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Date:   Mon, 3 Jul 2023 12:26:04 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Waiman Long <longman@redhat.com>
Cc:     Ingo Molnar <mingo@redhat.com>, Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Ben Segall <bsegall@google.com>, Mel Gorman <mgorman@suse.de>,
        Daniel Bristot de Oliveira <bristot@redhat.com>,
        Valentin Schneider <vschneid@redhat.com>,
        linux-kernel@vger.kernel.org, Phil Auld <pauld@redhat.com>,
        Brent Rowsell <browsell@redhat.com>,
        Peter Hunt <pehunt@redhat.com>
Subject: Re: [PATCH] sched/core: Use empty mask to reset cpumasks in
 sched_setaffinity()
Message-ID: <20230703102604.GC4253@hirez.programming.kicks-ass.net>
References: <20230628211637.1679348-1-longman@redhat.com>
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On Wed, Jun 28, 2023 at 05:16:37PM -0400, Waiman Long wrote:
> Since commit 8f9ea86fdf99 ("sched: Always preserve the user requested
> cpumask"), user provided CPU affinity via sched_setaffinity(2) is
> perserved even if the task is being moved to a different cpuset. However,
> that affinity is also being inherited by any subsequently created child
> processes which may not want or be aware of that affinity.
> 
> One way to solve this problem is to provide a way to back off from
> that user provided CPU affinity.  This patch implements such a scheme
> by using an empty cpumask to signal a reset of the cpumasks to the
> default as allowed by the current cpuset.
> 
> Before this patch, passing in an empty cpumask to sched_setaffinity(2)
> will return an EINVAL error. With this patch, an error will no longer
> be returned. Instead, the user_cpus_ptr that stores the user provided
> affinity, if set, will be cleared and the task's CPU affinity will be
> reset to that of the current cpuset. This reverts the cpumask change
> done by all the previous sched_setaffinity(2) calls.
> 

This is a user visible ABI change -- but with very limited motivation.
Why do we want this? Who will use this?

> Signed-off-by: Waiman Long <longman@redhat.com>
> ---
>  kernel/sched/core.c | 26 +++++++++++++++++++++-----
>  1 file changed, 21 insertions(+), 5 deletions(-)
> 
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index c52c2eba7c73..f4806d969fc9 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -8317,7 +8317,12 @@ __sched_setaffinity(struct task_struct *p, struct affinity_context *ctx)
>  	}
>  
>  	cpuset_cpus_allowed(p, cpus_allowed);
> -	cpumask_and(new_mask, ctx->new_mask, cpus_allowed);
> +
> +	/* Default to cpus_allowed with NULL new_mask */
> +	if (ctx->new_mask)
> +		cpumask_and(new_mask, ctx->new_mask, cpus_allowed);
> +	else
> +		cpumask_copy(new_mask, cpus_allowed);
>  
>  	ctx->new_mask = new_mask;
>  	ctx->flags |= SCA_CHECK;
> @@ -8366,6 +8371,7 @@ __sched_setaffinity(struct task_struct *p, struct affinity_context *ctx)
>  
>  long sched_setaffinity(pid_t pid, const struct cpumask *in_mask)
>  {
> +	bool reset_cpumasks = cpumask_empty(in_mask);
>  	struct affinity_context ac;
>  	struct cpumask *user_mask;
>  	struct task_struct *p;
> @@ -8403,13 +8409,23 @@ long sched_setaffinity(pid_t pid, const struct cpumask *in_mask)
>  		goto out_put_task;
>  
>  	/*
> -	 * With non-SMP configs, user_cpus_ptr/user_mask isn't used and
> -	 * alloc_user_cpus_ptr() returns NULL.
> +	 * If an empty cpumask is passed in, clear user_cpus_ptr, if set,
> +	 * and reset the current cpu affinity to the default for the
> +	 * current cpuset.
>  	 */
> -	user_mask = alloc_user_cpus_ptr(NUMA_NO_NODE);
> +	if (reset_cpumasks) {
> +		in_mask = NULL;	/* To be updated in __sched_setaffinity */
> +		user_mask = NULL;
> +	} else {
> +		/*
> +		 * With non-SMP configs, user_cpus_ptr/user_mask isn't used
> +		 * and alloc_user_cpus_ptr() returns NULL.
> +		 */
> +		user_mask = alloc_user_cpus_ptr(NUMA_NO_NODE);
> +	}
>  	if (user_mask) {
>  		cpumask_copy(user_mask, in_mask);
> -	} else if (IS_ENABLED(CONFIG_SMP)) {
> +	} else if (!reset_cpumasks && IS_ENABLED(CONFIG_SMP)) {
>  		retval = -ENOMEM;
>  		goto out_put_task;
>  	}
> -- 
> 2.31.1
>