task is not tied to its rq while enqueued. This decouples CPU selection
from queueing and allows sharing a scheduling queue across an arbitrary
subset of CPUs. This adds some complexities as a task may need to be
bounced between rq's right before it starts executing. See
dispatch_to_local_dsq() and move_task_to_local_dsq().
- One complication that arises from the above weak association between task
and rq is that synchronizing with dequeue() gets complicated as dequeue()
may happen anytime while the task is enqueued and the dispatch path might
need to release the rq lock to transfer the task. Solving this requires a
bit of complexity. See the logic around p->scx.sticky_cpu and
p->scx.ops_qseq.
- Both enable and disable paths are a bit complicated. The enable path
switches all tasks without blocking to avoid issues which can arise from
partially switched states (e.g. the switching task itself being starved).
The disable path can't trust the BPF scheduler at all, so it also has to
guarantee forward progress without blocking. See scx_ops_enable() and
scx_ops_disable_workfn().
- When sched_ext is disabled, static_branches are used to shut down the
entry points from hot paths.
v7: - scx_ops_bypass() was incorrectly and unnecessarily trying to grab
scx_ops_enable_mutex which can lead to deadlocks in the disable path.
Fixed.
- Fixed TASK_DEAD handling bug in scx_ops_enable() path which could lead
to use-after-free.
- Consolidated per-cpu variable usages and other cleanups.
v6: - SCX_NR_ONLINE_OPS replaced with SCX_OPI_*_BEGIN/END so that multiple
groups can be expressed. Later CPU hotplug operations are put into
their own group.
- SCX_OPS_DISABLING state is replaced with the new bypass mechanism
which allows temporarily putting the system into simple FIFO
scheduling mode bypassing the BPF scheduler. In addition to the shut
down path, this will also be used to isolate the BPF scheduler across
PM events. Enabling and disabling the bypass mode requires iterating
all runnable tasks. rq->scx.runnable_list addition is moved from the
later watchdog patch.
- ops.prep_enable() is replaced with ops.init_task() and
ops.enable/disable() are now called whenever the task enters and
leaves sched_ext instead of when the task becomes schedulable on
sched_ext and stops being so. A new operation - ops.exit_task() - is
called when the task stops being schedulable on sched_ext.
- scx_bpf_dispatch() can now be called from ops.select_cpu() too. This
removes the need for communicating local dispatch decision made by
ops.select_cpu() to ops.enqueue() via per-task storage.
SCX_KF_SELECT_CPU is added to support the change.
- SCX_TASK_ENQ_LOCAL which told the BPF scheudler that
scx_select_cpu_dfl() wants the task to be dispatched to the local DSQ
was removed. Instead, scx_bpf_select_cpu_dfl() now dispatches directly
if it finds a suitable idle CPU. If such behavior is not desired,
users can use scx_bpf_select_cpu_dfl() which returns the verdict in a
bool out param.
- scx_select_cpu_dfl() was mishandling WAKE_SYNC and could end up
queueing many tasks on a local DSQ which makes tasks to execute in
order while other CPUs stay idle which made some hackbench numbers
really bad. Fixed.
- The current state of sched_ext can now be monitored through files
under /sys/sched_ext instead of /sys/kernel/debug/sched/ext. This is
to enable monitoring on kernels which don't enable debugfs.
- sched_ext wasn't telling BPF that ops.dispatch()'s @prev argument may
be NULL and a BPF scheduler which derefs the pointer without checking
could crash the kernel. Tell BPF. This is currently a bit ugly. A
better way to annotate this is expected in the future.
- scx_exit_info updated to carry pointers to message buffers instead of
embedding them directly. This decouples buffer sizes from API so that
they can be changed without breaking compatibility.
- exit_code added to scx_exit_info. This is used to indicate different
exit conditions on non-error exits and will be used to handle e.g. CPU
hotplugs.
- The patch "sched_ext: Allow BPF schedulers to switch all eligible
tasks into sched_ext" is folded in and the interface is changed so
that partial switching is indicated with a new ops flag
%SCX_OPS_SWITCH_PARTIAL. This makes scx_bpf_switch_all() unnecessasry
and in turn SCX_KF_INIT. ops.init() is now called with
SCX_KF_SLEEPABLE.
- Code reorganized so that only the parts necessary to integrate with
the rest of the kernel are in the header files.
- Changes to reflect the BPF and other kernel changes including the
addition of bpf_sched_ext_ops.cfi_stubs.
v5: - To accommodate 32bit configs, p->scx.ops_state is now atomic_long_t
instead of atomic64_t and scx_dsp_buf_ent.qseq which uses
load_acquire/store_release is now unsigned long instead of u64.
- Fix the bug where bpf_scx_btf_struct_access() was allowing write
access to arbitrary fields.
