[PATCH v9 05/20] sched: Introduce task_cpu_possible_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>,
	Johannes Weiner <hannes@cmpxchg•org>,
	Ingo Molnar <mingo@redhat•com>,
	Juri Lelli <juri.lelli@redhat•com>,
	Vincent Guittot <vincent.guittot@linaro•org>,
	"Rafael J. Wysocki" <rjw@rjwysocki•net>,
	Dietmar Eggemann <dietmar.eggemann@arm•com>,
	Daniel Bristot de Oliveira <bristot@redhat•com>,
	Valentin Schneider <valentin.schneider@arm•com>,
	Mark Rutland <mark.rutland@arm•com>,
	kernel-team@android•com,
	Valentin Schneider <Valentin.Schneider@arm•com>
Subject: [PATCH v9 05/20] sched: Introduce task_cpu_possible_mask() to limit fallback rq selection
Date: Tue,  8 Jun 2021 19:02:58 +0100	[thread overview]
Message-ID: <20210608180313.11502-6-will@kernel.org> (raw)
In-Reply-To: <20210608180313.11502-1-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 a 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: Valentin Schneider <Valentin.Schneider@arm•com>
Reviewed-by: Quentin Perret <qperret@google•com>
Signed-off-by: Will Deacon <will@kernel•org>
---
 include/linux/mmu_context.h | 14 ++++++++++++++
 kernel/sched/core.c         |  5 ++---
 2 files changed, 16 insertions(+), 3 deletions(-)

diff --git a/include/linux/mmu_context.h b/include/linux/mmu_context.h
index 03dee12d2b61..b9b970f7ab45 100644
--- a/include/linux/mmu_context.h
+++ b/include/linux/mmu_context.h
@@ -14,4 +14,18 @@
 static inline void leave_mm(int cpu) { }
 #endif
 
+/*
+ * CPUs that are capable of running user task @p. Must contain at least one
+ * active CPU. It is assumed that the kernel can run on all CPUs, so calling
+ * this for a kernel thread is pointless.
+ *
+ * By default, we assume a sane, homogeneous system.
+ */
+#ifndef task_cpu_possible_mask
+# define task_cpu_possible_mask(p)	cpu_possible_mask
+# define task_cpu_possible(cpu, p)	true
+#else
+# define task_cpu_possible(cpu, p)	cpumask_test_cpu((cpu), task_cpu_possible_mask(p))
+#endif
+
 #endif
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 5226cc26a095..0c1b6f1a6c91 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1814,7 +1814,7 @@ static inline bool is_cpu_allowed(struct task_struct *p, int cpu)
 
 	/* Non kernel threads are not allowed during either online or offline. */
 	if (!(p->flags & PF_KTHREAD))
-		return cpu_active(cpu);
+		return cpu_active(cpu) && task_cpu_possible(cpu, p);
 
 	/* KTHREAD_IS_PER_CPU is always allowed. */
 	if (kthread_is_per_cpu(p))
@@ -2792,10 +2792,9 @@ static int select_fallback_rq(int cpu, struct task_struct *p)
 			 *
 			 * More yuck to audit.
 			 */
-			do_set_cpus_allowed(p, cpu_possible_mask);
+			do_set_cpus_allowed(p, task_cpu_possible_mask(p));
 			state = fail;
 			break;
-
 		case fail:
 			BUG();
 			break;
-- 
2.32.0.rc1.229.g3e70b5a671-goog


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