On Thu, Nov 19, 2020 at 11:07:09AM +0000, Will Deacon wrote: > On Thu, Nov 19, 2020 at 09:38:50AM +0000, Quentin Perret wrote: > > 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? > > I'm open to renaming it, so if nobody else has any better ideas then I'll > go with this. Ah, so in doing this I realised I don't like arch_cpu_possible_mask() so much because it makes it sound like a back-end to cpu_possible_mask, but the two are really different things. arch_task_cpu_possible_mask() might work? Will
