[PATCH v2 1/1] wq: Avoid using isolated cpus' timers on queue_delayed_work

[Leonardo Bras]

When __queue_delayed_work() is called, it chooses a cpu for handling the
timer interrupt. As of today, it will pick either the cpu passed as
parameter or the last cpu used for this.

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 cpu isolation is in
place, pick a random non-isolated cpu to handle the timer interrupt.

As an optimization, if the current cpu is not isolated, use it instead
of looking for another candidate.

Signed-off-by: Leonardo Bras <leobras@xxxxxxxxxx>
---
Changes since v1:
- Make sure the CPU is isolated for any value of "cpu"

Changes since RFC:
- Do not use the same cpu from the timer for queueing the work.
- If the current cpu is not isolated, use it's timer instead of
  looking for another candidate.

 kernel/workqueue.c | 14 +++++++++++---
 1 file changed, 11 insertions(+), 3 deletions(-)

diff --git a/kernel/workqueue.c b/kernel/workqueue.c
index 76e60faed8923..8dd7c01b326a4 100644
--- a/kernel/workqueue.c
+++ b/kernel/workqueue.c
@@ -1958,10 +1958,18 @@ static void __queue_delayed_work(int cpu, struct workqueue_struct *wq,
 	dwork->cpu = cpu;
 	timer->expires = jiffies + delay;
 
-	if (unlikely(cpu != WORK_CPU_UNBOUND))
+	if (housekeeping_enabled(HK_TYPE_TIMER)) {
+		/* If the current cpu is a housekeeping cpu, use it. */
+		cpu = smp_processor_id();
+		if (!housekeeping_test_cpu(cpu, HK_TYPE_TIMER))
+			cpu = housekeeping_any_cpu(HK_TYPE_TIMER);
 		add_timer_on(timer, cpu);
-	else
-		add_timer(timer);
+	} else {
+		if (likely(cpu == WORK_CPU_UNBOUND))
+			add_timer(timer);
+		else
+			add_timer_on(timer, cpu);
+	}
 }
 
 /**
-- 
2.43.0