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SubjectRe: sched: race between deactivate and switch sched_info accounting?
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On Fri, 2009-10-09 at 19:40 -0700, Paul Turner wrote:

This looks very funny, I would expect that whoever does activate() on
that task to do the sched_info*() muck?

The below patch looks very asymmetric in that regard.

> It's possible for our previously de-activated task to be re-activated by a
> remote cpu during lock balancing. We have to account for this manually
> since prev == next, yet the task just went through dequeue accounting.
>
> Signed-off-by: Paul Turner <pjt@google.com>
> ---
> kernel/sched.c | 15 ++++++++++++---
> 1 files changed, 12 insertions(+), 3 deletions(-)
>
> diff --git a/kernel/sched.c b/kernel/sched.c
> index ee61f45..6445d9d 100644
> --- a/kernel/sched.c
> +++ b/kernel/sched.c
> @@ -5381,7 +5381,7 @@ asmlinkage void __sched schedule(void)
> struct task_struct *prev, *next;
> unsigned long *switch_count;
> struct rq *rq;
> - int cpu;
> + int cpu, deactivated_prev = 0;
>
> need_resched:
> preempt_disable();
> @@ -5406,8 +5406,10 @@ need_resched_nonpreemptible:
> if (prev->state && !(preempt_count() & PREEMPT_ACTIVE)) {
> if (unlikely(signal_pending_state(prev->state, prev)))
> prev->state = TASK_RUNNING;
> - else
> + else {
> deactivate_task(rq, prev, 1);
> + deactivated_prev = 1;
> + }
> switch_count = &prev->nvcsw;
> }
>
> @@ -5434,8 +5436,15 @@ need_resched_nonpreemptible:
> */
> cpu = smp_processor_id();
> rq = cpu_rq(cpu);
> - } else
> + } else {
> + /*
> + * account for our previous task being re-activated by a
> + * remote cpu.
> + */
> + if (unlikely(deactivated_prev))
> + sched_info_switch(prev, prev);
> spin_unlock_irq(&rq->lock);
> + }
>
> post_schedule(rq);
>


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