linux-kernel - Re: [patch 2/2] sched/ext: Remove sched

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Message-ID: <87zfmojtpk.fsf@prevas.dk>
Date: Mon, 28 Oct 2024 13:30:15 +0100
From: Rasmus Villemoes <ravi@...vas.dk>
To: Thomas Gleixner <tglx@...utronix.de>
Cc: LKML <linux-kernel@...r.kernel.org>,  Peter Zijlstra
 <peterz@...radead.org>,  Tejun Heo <tj@...nel.org>,  David Vernet
 <void@...ifault.com>,  Ingo Molnar <mingo@...nel.org.com>,  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>,  Valentin Schneider <vschneid@...hat.com>,  Ingo Molnar
 <mingo@...nel.org>
Subject: Re: [patch 2/2] sched/ext: Remove sched_fork() hack

On Mon, Oct 28 2024, Thomas Gleixner <tglx@...utronix.de> wrote:

> Instead of solving the underlying problem of the double invocation of
> __sched_fork() for idle tasks, sched-ext decided to hack around the issue
> by partially clearing out the entity struct to preserve the already
> enqueued node. A provided analysis and solution has been ignored for four
> months.
>
> Now that someone else has taken care of cleaning it up, remove the
> disgusting hack and clear out the full structure.
>
> Fixes: f0e1a0643a59 ("sched_ext: Implement BPF extensible scheduler class")
> Signed-off-by: Thomas Gleixner <tglx@...utronix.de>
> Cc: Tejun Heo <tj@...nel.org>
> Cc: David Vernet <void@...ifault.com>
> Cc: Ingo Molnar <mingo@...nel.org.com>
> Cc: Peter Zijlstra <peterz@...radead.org>
> Cc: Juri Lelli <juri.lelli@...hat.com>
> Cc: Vincent Guittot <vincent.guittot@...aro.org>
> Cc: Dietmar Eggemann <dietmar.eggemann@....com>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Ben Segall <bsegall@...gle.com>
> Cc: Mel Gorman <mgorman@...e.de>
> Cc: Valentin Schneider <vschneid@...hat.com>
> ---
>  kernel/sched/ext.c |    7 +------
>  1 file changed, 1 insertion(+), 6 deletions(-)
>
> --- a/kernel/sched/ext.c
> +++ b/kernel/sched/ext.c
> @@ -3548,12 +3548,7 @@ static void scx_ops_exit_task(struct tas
>  
>  void init_scx_entity(struct sched_ext_entity *scx)
>  {
> -	/*
> -	 * init_idle() calls this function again after fork sequence is
> -	 * complete. Don't touch ->tasks_node as it's already linked.
> -	 */
> -	memset(scx, 0, offsetof(struct sched_ext_entity, tasks_node));
> -
> +	memset(scx, 0, sizeof(*scx));
>  	INIT_LIST_HEAD(&scx->dsq_list.node);
>  	RB_CLEAR_NODE(&scx->dsq_priq);
>  	scx->sticky_cpu = -1;

Should the "must be the last" comment in include/linux/sched/ext.h also
be removed?

Rasmus