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2017-08-31block, bfq: make lookup_next_entity push up vtime on expirationsPaolo Valente
To provide a very smooth service, bfq starts to serve a bfq_queue only if the queue is 'eligible', i.e., if the same queue would have started to be served in the ideal, perfectly fair system that bfq simulates internally. This is obtained by associating each queue with a virtual start time, and by computing a special system virtual time quantity: a queue is eligible only if the system virtual time has reached the virtual start time of the queue. Finally, bfq guarantees that, when a new queue must be set in service, there is always at least one eligible entity for each active parent entity in the scheduler. To provide this guarantee, the function __bfq_lookup_next_entity pushes up, for each parent entity on which it is invoked, the system virtual time to the minimum among the virtual start times of the entities in the active tree for the parent entity (more precisely, the push up occurs if the system virtual time happens to be lower than all such virtual start times). There is however a circumstance in which __bfq_lookup_next_entity cannot push up the system virtual time for a parent entity, even if the system virtual time is lower than the virtual start times of all the child entities in the active tree. It happens if one of the child entities is in service. In fact, in such a case, there is already an eligible entity, the in-service one, even if it may not be not present in the active tree (because in-service entities may be removed from the active tree). Unfortunately, in the last re-design of the hierarchical-scheduling engine, the reset of the pointer to the in-service entity for a given parent entity--reset to be done as a consequence of the expiration of the in-service entity--always happens after the function __bfq_lookup_next_entity has been invoked. This causes the function to think that there is still an entity in service for the parent entity, and then that the system virtual time cannot be pushed up, even if actually such a no-more-in-service entity has already been properly reinserted into the active tree (or in some other tree if no more active). Yet, the system virtual time *had* to be pushed up, to be ready to correctly choose the next queue to serve. Because of the lack of this push up, bfq may wrongly set in service a queue that had been speculatively pre-computed as the possible next-in-service queue, but that would no more be the one to serve after the expiration and the reinsertion into the active trees of the previously in-service entities. This commit addresses this issue by making __bfq_lookup_next_entity properly push up the system virtual time if an expiration is occurring. Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Tested-by: Lee Tibbert <lee.tibbert@gmail.com> Tested-by: Oleksandr Natalenko <oleksandr@natalenko.name> Signed-off-by: Jens Axboe <axboe@kernel.dk>
2017-08-28Merge tag 'v4.13-rc7' into for-4.14/block-postmergeJens Axboe
Linux 4.13-rc7 Signed-off-by: Jens Axboe <axboe@kernel.dk>
2017-08-11block,bfq: refactor device-idling logicPaolo Valente
The logic that decides whether to idle the device is scattered across three functions. Almost all of the logic is in the function bfq_bfqq_may_idle, but (1) part of the decision is made in bfq_update_idle_window, and (2) the function bfq_bfqq_must_idle may switch off idling regardless of the output of bfq_bfqq_may_idle. In addition, both bfq_update_idle_window and bfq_bfqq_must_idle make their decisions as a function of parameters that are used, for similar purposes, also in bfq_bfqq_may_idle. This commit addresses these issues by moving all the logic into bfq_bfqq_may_idle. Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Signed-off-by: Jens Axboe <axboe@kernel.dk>
2017-07-29block, bfq: consider also in_service_entity to state whether an entity is activePaolo Valente
