linux-kernel - Re: [PATCH 1/2] sched: Fix balance

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Date:   Fri, 11 Sep 2020 13:17:02 +0100
From:   Valentin Schneider <valentin.schneider@....com>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     mingo@...nel.org, vincent.guittot@...aro.org, tglx@...utronix.de,
        linux-kernel@...r.kernel.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bristot@...hat.com, swood@...hat.com
Subject: Re: [PATCH 1/2] sched: Fix balance_callback()


On 11/09/20 09:17, Peter Zijlstra wrote:
> The intent of balance_callback() has always been to delay executing
> balancing operations until the end of the current rq->lock section.
> This is because balance operations must often drop rq->lock, and that
> isn't safe in general.
>
> However, as noted by Scott, there were a few holes in that scheme;
> balance_callback() was called after rq->lock was dropped, which means
> another CPU can interleave and touch the callback list.
>

So that can be say __schedule() tail racing with some setprio; what's the
worst that can (currently) happen here? Something like say two consecutive
enqueuing of push_rt_tasks() to the callback list?

> Rework code to call the balance callbacks before dropping rq->lock
> where possible, and otherwise splice the balance list onto a local
> stack.
>
> This guarantees that the balance list must be empty when we take
> rq->lock. IOW, we'll only ever run our own balance callbacks.
>

Makes sense to me.

Reviewed-by: Valentin Schneider <valentin.schneider@....com>

> Reported-by: Scott Wood <swood@...hat.com>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>

[...]

> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1220,6 +1220,8 @@ static inline void rq_pin_lock(struct rq
>  #ifdef CONFIG_SCHED_DEBUG
>       rq->clock_update_flags &= (RQCF_REQ_SKIP|RQCF_ACT_SKIP);
>       rf->clock_update_flags = 0;
> +
> +	SCHED_WARN_ON(rq->balance_callback);

Clever!

>  #endif
>  }
>