workqueue: perform cpu down operations from low priority cpu_notifier()

Currently, all workqueue cpu hotplug operations run off
CPU_PRI_WORKQUEUE which is higher than normal notifiers.  This is to
ensure that workqueue is up and running while bringing up a CPU before
other notifiers try to use workqueue on the CPU.

Per-cpu workqueues are supposed to remain working and bound to the CPU
for normal CPU_DOWN_PREPARE notifiers.  This holds mostly true even
with workqueue offlining running with higher priority because
workqueue CPU_DOWN_PREPARE only creates a bound trustee thread which
runs the per-cpu workqueue without concurrency management without
explicitly detaching the existing workers.

However, if the trustee needs to create new workers, it creates
unbound workers which may wander off to other CPUs while
CPU_DOWN_PREPARE notifiers are in progress.  Furthermore, if the CPU
down is cancelled, the per-CPU workqueue may end up with workers which
aren't bound to the CPU.

While reliably reproducible with a convoluted artificial test-case
involving scheduling and flushing CPU burning work items from CPU down
notifiers, this isn't very likely to happen in the wild, and, even
when it happens, the effects are likely to be hidden by the following
successful CPU down.

Fix it by using different priorities for up and down notifiers - high
priority for up operations and low priority for down operations.

Workqueue cpu hotplug operations will soon go through further cleanup.

Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: stable@vger.kernel.org
Acked-by: "Rafael J. Wysocki" <rjw@sisk.pl>
workqueue: drop CPU_DYING notifier operation

Workqueue used CPU_DYING notification to mark GCWQ_DISASSOCIATED.
This was necessary because workqueue's CPU_DOWN_PREPARE happened
before other DOWN_PREPARE notifiers and workqueue needed to stay
associated across the rest of DOWN_PREPARE.

After the previous patch, workqueue's DOWN_PREPARE happens after
others and can set GCWQ_DISASSOCIATED directly.  Drop CPU_DYING and
let the trustee set GCWQ_DISASSOCIATED after disabling concurrency
management.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: "Rafael J. Wysocki" <rjw@sisk.pl>
workqueue: ROGUE workers are UNBOUND workers

Currently, WORKER_UNBOUND is used to mark workers for the unbound
global_cwq and WORKER_ROGUE is used to mark workers for disassociated
per-cpu global_cwqs.  Both are used to make the marked worker skip
concurrency management and the only place they make any difference is
in worker_enter_idle() where WORKER_ROGUE is used to skip scheduling
idle timer, which can easily be replaced with trustee state testing.

This patch replaces WORKER_ROGUE with WORKER_UNBOUND and drops
WORKER_ROGUE.  This is to prepare for removing trustee and handling
disassociated global_cwqs as unbound.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: "Rafael J. Wysocki" <rjw@sisk.pl>
workqueue: use mutex for global_cwq manager exclusion

POOL_MANAGING_WORKERS is used to ensure that at most one worker takes
the manager role at any given time on a given global_cwq.  Trustee
later hitched on it to assume manager adding blocking wait for the
bit.  As trustee already needed a custom wait mechanism, waiting for
MANAGING_WORKERS was rolled into the same mechanism.

Trustee is scheduled to be removed.  This patch separates out
MANAGING_WORKERS wait into per-pool mutex.  Workers use
mutex_trylock() to test for manager role and trustee uses mutex_lock()
to claim manager roles.

gcwq_claim/release_management() helpers are added to grab and release
manager roles of all pools on a global_cwq.  gcwq_claim_management()
always grabs pool manager mutexes in ascending pool index order and
uses pool index as lockdep subclass.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: "Rafael J. Wysocki" <rjw@sisk.pl>
workqueue: drop @bind from create_worker()

Currently, create_worker()'s callers are responsible for deciding
whether the newly created worker should be bound to the associated CPU
and create_worker() sets WORKER_UNBOUND only for the workers for the
unbound global_cwq.  Creation during normal operation is always via
maybe_create_worker() and @bind is true.  For workers created during
hotplug, @bind is false.

Normal operation path is planned to be used even while the CPU is
going through hotplug operations or offline and this static decision
won't work.

Drop @bind from create_worker() and decide whether to bind by looking
at GCWQ_DISASSOCIATED.  create_worker() will also set WORKER_UNBOUND
autmatically if disassociated.  To avoid flipping GCWQ_DISASSOCIATED
while create_worker() is in progress, the flag is now allowed to be
changed only while holding all manager_mutexes on the global_cwq.

