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_task_cpu_possible_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. Reviewed-by: Quentin Perret <qperret@xxxxxxxxxx> 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..99992d0beb65 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_task_cpu_possible_mask +#define arch_task_cpu_possible_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_task_cpu_possible_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_task_cpu_possible_mask(p)); state = fail; break; - case fail: BUG(); break; -- 2.29.2.454.gaff20da3a2-goog
