linux-ext4 - Re: [PATCH 1/4] sched: move IO scheduling accounting from io_schedule_timeout() to _

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Date:   Thu, 3 Nov 2016 21:03:45 +0530
From:   Pavan Kondeti <pkondeti@...eaurora.org>
To:     Tejun Heo <tj@...nel.org>
Cc:     torvalds@...ux-foundation.org, akpm@...ux-foundation.org,
        mingo@...hat.com, Peter Zijlstra <peterz@...radead.org>,
        axboe@...nel.dk, tytso@....edu, jack@...e.com,
        adilger.kernel@...ger.ca, linux-ext4@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, mingbo@...com
Subject: Re: [PATCH 1/4] sched: move IO scheduling accounting from
 io_schedule_timeout() to __schedule()

Hi Tejun,

> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 94732d1..f6baa38 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3336,11 +3336,17 @@ static void __sched notrace __schedule(bool preempt)
>         unsigned long *switch_count;
>         struct pin_cookie cookie;
>         struct rq *rq;
> -       int cpu;
> +       int cpu, in_iowait;
>
>         cpu = smp_processor_id();
>         rq = cpu_rq(cpu);
>         prev = rq->curr;
> +       in_iowait = prev->in_iowait;
> +
> +       if (in_iowait) {
> +               delayacct_blkio_start();
> +               atomic_inc(&rq->nr_iowait);
> +       }
>
>         schedule_debug(prev);
>
> @@ -3406,6 +3412,11 @@ static void __sched notrace __schedule(bool preempt)
>         }
>
>         balance_callback(rq);
> +
> +       if (in_iowait) {
> +               atomic_dec(&rq->nr_iowait);
> +               delayacct_blkio_end();
> +       }
>  }

I think, the nr_iowait update can go wrong here.

When the task migrates to a different CPU upon wakeup, this rq points
to a different CPU from the one on which nr_iowait is incremented
before.

Thanks,
Pavan

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