- Distinguish kfuncs which can be called from any sched_ext ops and from
anywhere. e.g. scx_bpf_pick_idle_cpu() can now be called only from
sched_ext ops.
- Rename "type" to "kind" in scx_exit_info to make it easier to use on
languages in which "type" is a reserved keyword.
- Since cff9b2332ab7 ("kernel/sched: Modify initial boot task idle
setup"), PF_IDLE is not set on idle tasks which haven't been online
yet which made scx_task_iter_next_filtered() include those idle tasks
in iterations leading to oopses. Update scx_task_iter_next_filtered()
to directly test p->sched_class against idle_sched_class instead of
using is_idle_task() which tests PF_IDLE.
- Other updates to match upstream changes such as adding const to
set_cpumask() param and renaming check_preempt_curr() to
wakeup_preempt().
v4: - SCHED_CHANGE_BLOCK replaced with the previous
sched_deq_and_put_task()/sched_enq_and_set_tsak() pair. This is
because upstream is adaopting a different generic cleanup mechanism.
Once that lands, the code will be adapted accordingly.
- task_on_scx() used to test whether a task should be switched into SCX,
which is confusing. Renamed to task_should_scx(). task_on_scx() now
tests whether a task is currently on SCX.
- scx_has_idle_cpus is barely used anymore and replaced with direct
check on the idle cpumask.
- SCX_PICK_IDLE_CORE added and scx_pick_idle_cpu() improved to prefer
fully idle cores.
- ops.enable() now sees up-to-date p->scx.weight value.
- ttwu_queue path is disabled for tasks on SCX to avoid confusing BPF
schedulers expecting ->select_cpu() call.
- Use cpu_smt_mask() instead of topology_sibling_cpumask() like the rest
of the scheduler.
v3: - ops.set_weight() added to allow BPF schedulers to track weight changes
without polling p->scx.weight.
- move_task_to_local_dsq() was losing SCX-specific enq_flags when
enqueueing the task on the target dsq because it goes through
activate_task() which loses the upper 32bit of the flags. Carry the
flags through rq->scx.extra_enq_flags.
- scx_bpf_dispatch(), scx_bpf_pick_idle_cpu(), scx_bpf_task_running()
and scx_bpf_task_cpu() now use the new KF_RCU instead of
KF_TRUSTED_ARGS to make it easier for BPF schedulers to call them.
- The kfunc helper access control mechanism implemented through
sched_ext_entity.kf_mask is improved. Now SCX_CALL_OP*() is always
used when invoking scx_ops operations.
v2: - balance_scx_on_up() is dropped. Instead, on UP, balance_scx() is
called from put_prev_taks_scx() and pick_next_task_scx() as necessary.
To determine whether balance_scx() should be called from
put_prev_task_scx(), SCX_TASK_DEQD_FOR_SLEEP flag is added. See the
comment in put_prev_task_scx() for details.
- sched_deq_and_put_task() / sched_enq_and_set_task() sequences replaced
with SCHED_CHANGE_BLOCK().
- Unused all_dsqs list removed. This was a left-over from previous
iterations.
- p->scx.kf_mask is added to track and enforce which kfunc helpers are
allowed. Also, init/exit sequences are updated to make some kfuncs
always safe to call regardless of the current BPF scheduler state.
Combined, this should make all the kfuncs safe.
- BPF now supports sleepable struct_ops operations. Hacky workaround
removed and operations and kfunc helpers are tagged appropriately.
- BPF now supports bitmask / cpumask helpers. scx_bpf_get_idle_cpumask()
and friends are added so that BPF schedulers can use the idle masks
with the generic helpers. This replaces the hacky kfunc helpers added
by a separate patch in V1.
- CONFIG_SCHED_CLASS_EXT can no longer be enabled if SCHED_CORE is
enabled. This restriction will be removed by a later patch which adds
core-sched support.
- Add MAINTAINERS entries and other misc changes.
Signed-off-by: Tejun Heo <tj@kernel.org>
Co-authored-by: David Vernet <dvernet@meta.com>
Acked-by: Josh Don <joshdon@google.com>
Acked-by: Hao Luo <haoluo@google.com>
Acked-by: Barret Rhoden <brho@google.com>
Cc: Andrea Righi <andrea.righi@canonical.com>
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This adds dummy implementations of sched_ext interfaces which interact with
the scheduler core and hook them in the correct places. As they're all
dummies, this doesn't cause any behavior changes. This is split out to help
reviewing.
v2: balance_scx_on_up() dropped. This will be handled in sched_ext proper.
Signed-off-by: Tejun Heo <tj@kernel.org>
Reviewed-by: David Vernet <dvernet@meta.com>
Acked-by: Josh Don <joshdon@google.com>
Acked-by: Hao Luo <haoluo@google.com>
Acked-by: Barret Rhoden <brho@google.com>
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