This requires that GCWQ_DISASSOCIATED is not cleared behind trustee's
back.  CPU_ONLINE no longer clears DISASSOCIATED before flushing
trustee, which clears DISASSOCIATED before rebinding remaining workers
if asked to release.  For cases where trustee isn't around, CPU_ONLINE
clears DISASSOCIATED after flushing trustee.  Also, now, first_idle
has UNBOUND set on creation which is explicitly cleared by CPU_ONLINE
while binding it.  These convolutions will soon be removed by further
simplification of CPU hotplug path.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: "Rafael J. Wysocki" <rjw@sisk.pl>
workqueue: reimplement CPU online rebinding to handle idle workers

Currently, if there are left workers when a CPU is being brough back
online, the trustee kills all idle workers and scheduled rebind_work
so that they re-bind to the CPU after the currently executing work is
finished.  This works for busy workers because concurrency management
doesn't try to wake up them from scheduler callbacks, which require
the target task to be on the local run queue.  The busy worker bumps
concurrency counter appropriately as it clears WORKER_UNBOUND from the
rebind work item and it's bound to the CPU before returning to the
idle state.

To reduce CPU on/offlining overhead (as many embedded systems use it
for powersaving) and simplify the code path, workqueue is planned to
be modified to retain idle workers across CPU on/offlining.  This
patch reimplements CPU online rebinding such that it can also handle
idle workers.

As noted earlier, due to the local wakeup requirement, rebinding idle
workers is tricky.  All idle workers must be re-bound before scheduler
callbacks are enabled.  This is achieved by interlocking idle
re-binding.  Idle workers are requested to re-bind and then hold until
all idle re-binding is complete so that no bound worker starts
executing work item.  Only after all idle workers are re-bound and
parked, CPU_ONLINE proceeds to release them and queue rebind work item
to busy workers thus guaranteeing scheduler callbacks aren't invoked
until all idle workers are ready.

worker_rebind_fn() is renamed to busy_worker_rebind_fn() and
idle_worker_rebind() for idle workers is added.  Rebinding logic is
moved to rebind_workers() and now called from CPU_ONLINE after
flushing trustee.  While at it, add CPU sanity check in
worker_thread().

Note that now a worker may become idle or the manager between trustee
release and rebinding during CPU_ONLINE.  As the previous patch
updated create_worker() so that it can be used by regular manager
while unbound and this patch implements idle re-binding, this is safe.

This prepares for removal of trustee and keeping idle workers across
CPU hotplugs.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: "Rafael J. Wysocki" <rjw@sisk.pl>
workqueue: don't butcher idle workers on an offline CPU

Currently, during CPU offlining, after all pending work items are
drained, the trustee butchers all workers.  Also, on CPU onlining
failure, workqueue_cpu_callback() ensures that the first idle worker
is destroyed.  Combined, these guarantee that an offline CPU doesn't
have any worker for it once all the lingering work items are finished.

This guarantee isn't really necessary and makes CPU on/offlining more
expensive than needs to be, especially for platforms which use CPU
hotplug for powersaving.

This patch lets offline CPUs removes idle worker butchering from the
trustee and let a CPU which failed onlining keep the created first
worker.  The first worker is created if the CPU doesn't have any
during CPU_DOWN_PREPARE and started right away.  If onlining succeeds,
the rebind_workers() call in CPU_ONLINE will rebind it like any other
workers.  If onlining fails, the worker is left alone till the next
try.

This makes CPU hotplugs cheaper by allowing global_cwqs to keep
workers across them and simplifies code.

Note that trustee doesn't re-arm idle timer when it's done and thus
the disassociated global_cwq will keep all workers until it comes back
online.  This will be improved by further patches.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: "Rafael J. Wysocki" <rjw@sisk.pl>
workqueue: remove CPU offline trustee

With the previous changes, a disassociated global_cwq now can run as
an unbound one on its own - it can create workers as necessary to
drain remaining works after the CPU has been brought down and manage
the number of workers using the usual idle timer mechanism making
trustee completely redundant except for the actual unbinding
operation.

This patch removes the trustee and let a disassociated global_cwq
manage itself.  Unbinding is moved to a work item (for CPU affinity)
which is scheduled and flushed from CPU_DONW_PREPARE.

This patch moves nr_running clearing outside gcwq and manager locks to
simplify the code.  As nr_running is unused at the point, this is
safe.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: "Rafael J. Wysocki" <rjw@sisk.pl>
workqueue: simplify CPU hotplug code

With trustee gone, CPU hotplug code can be simplified.

* gcwq_claim/release_management() now grab and release gcwq lock too
  respectively and gained _and_lock and _and_unlock postfixes.

* All CPU hotplug logic was implemented in workqueue_cpu_callback()
  which was called by workqueue_cpu_up/down_callback() for the correct
  priority.  This was because up and down paths shared a lot of logic,
  which is no longer true.  Remove workqueue_cpu_callback() and move
  all hotplug logic into the two actual callbacks.

This patch doesn't make any functional changes.

Signed-off-by: Tejun Heo <tj@kernel.org>
Acked-by: "Rafael J. Wysocki" <rjw@sisk.pl>