Marco Crivellari <marco.crivellari@suse.com> says:
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 128ea9f6cc ("workqueue: Add system_percpu_wq and system_dfl_wq")
- commit 930c2ea566 ("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 and system_unbound_wq
system_unbound_wq is to be used when locality is not required.
Because of that, system_unbound_wq has been replaced with
system_dfl_wq, to make clear it should be used when locality
is not required.
2) [P 2-3-4] 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!
Link: https://patch.msgid.link/20251031095643.74246-1-marco.crivellari@suse.com
Signed-off-by: Martin K. Petersen <martin.petersen@oracle.com>
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