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sched: access local runqueue directly in single_task_running

Commit 2ee507c ("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 f781951 ("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@linux.intel.com>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: <stable@vger.kernel.org>
Fixes: 2ee507c
Signed-off-by: Dominik Dingel <dingel@linux.vnet.ibm.com>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>
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Dominik Dingel authored and bonzini committed Sep 18, 2015
1 parent efe4d36 commit 00cc1633816de8c95f337608a1ea64e228faf771
Showing with 11 additions and 4 deletions.
  1. +11 −4 kernel/sched/core.c
@@ -2666,13 +2666,20 @@ unsigned long nr_running(void)
/*
* Check if only the current task is running on the cpu.
*
* 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 it correctly, for example:
*
* - from a non-preemptable section (of course)
*
* - from a thread that is bound to a single CPU
*
* - in a loop with very short iterations (e.g. a polling loop)
*/
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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