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

[Frederic Weisbecker <frederic@kernel.org>]

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@redhat.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
> 
>