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@xxxxxxxxxx> > --- > 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@xxxxxxxxxx? Thanks, Quentin
