linux-kernel - Re: [PATCH] irq_work: Replace wait loop with rcuwait_wait

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Message-ID: <ZuxRRZh-2NAlj96l@localhost.localdomain>
Date: Thu, 19 Sep 2024 18:28:53 +0200
From: Frederic Weisbecker <frederic@...nel.org>
To: Steven Davis <goldside000@...look.com>
Cc: akpm@...ux-foundation.org, ankur.a.arora@...cle.com,
	linux-kernel@...r.kernel.org, peterz@...radead.org,
	tglx@...utronix.de
Subject: Re: [PATCH] irq_work: Replace wait loop with rcuwait_wait_event

Le Thu, Sep 19, 2024 at 11:43:26AM -0400, Steven Davis a écrit :
> The previous implementation of irq_work_sync used a busy-wait
> loop with cpu_relax() to check the status of IRQ work. This
> approach, while functional, could lead to inefficiencies in
> CPU usage.
> 
> This commit replaces the busy-wait loop with the rcuwait_wait_event
> mechanism. This change leverages the RCU wait mechanism to handle
> waiting for IRQ work completion more efficiently, improving CPU
> responsiveness and reducing unnecessary CPU usage.
> 
> Signed-off-by: Steven Davis <goldside000@...look.com>
> ---
>  kernel/irq_work.c | 3 +--
>  1 file changed, 1 insertion(+), 2 deletions(-)
> 
> diff --git a/kernel/irq_work.c b/kernel/irq_work.c
> index 2f4fb336dda1..2b092a1d07a9 100644
> --- a/kernel/irq_work.c
> +++ b/kernel/irq_work.c
> @@ -295,8 +295,7 @@ void irq_work_sync(struct irq_work *work)
>  		return;
>  	}
>  
> -	while (irq_work_is_busy(work))
> -		cpu_relax();
> +	rcuwait_wait_event(&work->irqwait, !irq_work_is_busy(work), TASK_UNINTERRUPTIBLE);

Dan Carpenter brought to my attention this a few weeks ago for another problem.:

perf_remove_from_context() <- disables preempt
__perf_event_exit_context() <- disables preempt
-> __perf_remove_from_context()
   -> perf_group_detach()
      -> perf_put_aux_event()
         -> put_event()
            -> _free_event()
               -> irq_work_sync()