Groups of BFQ queues are represented by generic entities in BFQ. When a queue belonging to a parent entity is deactivated, the parent entity may need to be deactivated too, in case the deactivated queue was the only active queue for the parent entity. This deactivation may need to be propagated upwards if the entity belongs, in its turn, to a further higher-level entity, and so on. In particular, the upward propagation of deactivation stops at the first parent entity that remains active even if one of its child entities has been deactivated. To decide whether the last non-deactivation condition holds for a parent entity, BFQ checks whether the field next_in_service is still not NULL for the parent entity, after the deactivation of one of its child entity. If it is not NULL, then there are certainly other active entities in the parent entity, and deactivations can stop. Unfortunately, this check misses a corner case: if in_service_entity is not NULL, then next_in_service may happen to be NULL, although the parent entity is evidently active. This happens if: 1) the entity pointed by in_service_entity is the only active entity in the parent entity, and 2) according to the definition of next_in_service, the in_service_entity cannot be considered as next_in_service. See the comments on the definition of next_in_service for details on this second point. Hitting the above corner case causes crashes. To address this issue, this commit: 1) Extends the above check on only next_in_service to controlling both next_in_service and in_service_entity (if any of them is not NULL, then no further deactivation is performed) 2) Improves the (important) comments on how next_in_service is defined and updated; in particular it fixes a few rather obscure paragraphs Reported-by: Eric Wheeler <bfq-sched@lists.ewheeler.net> Reported-by: Rick Yiu <rick_yiu@htc.com> Reported-by: Tom X Nguyen <tom81094@gmail.com> Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Tested-by: Eric Wheeler <bfq-sched@lists.ewheeler.net> Tested-by: Rick Yiu <rick_yiu@htc.com> Tested-by: Laurentiu Nicola <lnicola@dend.ro> Tested-by: Tom X Nguyen <tom81094@gmail.com> Signed-off-by: Jens Axboe <axboe@kernel.dk>
2017-07-29block: use standard blktrace API to output cgroup info for debug notesShaohua Li
Currently cfq/bfq/blk-throttle output cgroup info in trace in their own way. Now we have standard blktrace API for this, so convert them to use it. Note, this changes the behavior a little bit. cgroup info isn't output by default, we only do this with 'blk_cgroup' option enabled. cgroup info isn't output as a string by default too, we only do this with 'blk_cgname' option enabled. Also cgroup info is output in different position of the note string. I think these behavior changes aren't a big issue (actually we make trace data shorter which is good), since the blktrace note is solely for debugging. Signed-off-by: Shaohua Li <shli@fb.com> Signed-off-by: Jens Axboe <axboe@kernel.dk>
2017-07-12bfq: fix typos in comments about B-WF2Q+ algorithmHou Tao
The start time of eligible entity should be less than or equal to the current virtual time, and the entity in idle tree has a finish time being greater than the current virtual time. Signed-off-by: Hou Tao <houtao1@huawei.com> Reviewed-by: Paolo Valente <paolo.valente@linaro.org> Signed-off-by: Jens Axboe <axboe@kernel.dk>
2017-07-03block, bfq: don't change ioprio class for a bfq_queue on a service treePaolo Valente
On each deactivation or re-scheduling (after being served) of a bfq_queue, BFQ invokes the function __bfq_entity_update_weight_prio(), to perform pending updates of ioprio, weight and ioprio class for the bfq_queue. BFQ also invokes this function on I/O-request dispatches, to raise or lower weights more quickly when needed, thereby improving latency. However, the entity representing the bfq_queue may be on the active (sub)tree of a service tree when this happens, and, although with a very low probability, the bfq_queue may happen to also have a pending change of its ioprio class. If both conditions hold when __bfq_entity_update_weight_prio() is invoked, then the entity moves to a sort of hybrid state: the new service tree for the entity, as returned by bfq_entity_service_tree(), differs from service tree on which the entity still is. The functions that handle activations and deactivations of entities do not cope with such a hybrid state (and would need to become more complex to cope). This commit addresses this issue by just making __bfq_entity_update_weight_prio() not perform also a possible pending change of ioprio class, when invoked on an I/O-request dispatch for a bfq_queue. Such a change is thus postponed to when __bfq_entity_update_weight_prio() is invoked on deactivation or re-scheduling of the bfq_queue. Reported-by: Marco Piazza <mpiazza@gmail.com> Reported-by: Laurentiu Nicola <lnicola@dend.ro> Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Tested-by: Marco Piazza <mpiazza@gmail.com> Signed-off-by: Jens Axboe <axboe@kernel.dk>
