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Subject[PATCH v4 10/14] sched: Introduce arch_task_cpu_possible_mask() to limit fallback rq selection
Date
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@google.com>
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..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
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