Rebinding workers of a per-cpu pool after a CPU comes online involves
a lot of back-and-forth mostly because only the task itself could
adjust CPU affinity if PF_THREAD_BOUND was set.

As CPU_ONLINE itself couldn't adjust affinity, it had to somehow
coerce the workers themselves to perform set_cpus_allowed_ptr().  Due
to the various states a worker can be in, this led to three different
paths a worker may be rebound.  worker->rebind_work is queued to busy
workers.  Idle ones are signaled by unlinking worker->entry and call
idle_worker_rebind().  The manager isn't covered by either and
implements its own mechanism.

PF_THREAD_BOUND has been relaced with PF_NO_SETAFFINITY and CPU_ONLINE
itself now can manipulate CPU affinity of workers.  This patch
replaces the existing rebind mechanism with direct one where
CPU_ONLINE iterates over all workers using for_each_pool_worker(),
restores CPU affinity, and clears WORKER_UNBOUND.

There are a couple subtleties.  All bound idle workers should have
their runqueues set to that of the bound CPU; however, if the target
task isn't running, set_cpus_allowed_ptr() just updates the
cpus_allowed mask deferring the actual migration to when the task
wakes up.  This is worked around by waking up idle workers after
restoring CPU affinity before any workers can become bound.

Another subtlety is stems from matching @pool->nr_running with the
number of running unbound workers.  While DISASSOCIATED, all workers
are unbound and nr_running is zero.  As workers become bound again,
nr_running needs to be adjusted accordingly; however, there is no good
way to tell whether a given worker is running without poking into
scheduler internals.  Instead of clearing UNBOUND directly,
rebind_workers() replaces UNBOUND with another new NOT_RUNNING flag -
REBOUND, which will later be cleared by the workers themselves while
preparing for the next round of work item execution.  The only change
needed for the workers is clearing REBOUND along with PREP.

* This patch leaves for_each_busy_worker() without any user.  Removed.

* idle_worker_rebind(), busy_worker_rebind_fn(), worker->rebind_work
  and rebind logic in manager_workers() removed.

* worker_thread() now looks at WORKER_DIE instead of testing whether
  @worker->entry is empty to determine whether it needs to do
  something special as dying is the only special thing now.
Signed-off-by: NTejun Heo <tj@kernel.org>
Reviewed-by: NLai Jiangshan <laijs@cn.fujitsu.com>

Workqueue is mixing unsigned int and int for @cpu variables.  There's
no point in using unsigned int for cpus - many of cpu related APIs
take int anyway.  Consistently use int for @cpu variables so that we
can use negative values to mark special ones.

This patch doesn't introduce any visible behavior changes.
Signed-off-by: NTejun Heo <tj@kernel.org>
Reviewed-by: NLai Jiangshan <laijs@cn.fujitsu.com>

Rescuers visit different worker_pools to process work items from pools
under pressure.  Currently, rescuer->pool is updated outside any
locking and when an outsider looks at a rescuer, there's no way to
tell when and whether rescuer->pool is gonna change.  While this
doesn't currently cause any problem, it is nasty.

With recent worker_maybe_bind_and_lock() changes, we can move
rescuer->pool updates inside pool locks such that if rescuer->pool
equals a locked pool, it's guaranteed to stay that way until the pool
is unlocked.

Move rescuer->pool inside pool->lock.

This patch doesn't introduce any visible behavior difference.

tj: Updated the description.
Signed-off-by: NLai Jiangshan <laijs@cn.fujitsu.com>
Signed-off-by: NTejun Heo <tj@kernel.org>

workqueue has moved away from global_cwqs to worker_pools and with the
scheduled custom worker pools, wforkqueues will be associated with
pools which don't have anything to do with CPUs.  The workqueue code
went through significant amount of changes recently and mass renaming
isn't likely to hurt much additionally.  Let's replace 'cpu' with
'pool' so that it reflects the current design.

* s/struct cpu_workqueue_struct/struct pool_workqueue/
* s/cpu_wq/pool_wq/
* s/cwq/pwq/

This patch is purely cosmetic.
Signed-off-by: NTejun Heo <tj@kernel.org>

global_cwq is now nothing but a container for per-cpu standard
worker_pools.  Declare the worker pools directly as
cpu/unbound_std_worker_pools[] and remove global_cwq.

* ____cacheline_aligned_in_smp moved from global_cwq to worker_pool.
  This probably would have made sense even before this change as we
  want each pool to be aligned.

* get_gcwq() is replaced with std_worker_pools() which returns the
  pointer to the standard pool array for a given CPU.

* __alloc_workqueue_key() updated to use get_std_worker_pool() instead
  of open-coding pool determination.

This is part of an effort to remove global_cwq and make worker_pool
the top level abstraction, which in turn will help implementing worker
pools with user-specified attributes.

v2: Joonsoo pointed out that it'd better to align struct worker_pool
    rather than the array so that every pool is aligned.
Signed-off-by: NTejun Heo <tj@kernel.org>
Reviewed-by: NLai Jiangshan <laijs@cn.fujitsu.com>
Cc: Joonsoo Kim <js1304@gmail.com>

This function queries whether %current is an async worker executing an
async item.  This will be used to implement warning on synchronous
request_module() from async workers.
Signed-off-by: NTejun Heo <tj@kernel.org>

This will be used to implement an inline function to query whether
%current is a workqueue worker and, if so, allow determining which
work item it's executing.
Signed-off-by: NTejun Heo <tj@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>

Workqueue wants to expose more interface internal to kernel/.  Instead
of adding a new header file, repurpose kernel/workqueue_sched.h.
Rename it to workqueue_internal.h and add include protector.

This patch doesn't introduce any functional changes.
Signed-off-by: NTejun Heo <tj@kernel.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>