linux-kernel - Re: [PATCH v2 03/12] sched: Optimize ttwu() / select_task

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <CAKfTPtBjtHdFUNCfe4+iiRXf570_GSHPvRtuhzHndf0PD4fmJQ@mail.gmail.com>
Date: Thu, 10 Jul 2025 18:47:49 +0200
From: Vincent Guittot <vincent.guittot@...aro.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: mingo@...hat.com, juri.lelli@...hat.com, dietmar.eggemann@....com, 
	rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de, vschneid@...hat.com, 
	clm@...a.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2 03/12] sched: Optimize ttwu() / select_task_rq()

On Wed, 2 Jul 2025 at 14:12, Peter Zijlstra <peterz@...radead.org> wrote:
>
> Optimize ttwu() by pushing select_idle_siblings() up above waiting for
> on_cpu(). This allows making use of the cycles otherwise spend waiting
> to search for an idle CPU.
>
> One little detail is that since the task we're looking for an idle CPU
> for might still be on the CPU, that CPU won't report as running the
> idle task, and thus won't find his own CPU idle, even when it is.
>
> To compensate, remove the 'rq->curr == rq->idle' condition from
> idle_cpu() -- it doesn't really make sense anyway.
>
> Additionally, Chris found (concurrently) that perf-c2c reported that
> test as being a cache-miss monster.
>
> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> Link: https://lkml.kernel.org/r/20250520101727.620602459@infradead.org

Reviewed-by: Vincent Guittot <vincent.guittot@...aro.org>


> ---
>  kernel/sched/core.c     |    5 +++--
>  kernel/sched/syscalls.c |    3 ---
>  2 files changed, 3 insertions(+), 5 deletions(-)
>
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -3593,7 +3593,7 @@ int select_task_rq(struct task_struct *p
>                 cpu = p->sched_class->select_task_rq(p, cpu, *wake_flags);
>                 *wake_flags |= WF_RQ_SELECTED;
>         } else {
> -               cpu = cpumask_any(p->cpus_ptr);
> +               cpu = task_cpu(p);
>         }
>
>         /*
> @@ -4309,6 +4309,8 @@ int try_to_wake_up(struct task_struct *p
>                     ttwu_queue_wakelist(p, task_cpu(p), wake_flags))
>                         break;
>
> +               cpu = select_task_rq(p, p->wake_cpu, &wake_flags);
> +
>                 /*
>                  * If the owning (remote) CPU is still in the middle of schedule() with
>                  * this task as prev, wait until it's done referencing the task.
> @@ -4320,7 +4322,6 @@ int try_to_wake_up(struct task_struct *p
>                  */
>                 smp_cond_load_acquire(&p->on_cpu, !VAL);
>
> -               cpu = select_task_rq(p, p->wake_cpu, &wake_flags);
>                 if (task_cpu(p) != cpu) {
>                         if (p->in_iowait) {
>                                 delayacct_blkio_end(p);
> --- a/kernel/sched/syscalls.c
> +++ b/kernel/sched/syscalls.c
> @@ -203,9 +203,6 @@ int idle_cpu(int cpu)
>  {
>         struct rq *rq = cpu_rq(cpu);
>
> -       if (rq->curr != rq->idle)
> -               return 0;
> -
>         if (rq->nr_running)
>                 return 0;
>
>
>