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Date:   Fri, 21 May 2021 18:03:44 +0200
From:   Peter Zijlstra <peterz@infradead.org>
To:     Will Deacon <will@kernel.org>
Cc:     linux-arm-kernel@lists.infradead.org, linux-arch@vger.kernel.org,
        linux-kernel@vger.kernel.org,
        Catalin Marinas <catalin.marinas@arm.com>,
        Marc Zyngier <maz@kernel.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Morten Rasmussen <morten.rasmussen@arm.com>,
        Qais Yousef <qais.yousef@arm.com>,
        Suren Baghdasaryan <surenb@google.com>,
        Quentin Perret <qperret@google.com>, Tejun Heo <tj@kernel.org>,
        Li Zefan <lizefan@huawei.com>,
        Johannes Weiner <hannes@cmpxchg.org>,
        Ingo Molnar <mingo@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        "Rafael J. Wysocki" <rjw@rjwysocki.net>, kernel-team@android.com
Subject: Re: [PATCH v6 06/21] sched: Introduce task_cpu_possible_mask() to
 limit fallback rq selection
Message-ID: <20210521160344.GJ5618@worktop.programming.kicks-ass.net>
References: <20210518094725.7701-1-will@kernel.org>
 <20210518094725.7701-7-will@kernel.org>
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On Tue, May 18, 2021 at 10:47:10AM +0100, Will Deacon wrote:
> diff --git a/include/linux/mmu_context.h b/include/linux/mmu_context.h
> index 03dee12d2b61..bc4ac3c525e6 100644
> --- a/include/linux/mmu_context.h
> +++ b/include/linux/mmu_context.h
> @@ -14,4 +14,12 @@
>  static inline void leave_mm(int cpu) { }
>  #endif
>  
> +/*
> + * CPUs that are capable of running task @p. By default, we assume a sane,
> + * homogeneous system. Must contain at least one active CPU.
> + */
> +#ifndef task_cpu_possible_mask
> +# define task_cpu_possible_mask(p)	cpu_possible_mask
> +#endif

#ifndef task_cpu_possible_mask
# define task_cpu_possible_mask(p)	cpu_possible_mask
# define task_cpu_possible(cpu, p)	true
#else
# define task_cpu_possible(cpu, p)	cpumask_test_cpu((cpu), task_cpu_possible_mask(p))
#endif

> +
>  #endif
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 5226cc26a095..482f7fdca0e8 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1813,8 +1813,11 @@ static inline bool is_cpu_allowed(struct task_struct *p, int cpu)
>  		return cpu_online(cpu);
>  
>  	/* Non kernel threads are not allowed during either online or offline. */
>  	if (!(p->flags & PF_KTHREAD))
> -		return cpu_active(cpu);
+		return cpu_active(cpu) && task_cpu_possible(cpu, p);

>  	/* KTHREAD_IS_PER_CPU is always allowed. */
>  	if (kthread_is_per_cpu(p))

Would something like that make sense?