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    SubjectRe: [PATCH] sched: RCU-protect __set_task_cpu() in set_task_cpu()
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    On Mon, 2011-06-06 at 18:46 +0200, Oleg Nesterov wrote:
    > On 06/06, Peter Zijlstra wrote:
    > >
    > > You're right, p->pi_lock for wakeups, rq->lock for runnable tasks.
    >
    > Good, thanks.
    >
    > Help! I have another question.
    >
    > try_to_wake_up:
    >
    > raw_spin_lock_irqsave(&p->pi_lock, flags);
    > if (!(p->state & state))
    > goto out;
    >
    > cpu = task_cpu(p);
    >
    > if (p->on_rq && ttwu_remote(p, wake_flags))
    > goto stat;
    >
    > This doesn't look a bit confusing, we can't trust "cpu = task_cpu" before
    > we check ->on_rq. OK, not a problem, this cpu number can only be used in
    > ttwu_stat(cpu).
    >
    > But ttwu_stat(cpu) in turn does
    >
    > if (cpu != task_cpu(p))
    > schedstat_inc(p, se.statistics.nr_wakeups_migrate);
    >
    > Ignoring the theoretical races with pull_task/etc, how it is possible
    > that cpu != task_cpu(p) ? Another caller is try_to_wake_up_local(), it
    > obviously can't trigger this case.
    >
    > This looks broken to me. Looking at its name, I guess nr_wakeups_migrate
    > should be incremented if ttwu does set_task_cpu(), correct?
    >
    > IOW. Don't we need something like the (untested/ucompiled) patch below?
    > _If_ I am right, I can resend it with the changelog/etc but please feel
    > free to make another fix.

    You're right, I spotted the same a few days ago which resulted in:

    ---
    commit f339b9dc1f03591761d5d930800db24bc0eda1e1
    Author: Peter Zijlstra <a.p.zijlstra@chello.nl>
    Date: Tue May 31 10:49:20 2011 +0200

    sched: Fix schedstat.nr_wakeups_migrate

    While looking over the code I found that with the ttwu rework the
    nr_wakeups_migrate test broke since we now switch cpus prior to
    calling ttwu_stat(), hence the test is always true.

    Cure this by passing the migration state in wake_flags. Also move the
    whole test under CONFIG_SMP, its hard to migrate tasks on UP :-)

    Signed-off-by: Peter Zijlstra <a.p.zijlstra@chello.nl>
    Link: http://lkml.kernel.org/n/tip-pwwxl7gdqs5676f1d4cx6pj7@git.kernel.org
    Signed-off-by: Ingo Molnar <mingo@elte.hu>

    diff --git a/include/linux/sched.h b/include/linux/sched.h
    index 8da84b7..483c1ed 100644
    --- a/include/linux/sched.h
    +++ b/include/linux/sched.h
    @@ -1063,6 +1063,7 @@ struct sched_domain;
    */
    #define WF_SYNC 0x01 /* waker goes to sleep after wakup */
    #define WF_FORK 0x02 /* child wakeup after fork */
    +#define WF_MIGRATED 0x04 /* internal use, task got migrated */

    #define ENQUEUE_WAKEUP 1
    #define ENQUEUE_HEAD 2
    diff --git a/kernel/sched.c b/kernel/sched.c
    index 49cc70b..2fe98ed 100644
    --- a/kernel/sched.c
    +++ b/kernel/sched.c
    @@ -2447,6 +2447,10 @@ ttwu_stat(struct task_struct *p, int cpu, int wake_flags)
    }
    rcu_read_unlock();
    }
    +
    + if (wake_flags & WF_MIGRATED)
    + schedstat_inc(p, se.statistics.nr_wakeups_migrate);
    +
    #endif /* CONFIG_SMP */

    schedstat_inc(rq, ttwu_count);
    @@ -2455,9 +2459,6 @@ ttwu_stat(struct task_struct *p, int cpu, int wake_flags)
    if (wake_flags & WF_SYNC)
    schedstat_inc(p, se.statistics.nr_wakeups_sync);

    - if (cpu != task_cpu(p))
    - schedstat_inc(p, se.statistics.nr_wakeups_migrate);
    -
    #endif /* CONFIG_SCHEDSTATS */
    }

    @@ -2675,8 +2676,10 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags)
    p->sched_class->task_waking(p);

    cpu = select_task_rq(p, SD_BALANCE_WAKE, wake_flags);
    - if (task_cpu(p) != cpu)
    + if (task_cpu(p) != cpu) {
    + wake_flags |= WF_MIGRATED;
    set_task_cpu(p, cpu);
    + }
    #endif /* CONFIG_SMP */

    ttwu_queue(p, cpu);


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