diff mbox series

[v8,05/19] sched: Introduce task_cpu_possible_mask() to limit fallback rq selection

Message ID 20210602164719.31777-6-will@kernel.org (mailing list archive)
State New, archived
Headers show
Series Add support for 32-bit tasks on asymmetric AArch32 systems | expand

Commit Message

Will Deacon June 2, 2021, 4:47 p.m. UTC 
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: Quentin Perret <qperret@google.com>
Signed-off-by: Will Deacon <will@kernel.org>
---
 include/linux/mmu_context.h | 11 +++++++++++
 kernel/sched/core.c         |  5 ++---
 2 files changed, 13 insertions(+), 3 deletions(-)

Comments

Valentin Schneider June 4, 2021, 5:10 p.m. UTC | #1 
On 02/06/21 17:47, 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 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.
>

Nit: Some uses of this mask (cpu_is_allowed(), __set_cpus_allowed_ptr())
don't apply to kthreads. This makes sense for the 32-bit@EL0 faff, but it
wouldn't hurt to point this out somewhere IMO.

Also, that's an odd place for the definitions, but IIRC there isn't a much
better choice.

Reviewed-by: Valentin Schneider <Valentin.Schneider@arm.com>
Will Deacon June 7, 2021, 5:04 p.m. UTC | #2 
On Fri, Jun 04, 2021 at 06:10:46PM +0100, Valentin Schneider wrote:
> On 02/06/21 17:47, 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 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.
> >
> 
> Nit: Some uses of this mask (cpu_is_allowed(), __set_cpus_allowed_ptr())
> don't apply to kthreads. This makes sense for the 32-bit@EL0 faff, but it
> wouldn't hurt to point this out somewhere IMO.

That's a good point: even after these patches, we still assume the kernel
(and therefore kthreads) can run on all CPUs. I'll expand the comment.

> Also, that's an odd place for the definitions, but IIRC there isn't a much
> better choice.

Short of adding a new header just for this, I couldn't find anything, no.

> Reviewed-by: Valentin Schneider <Valentin.Schneider@arm.com>

Thanks!

Will
diff mbox series

Patch

diff --git a/include/linux/mmu_context.h b/include/linux/mmu_context.h
index 03dee12d2b61..1a599ba3524f 100644
--- a/include/linux/mmu_context.h
+++ b/include/linux/mmu_context.h
@@ -14,4 +14,15 @@ 
 static inline void leave_mm(int cpu) { }
 #endif
 
+/*
+ * CPUs that are capable of running task @p. By default, we assume a sane,
+ * homogeneous system. Must contain at least one active CPU.
+ */
+#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;