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    SubjectRe: [PATCH v3 10/14] sched: Introduce arch_cpu_allowed_mask() to limit fallback rq selection
    On Friday 13 Nov 2020 at 09:37:15 (+0000), Will Deacon wrote:
    > Asymmetric systems may not offer the same level of userspace ISA support
    > across all CPUs, meaning that some applications cannot be executed by
    > some CPUs. As a concrete example, upcoming arm64 big.LITTLE designs do
    > not feature support for 32-bit applications on both clusters.
    >
    > On such a system, we must take care not to migrate a task to an
    > unsupported CPU when forcefully moving tasks in select_fallback_rq()
    > in response to a CPU hot-unplug operation.
    >
    > Introduce an arch_cpu_allowed_mask() hook which, given a task argument,
    > allows an architecture to return a cpumask of CPUs that are capable of
    > executing that task. The default implementation returns the
    > cpu_possible_mask, since sane machines do not suffer from per-cpu ISA
    > limitations that affect scheduling. The new mask is used when selecting
    > the fallback runqueue as a last resort before forcing a migration to the
    > first active CPU.
    >
    > Signed-off-by: Will Deacon <will@kernel.org>
    > ---
    > kernel/sched/core.c | 13 ++++++++++---
    > 1 file changed, 10 insertions(+), 3 deletions(-)
    >
    > diff --git a/kernel/sched/core.c b/kernel/sched/core.c
    > index 818c8f7bdf2a..8df38ebfe769 100644
    > --- a/kernel/sched/core.c
    > +++ b/kernel/sched/core.c
    > @@ -1696,6 +1696,11 @@ void check_preempt_curr(struct rq *rq, struct task_struct *p, int flags)
    >
    > #ifdef CONFIG_SMP
    >
    > +/* Must contain at least one active CPU */
    > +#ifndef arch_cpu_allowed_mask
    > +#define arch_cpu_allowed_mask(p) cpu_possible_mask
    > +#endif
    > +
    > /*
    > * Per-CPU kthreads are allowed to run on !active && online CPUs, see
    > * __set_cpus_allowed_ptr() and select_fallback_rq().
    > @@ -1708,7 +1713,10 @@ static inline bool is_cpu_allowed(struct task_struct *p, int cpu)
    > if (is_per_cpu_kthread(p))
    > return cpu_online(cpu);
    >
    > - return cpu_active(cpu);
    > + if (!cpu_active(cpu))
    > + return false;
    > +
    > + return cpumask_test_cpu(cpu, arch_cpu_allowed_mask(p));
    > }
    >
    > /*
    > @@ -2361,10 +2369,9 @@ static int select_fallback_rq(int cpu, struct task_struct *p)
    > }
    > fallthrough;
    > case possible:
    > - do_set_cpus_allowed(p, cpu_possible_mask);
    > + do_set_cpus_allowed(p, arch_cpu_allowed_mask(p));

    Nit: I'm wondering if this should be called arch_cpu_possible_mask()
    instead?

    In any case:

    Reviewed-by: Quentin Perret <qperret@google.com?

    Thanks,
    Quentin

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