linux-kernel - Re: [PATCH 17/22] sched/core: fix opencoded cpumask_any

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Date:   Tue, 10 May 2022 18:37:07 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Yury Norov <yury.norov@...il.com>
Cc:     Andy Shevchenko <andriy.shevchenko@...ux.intel.com>,
        David Laight <David.Laight@...LAB.COM>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Joe Perches <joe@...ches.com>,
        Julia Lawall <Julia.Lawall@...ia.fr>,
        Michał Mirosław <mirq-linux@...e.qmqm.pl>,
        Nicholas Piggin <npiggin@...il.com>,
        Nicolas Palix <nicolas.palix@...g.fr>,
        Rasmus Villemoes <linux@...musvillemoes.dk>,
        Matti Vaittinen <Matti.Vaittinen@...rohmeurope.com>,
        linux-kernel@...r.kernel.org, Ben Segall <bsegall@...gle.com>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Mel Gorman <mgorman@...e.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        Valentin Schneider <vschneid@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>
Subject: Re: [PATCH 17/22] sched/core: fix opencoded cpumask_any_but()

On Tue, May 10, 2022 at 08:47:45AM -0700, Yury Norov wrote:
> sched_core_cpu_starting() and sched_core_cpu_deactivate() implement
> opencoded cpumask_any_but(). Fix it.
> 
> CC: Ben Segall <bsegall@...gle.com>
> CC: Daniel Bristot de Oliveira <bristot@...hat.com>
> CC: Dietmar Eggemann <dietmar.eggemann@....com>
> CC: Ingo Molnar <mingo@...hat.com>
> CC: Juri Lelli <juri.lelli@...hat.com>
> CC: Mel Gorman <mgorman@...e.de>
> CC: Peter Zijlstra <peterz@...radead.org>
> CC: Steven Rostedt <rostedt@...dmis.org>
> CC: Valentin Schneider <vschneid@...hat.com>
> CC: Vincent Guittot <vincent.guittot@...aro.org>
> CC: linux-kernel@...r.kernel.org
> Signed-off-by: Yury Norov <yury.norov@...il.com>
> ---
>  kernel/sched/core.c | 33 +++++++++++++--------------------
>  1 file changed, 13 insertions(+), 20 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index f5ebc392493d..9700001948d0 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -6125,7 +6125,7 @@ static void queue_core_balance(struct rq *rq)
>  static void sched_core_cpu_starting(unsigned int cpu)
>  {
>  	const struct cpumask *smt_mask = cpu_smt_mask(cpu);
> -	struct rq *rq = cpu_rq(cpu), *core_rq = NULL;
> +	struct rq *rq = cpu_rq(cpu), *core_rq;
>  	unsigned long flags;
>  	int t;
>  
> @@ -6138,19 +6138,16 @@ static void sched_core_cpu_starting(unsigned int cpu)
>  		goto unlock;
>  
>  	/* find the leader */
> -	for_each_cpu(t, smt_mask) {
> -		if (t == cpu)
> -			continue;
> -		rq = cpu_rq(t);
> -		if (rq->core == rq) {
> -			core_rq = rq;
> -			break;
> -		}
> -	}
> +	t = cpumask_any_but(smt_mask, cpu);
> +	if (t >= nr_cpu_ids)
> +		goto unlock;
>  
> -	if (WARN_ON_ONCE(!core_rq)) /* whoopsie */
> +	rq = cpu_rq(t);
> +	if (WARN_ON_ONCE(rq->core != rq)) /* whoopsie */
>  		goto unlock;
>  
> +	core_rq = rq;
> +
>  	/* install and validate core_rq */
>  	for_each_cpu(t, smt_mask) {
>  		rq = cpu_rq(t);

I don't think this is equivalent. Imagine SMT4, with:

  rqN->core_rq = rq0

Now, further suppose smt0-2 are online and we're about to online smt3.
Then t above is free to be smt2, which then results in insta triggering:

+	if (WARN_ON_ONCE(rq->core != rq)) /* whoopsie */

You seem to have lost how the first loop searches for rq->core.

Please, be more careful. Also, all of this is super cold path don't
bother with optimizations. Much of the patches you have in this series
fall under that.