[PATCH V3 0/7] workqueue: Factor the way to assign rescuer work

[Lai Jiangshan <jiangshanlai@gmail.com>]

From: Lai Jiangshan <jiangshan.ljs@antgroup.com>

Previously, the rescuer scanned for all matching work items at once and
processed them within a single rescuer thread, which could cause one
blocking work item to stall all others.

Make the rescuer process work items one-by-one instead of slurping all
matches in a single pass.

Break the rescuer loop after finding and processing the first matching
work item, then restart the search to pick up the next. This gives
normal worker threads a chance to process other items which gives them
the opportinity to be processed instead of waiting on the rescuer's
queue and prevents a blocking work item from stalling the rest once
memory pressure is relieved.

Introduce a dummy cursor  work item to avoid potentially O(N^2)
rescans of the work list.  The marker records the resume position for
the next scan, eliminating redundant traversals.


Change from v2:

 Renames.

 Change WARN_ON_ONCE() conditions in assign_work().

 Remove the check whether the cursor is on the list in send_mayday()
 which has the benefit of enabling the ability to only process a
 limit number of work items per run and still allow the cursor in
 worklist between runs.

 Also remove the insertion of the cursor in send_mayday() and contain
 the code of handling the cursor in assign[_rescuer]_work().

 Split patch.

V2: https://lore.kernel.org/lkml/20251118093832.81920-1-jiangshanlai@gmail.com/

Changed from v1:

Insert the marker and use it purely as iteration marker as Tejun
request.
  It is hard to maintain the proper state of it, expecially maintaining
  it only in pool->worklist which also requires an unlikely path in the
  worker_thread() path.

Add an unlikely path in the worker_thread() path(in assign_work()).

Add other code in other place to make it not be treated like a work
item.  I'm sure all the paths have been covered now, but I still feel
a bit nevous about it for future changes.

Extra the code to assign work in the rescuer_thread() out as
assign_rescue_work().

V1: https://lore.kernel.org/lkml/20251113163426.2950-1-jiangshanlai@gmail.com/

Lai Jiangshan (7):
  workqueue: Factor out assign_rescuer_work()
  workqueue: Only assign rescuer work when really needed
  workqueue: Don't rely on wq->rescuer to stop rescuer
  workqueue: Loop over in rescuer until all its work is done
  workqueue: Process rescuer work items one-by-one using a cursor
  workqueue: Limit number of processed works in rescuer per turn
  workqueue: Process extra works in rescuer when there are no more to
    rescue

 kernel/workqueue.c | 146 ++++++++++++++++++++++++++++++++-------------
 1 file changed, 103 insertions(+), 43 deletions(-)

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2.19.1.6.gb485710b