Messages in this thread Patch in this message | | | Date | Thu, 24 Sep 2020 13:50:42 +0200 | From | Peter Zijlstra <> | Subject | Re: sched: rq->nr_iowait transiently going negative after the recent p->on_cpu optimization |
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On Fri, Sep 18, 2020 at 01:27:59PM -0400, Tejun Heo wrote: > Hello, > > Peter, I noticed /proc/stat::procs_blocked going U64_MAX transiently once in > the blue moon without any other persistent issues. After looking at the code > with Rik for a bit, the culprit seems to be c6e7bd7afaeb ("sched/core: > Optimize ttwu() spinning on p->on_cpu") - it changed where ttwu dec's > nr_iowait and it looks like that can happen before the target task gets to > inc.
Hurmph.. I suppose you're right :/ And this is an actual problem?
I think the below should cure that, but blergh, not nice. If you could confirm, I'll try and think of something nicer.
diff --git a/kernel/sched/core.c b/kernel/sched/core.c index ebb90572e10d..259a4ae8ab8e 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -2505,7 +2505,12 @@ ttwu_do_activate(struct rq *rq, struct task_struct *p, int wake_flags, #ifdef CONFIG_SMP if (wake_flags & WF_MIGRATED) en_flags |= ENQUEUE_MIGRATED; + else #endif + if (p->in_iowait) { + delayacct_blkio_end(p); + atomic_dec(&task_rq(p)->nr_iowait); + } activate_task(rq, p, en_flags); ttwu_do_wakeup(rq, p, wake_flags, rf); @@ -2892,11 +2897,6 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags) if (READ_ONCE(p->on_rq) && ttwu_runnable(p, wake_flags)) goto unlock; - if (p->in_iowait) { - delayacct_blkio_end(p); - atomic_dec(&task_rq(p)->nr_iowait); - } - #ifdef CONFIG_SMP /* * Ensure we load p->on_cpu _after_ p->on_rq, otherwise it would be @@ -2967,6 +2967,11 @@ try_to_wake_up(struct task_struct *p, unsigned int state, int wake_flags) cpu = select_task_rq(p, p->wake_cpu, SD_BALANCE_WAKE, wake_flags); if (task_cpu(p) != cpu) { + if (p->in_iowait) { + delayacct_blkio_end(p); + atomic_dec(&task_rq(p)->nr_iowait); + } + wake_flags |= WF_MIGRATED; psi_ttwu_dequeue(p); set_task_cpu(p, cpu);
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