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Message-ID: <Zv_hk59xfNvzlw3B@localhost.localdomain>
Date: Fri, 4 Oct 2024 14:37:39 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: Leonardo Bras <leobras@...hat.com>
Cc: Tejun Heo <tj@...nel.org>, Lai Jiangshan <jiangshanlai@...il.com>,
linux-kernel@...r.kernel.org, Junyao Zhao <junzhao@...hat.com>
Subject: Re: [PATCH v2 1/1] wq: Avoid using isolated cpus' timers on
queue_delayed_work
Hi,
Le Mon, Jan 29, 2024 at 10:00:46PM -0300, Leonardo Bras a écrit :
> 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@...hat.com>
> ---
> 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() already picks up a housekeeping CPU. So why is it needed?
Thanks.
> 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
>
>
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