linux-kernel - Re: [PATCH] sched: access local runqueue directly in single_task

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Date:	Fri, 18 Sep 2015 13:26:53 +0200
From:	Paolo Bonzini <pbonzini@...hat.com>
To:	Dominik Dingel <dingel@...ux.vnet.ibm.com>,
	Peter Zijlstra <peterz@...radead.org>
Cc:	kvm@...r.kernel.org, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...hat.com>,
	Tim Chen <tim.c.chen@...ux.intel.com>, stable@...r.kernel.org
Subject: Re: [PATCH] sched: access local runqueue directly in
 single_task_running



On 18/09/2015 11:27, Dominik Dingel wrote:
> Commit 2ee507c47293 ("sched: Add function single_task_running to let a task
> check if it is the only task running on a cpu") referenced the current
> runqueue with the smp_processor_id.  When CONFIG_DEBUG_PREEMPT is enabled,
> that is only allowed if preemption is disabled or the currrent task is
> bound to the local cpu (e.g. kernel worker).
> 
> With commit f78195129963 ("kvm: add halt_poll_ns module parameter") KVM
> calls single_task_running. If CONFIG_DEBUG_PREEMPT is enabled that
> generates a lot of kernel messages.
> 
> To avoid adding preemption in that cases, as it would limit the usefulness,
> we change single_task_running to access directly the cpu local runqueue.
> 
> Cc: Tim Chen <tim.c.chen@...ux.intel.com>
> Suggested-by: Peter Zijlstra <peterz@...radead.org>
> Cc: <stable@...r.kernel.org> # 4.2.x
> Signed-off-by: Dominik Dingel <dingel@...ux.vnet.ibm.com>
> ---
>  kernel/sched/core.c | 8 ++++----
>  1 file changed, 4 insertions(+), 4 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 78b4bad10..5bfad0b 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2614,13 +2614,13 @@ unsigned long nr_running(void)
>  
>  /*
>   * Check if only the current task is running on the cpu.
> + *
> + * Caution result is subject to time-of-check-to-time-of-use race,
> + * every caller is responsible to set up additional fences if necessary.

Let's expand it a bit more:

 * Caution: this function does not check that the caller has disabled
 * preemption, thus the result might have a time-of-check-to-time-of-use
 * race.  The caller is responsible to use this correctly, for example:
 *
 * - use it from a non-preemptable section
 *
 * - use it from a thread that is bound to a single CPU
 *
 * - use it in a loop where each iteration takes very little time
 *   (e.g. a polling loop)
 */

I'll include it in my pull request.

Paolo

>   */
>  bool single_task_running(void)
>  {
> -	if (cpu_rq(smp_processor_id())->nr_running == 1)
> -		return true;
> -	else
> -		return false;
> +	return raw_rq()->nr_running == 1;
>  }
>  EXPORT_SYMBOL(single_task_running);
>  
> 
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