2017-06-08block, bfq: access and cache blkg data only when safePaolo Valente
In blk-cgroup, operations on blkg objects are protected with the request_queue lock. This is no more the lock that protects I/O-scheduler operations in blk-mq. In fact, the latter are now protected with a finer-grained per-scheduler-instance lock. As a consequence, although blkg lookups are also rcu-protected, blk-mq I/O schedulers may see inconsistent data when they access blkg and blkg-related objects. BFQ does access these objects, and does incur this problem, in the following case. The blkg_lookup performed in bfq_get_queue, being protected (only) through rcu, may happen to return the address of a copy of the original blkg. If this is the case, then the blkg_get performed in bfq_get_queue, to pin down the blkg, is useless: it does not prevent blk-cgroup code from destroying both the original blkg and all objects directly or indirectly referred by the copy of the blkg. BFQ accesses these objects, which typically causes a crash for NULL-pointer dereference of memory-protection violation. Some additional protection mechanism should be added to blk-cgroup to address this issue. In the meantime, this commit provides a quick temporary fix for BFQ: cache (when safe) blkg data that might disappear right after a blkg_lookup. In particular, this commit exploits the following facts to achieve its goal without introducing further locks. Destroy operations on a blkg invoke, as a first step, hooks of the scheduler associated with the blkg. And these hooks are executed with bfqd->lock held for BFQ. As a consequence, for any blkg associated with the request queue an instance of BFQ is attached to, we are guaranteed that such a blkg is not destroyed, and that all the pointers it contains are consistent, while that instance is holding its bfqd->lock. A blkg_lookup performed with bfqd->lock held then returns a fully consistent blkg, which remains consistent until this lock is held. In more detail, this holds even if the returned blkg is a copy of the original one. Finally, also the object describing a group inside BFQ needs to be protected from destruction on the blkg_free of the original blkg (which invokes bfq_pd_free). This commit adds private refcounting for this object, to let it disappear only after no bfq_queue refers to it any longer. This commit also removes or updates some stale comments on locking issues related to blk-cgroup operations. Reported-by: Tomas Konir <tomas.konir@gmail.com> Reported-by: Lee Tibbert <lee.tibbert@gmail.com> Reported-by: Marco Piazza <mpiazza@gmail.com> Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Tested-by: Tomas Konir <tomas.konir@gmail.com> Tested-by: Lee Tibbert <lee.tibbert@gmail.com> Tested-by: Marco Piazza <mpiazza@gmail.com> Signed-off-by: Jens Axboe <axboe@fb.com>
2017-04-20bfq: fix compile error if CONFIG_CGROUPS=nJens Axboe
If we don't have CGROUPS enabled, the compile ends in the following misery: In file included from ../block/bfq-iosched.c:105:0: ../block/bfq-iosched.h:819:22: error: array type has incomplete element type extern struct cftype bfq_blkcg_legacy_files[]; ^ ../block/bfq-iosched.h:820:22: error: array type has incomplete element type extern struct cftype bfq_blkg_files[]; ^ Move the declarations under the right ifdef. Reported-by: Randy Dunlap <rdunlap@infradead.org> Signed-off-by: Jens Axboe <axboe@fb.com>
2017-04-19block, bfq: split bfq-iosched.c into multiple source filesPaolo Valente
The BFQ I/O scheduler features an optimal fair-queuing (proportional-share) scheduling algorithm, enriched with several mechanisms to boost throughput and reduce latency for interactive and real-time applications. This makes BFQ a large and complex piece of code. This commit addresses this issue by splitting BFQ into three main, independent components, and by moving each component into a separate source file: 1. Main algorithm: handles the interaction with the kernel, and decides which requests to dispatch; it uses the following two further components to achieve its goals. 2. Scheduling engine (Hierarchical B-WF2Q+ scheduling algorithm): computes the schedule, using weights and budgets provided by the above component. 3. cgroups support: handles group operations (creation, destruction, move, ...). Signed-off-by: Paolo Valente <paolo.valente@linaro.org> Signed-off-by: Jens Axboe <axboe@fb.com>