diff mbox series

[v6,06/21] sched: Introduce task_cpu_possible_mask() to limit fallback rq selection

Message ID 20210518094725.7701-7-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 May 18, 2021, 9:47 a.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 |  8 ++++++++
 kernel/sched/core.c         | 10 ++++++----
 2 files changed, 14 insertions(+), 4 deletions(-)

Comments

Peter Zijlstra May 21, 2021, 4:03 p.m. UTC | #1 
On Tue, May 18, 2021 at 10:47:10AM +0100, Will Deacon wrote:
> diff --git a/include/linux/mmu_context.h b/include/linux/mmu_context.h
> index 03dee12d2b61..bc4ac3c525e6 100644
> --- a/include/linux/mmu_context.h
> +++ b/include/linux/mmu_context.h
> @@ -14,4 +14,12 @@
>  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
> +#endif

#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..482f7fdca0e8 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -1813,8 +1813,11 @@ static inline bool is_cpu_allowed(struct task_struct *p, int cpu)
>  		return cpu_online(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))

Would something like that make sense?
Will Deacon May 24, 2021, 12:17 p.m. UTC | #2 
On Fri, May 21, 2021 at 06:03:44PM +0200, Peter Zijlstra wrote:
> On Tue, May 18, 2021 at 10:47:10AM +0100, Will Deacon wrote:
> > diff --git a/include/linux/mmu_context.h b/include/linux/mmu_context.h
> > index 03dee12d2b61..bc4ac3c525e6 100644
> > --- a/include/linux/mmu_context.h
> > +++ b/include/linux/mmu_context.h
> > @@ -14,4 +14,12 @@
> >  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
> > +#endif
> 
> #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..482f7fdca0e8 100644
> > --- a/kernel/sched/core.c
> > +++ b/kernel/sched/core.c
> > @@ -1813,8 +1813,11 @@ static inline bool is_cpu_allowed(struct task_struct *p, int cpu)
> >  		return cpu_online(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))
> 
> Would something like that make sense?

I think this is probably the only place that we could use the helper, but
it's also one of the places where architectures that don't have to worry
about asymmetry end up with the check so, yes, I'll do that for v7.

Will
diff mbox series

Patch

diff --git a/include/linux/mmu_context.h b/include/linux/mmu_context.h
index 03dee12d2b61..bc4ac3c525e6 100644
--- a/include/linux/mmu_context.h
+++ b/include/linux/mmu_context.h
@@ -14,4 +14,12 @@ 
 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
+#endif
+
 #endif
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 5226cc26a095..482f7fdca0e8 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -1813,8 +1813,11 @@  static inline bool is_cpu_allowed(struct task_struct *p, int cpu)
 		return cpu_online(cpu);
 
 	/* Non kernel threads are not allowed during either online or offline. */
-	if (!(p->flags & PF_KTHREAD))
-		return cpu_active(cpu);
+	if (!(p->flags & PF_KTHREAD)) {
+		if (cpu_active(cpu))
+			return cpumask_test_cpu(cpu, task_cpu_possible_mask(p));
+		return false;
+	}
 
 	/* KTHREAD_IS_PER_CPU is always allowed. */
 	if (kthread_is_per_cpu(p))
@@ -2792,10 +2795,9 @@  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;