[PATCH 0/2] replace system_wq with system_percpu_wq, add WQ_PERCPU to alloc_workqueue

[Marco Crivellari <marco.crivellari@suse.com>]

Hi,

=== Current situation: problems ===

Let's consider a nohz_full system with isolated CPUs: wq_unbound_cpumask is
set to the housekeeping CPUs, for !WQ_UNBOUND the local CPU is selected.

This leads to different scenarios if a work item is scheduled on an
isolated CPU where "delay" value is 0 or greater then 0:
        schedule_delayed_work(, 0);

This will be handled by __queue_work() that will queue the work item on the
current local (isolated) CPU, while:

        schedule_delayed_work(, 1);

Will move the timer on an housekeeping CPU, and schedule the work there.

Currently if a user enqueue a work item using schedule_delayed_work() the
used wq is "system_wq" (per-cpu wq) while queue_delayed_work() use
WORK_CPU_UNBOUND (used when a cpu is not specified). The same applies to
schedule_work() that is using system_wq and queue_work(), that makes use
again of WORK_CPU_UNBOUND.

This lack of consistency cannot be addressed without refactoring the API.

=== Recent changes to the WQ API ===

The following, address the recent changes in the Workqueue API:

- commit 128ea9f6ccfb ("workqueue: Add system_percpu_wq and system_dfl_wq")
- commit 930c2ea566af ("workqueue: Add new WQ_PERCPU flag")

The old workqueues will be removed in a future release cycle.

=== Introduced Changes by this series ===

1) [P 1] Replace uses of system_wq

    system_wq is a per-CPU workqueue, but his name is not clear.
    Because of that, system_wq has been replaced with system_percpu_wq.

2) [P 2] WQ_PERCPU added to alloc_workqueue()

    This change adds a new WQ_PERCPU flag to explicitly request
    alloc_workqueue() to be per-cpu when WQ_UNBOUND has not been specified.


Thanks!


Marco Crivellari (2):
  xen/events: replace use of system_wq with system_percpu_wq
  xen: privcmd: WQ_PERCPU added to alloc_workqueue users

 drivers/xen/events/events_base.c | 6 +++---
 drivers/xen/privcmd.c            | 3 ++-
 2 files changed, 5 insertions(+), 4 deletions(-)

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2.51.1