[PATCH v3 10/14] sched: Introduce arch_cpu_allowed_mask() to limit fallback rq selection

From: Will Deacon <will@kernel.org>
To: linux-arm-kernel@lists.infradead.org
Cc: linux-arch@vger.kernel.org, linux-kernel@vger.kernel.org,
	Will Deacon <will@kernel.org>,
	Catalin Marinas <catalin.marinas@arm.com>,
	Marc Zyngier <maz@kernel.org>,
	Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
	Peter Zijlstra <peterz@infradead.org>,
	Morten Rasmussen <morten.rasmussen@arm.com>,
	Qais Yousef <qais.yousef@arm.com>,
	Suren Baghdasaryan <surenb@google.com>,
	Quentin Perret <qperret@google.com>, Tejun Heo <tj@kernel.org>,
	Li Zefan <lizefan@huawei.com>,
	Johannes Weiner <hannes@cmpxchg.org>,
	Ingo Molnar <mingo@redhat.com>,
	Juri Lelli <juri.lelli@redhat.com>,
	Vincent Guittot <vincent.guittot@linaro.org>,
	kernel-team@android.com
Subject: [PATCH v3 10/14] sched: Introduce arch_cpu_allowed_mask() to limit fallback rq selection
Date: Fri, 13 Nov 2020 09:37:15 +0000	[thread overview]
Message-ID: <20201113093720.21106-11-will@kernel.org> (raw)
In-Reply-To: <20201113093720.21106-1-will@kernel.org>

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));
 			state = fail;
 			break;
-
 		case fail:
 			BUG();
 			break;
-- 
2.29.2.299.gdc1121823c-goog

