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@xxxxxxxxxxxxxxx>
> Suggested-by: Peter Zijlstra <peterz@xxxxxxxxxxxxx>
> Cc: <stable@xxxxxxxxxxxxxxx> # 4.2.x
> Signed-off-by: Dominik Dingel <dingel@xxxxxxxxxxxxxxxxxx>
> ---
> 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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