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Message-ID: <b5ebf7e2-19ee-962c-d580-dae717d1c977@redhat.com>
Date: Fri, 20 Jan 2023 21:14:43 -0500
From: Waiman Long <longman@...hat.com>
To: 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>,
Daniel Bristot de Oliveira <bristot@...hat.com>,
Valentin Schneider <vschneid@...hat.com>,
Will Deacon <will@...nel.org>
Cc: Phil Auld <pauld@...hat.com>,
Linus Torvalds <torvalds@...ux-foundation.org>,
linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched: Store restrict_cpus_allowed_ptr() call state
On 1/20/23 16:08, Waiman Long wrote:
> The user_cpus_ptr field was originally added by commit b90ca8badbd1
> ("sched: Introduce task_struct::user_cpus_ptr to track requested
> affinity"). It was used only by arm64 arch due to possible asymmetric
> CPU setup.
>
> Since commit 8f9ea86fdf99 ("sched: Always preserve the user requested
> cpumask"), task_struct::user_cpus_ptr is repurposed to store user
> requested cpu affinity specified in the sched_setaffinity().
>
> This results in a performance regression in an arm64 system when booted
> with "allow_mismatched_32bit_el0" on the command-line. The arch code will
> (amongst other things) calls force_compatible_cpus_allowed_ptr() and
> relax_compatible_cpus_allowed_ptr() when exec()'ing a 32-bit or a 64-bit
> task respectively. Now a call to relax_compatible_cpus_allowed_ptr()
> will always result in a __sched_setaffinity() call whether there is a
> previous force_compatible_cpus_allowed_ptr() call or not.
>
> In order to fix this regression, a new scheduler flag
> task_struct::cpus_allowed_restricted is now added to track if
> force_compatible_cpus_allowed_ptr() has been called before or not. This
> patch also updates the comments in force_compatible_cpus_allowed_ptr()
> and relax_compatible_cpus_allowed_ptr() and handles their interaction
> with sched_setaffinity().
>
> Fixes: 8f9ea86fdf99 ("sched: Always preserve the user requested cpumask")
> Reported-by: Will Deacon <will@...nel.org>
> Signed-off-by: Waiman Long <longman@...hat.com>
> ---
> include/linux/sched.h | 3 +++
> kernel/sched/core.c | 55 ++++++++++++++++++++++++++++++++++++-------
> kernel/sched/sched.h | 2 ++
> 3 files changed, 51 insertions(+), 9 deletions(-)
>
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index 853d08f7562b..f93f62a1f858 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -886,6 +886,9 @@ struct task_struct {
> unsigned sched_contributes_to_load:1;
> unsigned sched_migrated:1;
>
> + /* restrict_cpus_allowed_ptr() bit, serialized by scheduler locks */
> + unsigned cpus_allowed_restricted:1;
> +
> /* Force alignment to the next boundary: */
> unsigned :0;
>
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index bb1ee6d7bdde..c334c1971e81 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -2999,15 +2999,49 @@ static int __set_cpus_allowed_ptr(struct task_struct *p,
> struct rq *rq;
>
> rq = task_rq_lock(p, &rf);
> +
> + if (ctx->flags & (SCA_CLR_RESTRICT | SCA_SET_RESTRICT)) {
> + p->cpus_allowed_restricted = 0;
> + } else if (p->cpus_allowed_restricted) {
> + /*
> + * If force_compatible_cpus_allowed_ptr() has been called,
> + * we can't extend cpumask to beyond what is in cpus_mask.
> + */
> + if (!cpumask_and(rq->scratch_mask, ctx->new_mask,
> + &p->cpus_mask)) {
> + task_rq_unlock(rq, p, &rf);
> + return -EINVAL;
> + }
> +
> + /*
> + * In the unlikely event that sched_setaffinity() has been
> + * called on a cpu with cpus_allowed_restricted flag set,
> + * we have to store in user_cpus_ptr a restricted version
> + * of cpu affinity mask that won't be restored when
> + * relax_compatible_cpus_allowed_ptr() is called.
