workqueue: Make unbound workqueues to use per-cpu pool_workqueues

A pwq (pool_workqueue) represents an association between a workqueue and a
worker_pool. When a work item is queued, the workqueue selects the pwq to
use, which in turn determines the pool, and queues the work item to the pool
through the pwq. pwq is also what implements the maximum concurrency limit -
@max_active.

As a per-cpu workqueue should be assocaited with a different worker_pool on
each CPU, it always had per-cpu pwq's that are accessed through wq->cpu_pwq.
However, unbound workqueues were sharing a pwq within each NUMA node by
default. The sharing has several downsides:

* Because @max_active is per-pwq, the meaning of @max_active changes
  depending on the machine configuration and whether workqueue NUMA locality
  support is enabled.

* Makes per-cpu and unbound code deviate.

* Gets in the way of making workqueue CPU locality awareness more flexible.

This patch makes unbound workqueues use per-cpu pwq's the same way per-cpu
workqueues do by making the following changes:

* wq->numa_pwq_tbl[] is removed and unbound workqueues now use wq->cpu_pwq
  just like per-cpu workqueues. wq->cpu_pwq is now RCU protected for unbound
  workqueues.

* numa_pwq_tbl_install() is renamed to install_unbound_pwq() and installs
  the specified pwq to the target CPU's wq->cpu_pwq.

* apply_wqattrs_prepare() now always allocates a separate pwq for each CPU
  unless the workqueue is ordered. If ordered, all CPUs use wq->dfl_pwq.
  This makes the return value of wq_calc_node_cpumask() unnecessary. It now
  returns void.

* @max_active now means the same thing for both per-cpu and unbound
  workqueues. WQ_UNBOUND_MAX_ACTIVE now equals WQ_MAX_ACTIVE and
  documentation is updated accordingly. WQ_UNBOUND_MAX_ACTIVE is no longer
  used in workqueue implementation and will be removed later.

* All unbound pwq operations which used to be per-numa-node are now per-cpu.

For most unbound workqueue users, this shouldn't cause noticeable changes.
Work item issue and completion will be a small bit faster, flush_workqueue()
would become a bit more expensive, and the total concurrency limit would
likely become higher. All @max_active==1 use cases are currently being
audited for conversion into alloc_ordered_workqueue() and they shouldn't be
affected once the audit and conversion is complete.

One area where the behavior change may be more noticeable is
workqueue_congested() as the reported congestion state is now per CPU
instead of NUMA node. There are only two users of this interface -
drivers/infiniband/hw/hfi1 and net/smc. Maintainers of both subsystems are
cc'd. Inputs on the behavior change would be very much appreciated.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: Dennis Dalessandro <dennis.dalessandro@cornelisnetworks.com>
Cc: Jason Gunthorpe <jgg@ziepe.ca>
Cc: Leon Romanovsky <leon@kernel.org>
Cc: Karsten Graul <kgraul@linux.ibm.com>
Cc: Wenjia Zhang <wenjia@linux.ibm.com>
Cc: Jan Karcher <jaka@linux.ibm.com>

1 files changed, 10 insertions, 11 deletions

diff --git a/Documentation/core-api/workqueue.rst b/Documentation/core-api/workqueue.rst
index a4c9b9d1905f..8e541c5d8fa9 100644
--- a/Documentation/core-api/workqueue.rst
+++ b/Documentation/core-api/workqueue.rst
@@ -220,17 +220,16 @@ resources, scheduled and executed.
 ``max_active``
 --------------
 
-``@max_active`` determines the maximum number of execution contexts
-per CPU which can be assigned to the work items of a wq.  For example,
-with ``@max_active`` of 16, at most 16 work items of the wq can be
-executing at the same time per CPU.
-
-Currently, for a bound wq, the maximum limit for ``@max_active`` is
-512 and the default value used when 0 is specified is 256.  For an
-unbound wq, the limit is higher of 512 and 4 *
-``num_possible_cpus()``.  These values are chosen sufficiently high
-such that they are not the limiting factor while providing protection
-in runaway cases.
+``@max_active`` determines the maximum number of execution contexts per
+CPU which can be assigned to the work items of a wq. For example, with
+``@max_active`` of 16, at most 16 work items of the wq can be executing
+at the same time per CPU. This is always a per-CPU attribute, even for
+unbound workqueues.
+
+The maximum limit for ``@max_active`` is 512 and the default value used
+when 0 is specified is 256. These values are chosen sufficiently high
+such that they are not the limiting factor while providing protection in
+runaway cases.
 
 The number of active work items of a wq is usually regulated by the
 users of the wq, more specifically, by how many work items the users