linux-kernel - Re: [PATCH v6 07/21] cpuset: Don't use the cpu_possible

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Message-ID: <20210521173934.pjcv37j63odtsrp6@e107158-lin.cambridge.arm.com>
Date:   Fri, 21 May 2021 18:39:34 +0100
From:   Qais Yousef <qais.yousef@....com>
To:     Will Deacon <will@...nel.org>
Cc:     linux-arm-kernel@...ts.infradead.org, linux-arch@...r.kernel.org,
        linux-kernel@...r.kernel.org,
        Catalin Marinas <catalin.marinas@....com>,
        Marc Zyngier <maz@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Morten Rasmussen <morten.rasmussen@....com>,
        Suren Baghdasaryan <surenb@...gle.com>,
        Quentin Perret <qperret@...gle.com>, Tejun Heo <tj@...nel.org>,
        Li Zefan <lizefan@...wei.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Ingo Molnar <mingo@...hat.com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>, kernel-team@...roid.com
Subject: Re: [PATCH v6 07/21] cpuset: Don't use the cpu_possible_mask as a
 last resort for cgroup v1

On 05/18/21 10:47, Will Deacon wrote:
> If the scheduler cannot find an allowed CPU for a task,
> cpuset_cpus_allowed_fallback() will widen the affinity to cpu_possible_mask
> if cgroup v1 is in use.
> 
> In preparation for allowing architectures to provide their own fallback
> mask, just return early if we're either using cgroup v1 or we're using
> cgroup v2 with a mask that contains invalid CPUs. This will allow
> select_fallback_rq() to figure out the mask by itself.
> 
> Cc: Li Zefan <lizefan@...wei.com>
> Cc: Tejun Heo <tj@...nel.org>
> Cc: Johannes Weiner <hannes@...xchg.org>
> Reviewed-by: Quentin Perret <qperret@...gle.com>
> Signed-off-by: Will Deacon <will@...nel.org>
> ---
>  include/linux/cpuset.h |  1 +
>  kernel/cgroup/cpuset.c | 12 ++++++++++--
>  2 files changed, 11 insertions(+), 2 deletions(-)
> 
> diff --git a/include/linux/cpuset.h b/include/linux/cpuset.h
> index 04c20de66afc..ed6ec677dd6b 100644
> --- a/include/linux/cpuset.h
> +++ b/include/linux/cpuset.h
> @@ -15,6 +15,7 @@
>  #include <linux/cpumask.h>
>  #include <linux/nodemask.h>
>  #include <linux/mm.h>
> +#include <linux/mmu_context.h>
>  #include <linux/jump_label.h>
>  
>  #ifdef CONFIG_CPUSETS
> diff --git a/kernel/cgroup/cpuset.c b/kernel/cgroup/cpuset.c
> index a945504c0ae7..8c799260a4a2 100644
> --- a/kernel/cgroup/cpuset.c
> +++ b/kernel/cgroup/cpuset.c
> @@ -3322,9 +3322,17 @@ void cpuset_cpus_allowed(struct task_struct *tsk, struct cpumask *pmask)
>  
>  void cpuset_cpus_allowed_fallback(struct task_struct *tsk)
>  {
> +	const struct cpumask *cs_mask;
> +	const struct cpumask *possible_mask = task_cpu_possible_mask(tsk);
> +
>  	rcu_read_lock();
> -	do_set_cpus_allowed(tsk, is_in_v2_mode() ?
> -		task_cs(tsk)->cpus_allowed : cpu_possible_mask);
> +	cs_mask = task_cs(tsk)->cpus_allowed;
> +
> +	if (!is_in_v2_mode() || !cpumask_subset(cs_mask, possible_mask))
> +		goto unlock; /* select_fallback_rq will try harder */
> +
> +	do_set_cpus_allowed(tsk, cs_mask);

Shouldn't we take the intersection with possible_mask like we discussed before?

	https://lore.kernel.org/lkml/20201217145954.GA17881@willie-the-truck/

Thanks

--
Qais Yousef

> +unlock:
>  	rcu_read_unlock();
>  
>  	/*
> -- 
> 2.31.1.751.gd2f1c929bd-goog
>