linux-kernel - Re: [PATCH] sched/smt: Call sched_core_cpu

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Message-ID: <xhsmhy11nb8mg.mognet@vschneid-thinkpadt14sgen2i.remote.csb>
Date: Wed, 13 Nov 2024 13:50:31 +0100
From: Valentin Schneider <vschneid@...hat.com>
To: Andrea Ciprietti <ciprietti@...gle.com>, linux-kernel@...r.kernel.org
Cc: Andrea Ciprietti <ciprietti@...gle.com>, Ingo Molnar <mingo@...hat.com>,
 Peter Zijlstra <peterz@...radead.org>, Juri Lelli <juri.lelli@...hat.com>,
 Vincent Guittot <vincent.guittot@...aro.org>, Dietmar Eggemann
 <dietmar.eggemann@....com>, Steven Rostedt <rostedt@...dmis.org>, Ben
 Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>, Josh Don
 <joshdon@...gle.com>
Subject: Re: [PATCH] sched/smt: Call sched_core_cpu_deactivate() after error
 is handled

On 08/11/24 12:11, Andrea Ciprietti wrote:
> In sched_cpu_deactivate(), the error path restores most of the initial
> state before returning, but, if CONFIG_SCHED_SMT is defined, it does not
> undo the previous call to sched_core_cpu_deactivate().
>
> There is no easy way to invert such function since it overrides some shared
> state of the leader CPU. Instead, simply move the call past the error
> handling code. This is fine since cpuset_cpu_inactive() performs unrelated
> operations.
>
> Note that there's no need for #ifdef since cpuset_core_* have empty
> definitions when CONFIG_SCHED_SMT is not set (in which case this patch is a
> no-op).
>
> Fixes: 3c474b3239f1 ("sched: Fix Core-wide rq->lock for uninitialized CPUs")
> Cc: stable@...nel.org
> Suggested-by: Josh Don <joshdon@...gle.com>
> Signed-off-by: Andrea Ciprietti <ciprietti@...gle.com>
> ---
>  kernel/sched/core.c | 5 +----
>  1 file changed, 1 insertion(+), 4 deletions(-)
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 719e0ed1e976..4d55bc243ae5 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -8187,10 +8187,6 @@ int sched_cpu_deactivate(unsigned int cpu)
>  	 */
>  	sched_smt_present_dec(cpu);
>  
> -#ifdef CONFIG_SCHED_SMT
> -	sched_core_cpu_deactivate(cpu);
> -#endif
> -
>  	if (!sched_smp_initialized)
>  		return 0;

                ^^^^^^^^
You're moving the call to the other side of this, however I don't recall
that hot-unplug can happen before sched_init_smp(). Built-in rcutorture
wouldn't start flipping CPUs until device_initcall's, which happens later
in kernel_init_freeable():

  kernel_init_freeable()
  `\
    smp_init() <- boots all secondaries
    sched_init_smp()
    do_basic_setup()
    `\
      do_initcalls()

So this looks okay, but needs a mention in the changelog IMO.

>
> @@ -8204,6 +8200,7 @@ int sched_cpu_deactivate(unsigned int cpu)
>  		sched_update_numa(cpu, true);
>  		return ret;
>  	}
> +	sched_core_cpu_deactivate(cpu);
>  	sched_domains_numa_masks_clear(cpu);
>  	return 0;
>  }
> -- 
> 2.47.0.277.g8800431eea-goog