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Subject[RFC PATCH v1 1/1] wq: Avoid using isolated cpus' timers on unbounded queue_delayed_work
Date
When __queue_delayed_work() is called with WORK_CPU_UNBOUND, it means any
cpu is able to run the work, as well as any cpu timer is able to be used.

This is not good if a system does use CPU isolation, because it can take
away some valuable cpu time to:
1 - deal with the timer interrupt,
2 - schedule-out the desired task,
3 - queue work on a random workqueue, and
4 - schedule the desired task back to the cpu.

So to fix this, during __queue_delayed_work(), if both:
- Work is not cpu-bounded,
- CPU isolation is in place,
then pick a random non-isolated cpu to use both the timer and the
system per-cpu workqueue.

Signed-off-by: Leonardo Bras <leobras@redhat.com>
---
kernel/workqueue.c | 8 ++++++++
1 file changed, 8 insertions(+)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 76e60faed8923..0c50f41d9f95e 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1954,6 +1954,14 @@ static void __queue_delayed_work(int cpu, struct workqueue_struct *wq,
return;
}

+ /*
+ * If the work is cpu-unbound, and cpu isolation is in place, only
+ * schedule use timers from housekeeping cpus. In favor of avoiding
+ * cacheline bouncing, run the WQ in the same cpu as the timer.
+ */
+ if (cpu == WORK_CPU_UNBOUND && housekeeping_enabled(HK_TYPE_TIMER))
+ cpu = housekeeping_any_cpu(HK_TYPE_TIMER);
+
dwork->wq = wq;
dwork->cpu = cpu;
timer->expires = jiffies + delay;
--
2.43.0

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