> + *
> + * Note that we don't need to do further user_cpus_ptr
> + * masking below as cpus_mask should be a subset of
> + * user_cpus_ptr if set.
> + */
> + if (ctx->flags & SCA_USER)
> + cpumask_copy((struct cpumask *)ctx->new_mask, rq->scratch_mask);
Sorry, that part is wrong. I forgot there were two masks in ctx. Will
send out a v2.
Cheers,
Longman
> + else
> + ctx->new_mask = rq->scratch_mask;
> + }
> +
> /*
> * Masking should be skipped if SCA_USER or any of the SCA_MIGRATE_*
> - * flags are set.
> + * flags are set or when cpus_allowed_restricted flag has been set.
> */
> - if (p->user_cpus_ptr &&
> + if (p->user_cpus_ptr && !p->cpus_allowed_restricted &&
> !(ctx->flags & (SCA_USER | SCA_MIGRATE_ENABLE | SCA_MIGRATE_DISABLE)) &&
> cpumask_and(rq->scratch_mask, ctx->new_mask, p->user_cpus_ptr))
> ctx->new_mask = rq->scratch_mask;
>
> + if (ctx->flags & SCA_SET_RESTRICT)
> + p->cpus_allowed_restricted = 1;
> +
> return __set_cpus_allowed_ptr_locked(p, ctx, rq, &rf);
> }
>
> @@ -3025,8 +3059,8 @@ EXPORT_SYMBOL_GPL(set_cpus_allowed_ptr);
> /*
> * Change a given task's CPU affinity to the intersection of its current
> * affinity mask and @subset_mask, writing the resulting mask to @new_mask.
> - * If user_cpus_ptr is defined, use it as the basis for restricting CPU
> - * affinity or use cpu_online_mask instead.
> + * The cpus_allowed_restricted bit is set to indicate to a later
> + * relax_compatible_cpus_allowed_ptr() call to relax the cpumask.
> *
> * If the resulting mask is empty, leave the affinity unchanged and return
> * -EINVAL.
> @@ -3037,7 +3071,7 @@ static int restrict_cpus_allowed_ptr(struct task_struct *p,
> {
> struct affinity_context ac = {
> .new_mask = new_mask,
> - .flags = 0,
> + .flags = SCA_SET_RESTRICT,
> };
> struct rq_flags rf;
> struct rq *rq;
> @@ -3069,9 +3103,8 @@ static int restrict_cpus_allowed_ptr(struct task_struct *p,
>
> /*
> * Restrict the CPU affinity of task @p so that it is a subset of
> - * task_cpu_possible_mask() and point @p->user_cpus_ptr to a copy of the
> - * old affinity mask. If the resulting mask is empty, we warn and walk
> - * up the cpuset hierarchy until we find a suitable mask.
> + * task_cpu_possible_mask(). If the resulting mask is empty, we warn
> + * and walk up the cpuset hierarchy until we find a suitable mask.
> */
> void force_compatible_cpus_allowed_ptr(struct task_struct *p)
> {
> @@ -3126,10 +3159,14 @@ void relax_compatible_cpus_allowed_ptr(struct task_struct *p)
> {
> struct affinity_context ac = {
> .new_mask = task_user_cpus(p),
> - .flags = 0,
> + .flags = SCA_CLR_RESTRICT,
> };
> int ret;
>
> + /* Return if no previous force_compatible_cpus_allowed_ptr() call */
> + if (!data_race(p->cpus_allowed_restricted))
> + return;
> +
> /*
> * Try to restore the old affinity mask with __sched_setaffinity().
> * Cpuset masking will be done there too.
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 771f8ddb7053..adcef29d5479 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -2293,6 +2293,8 @@ extern struct task_struct *pick_next_task_idle(struct rq *rq);
> #define SCA_MIGRATE_DISABLE 0x02
> #define SCA_MIGRATE_ENABLE 0x04
> #define SCA_USER 0x08
> +#define SCA_CLR_RESTRICT 0x10
> +#define SCA_SET_RESTRICT 0x20
>
> #ifdef CONFIG_SMP
>
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