2020-12-03 06:10:43

by Crystal Wood

[permalink] [raw]
Subject: [PATCH] sched/fair: Don't migrate with src_cpu == dst_cpu

Besides being a waste of time to try to move tasks to where they already
are, this avoids triggering the WARN_ON_ONCE(is_migration_disabled(p))
in set_task_cpu().

Signed-off-by: Scott Wood <[email protected]>
---
Patch is against tip/master. Assertion was seen by running rteval on the
RT tree.

kernel/sched/fair.c | 3 ++-
1 file changed, 2 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index e7e21ac479a2..f443626164d4 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7574,7 +7574,8 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)

/* Prevent to re-select dst_cpu via env's CPUs: */
for_each_cpu_and(cpu, env->dst_grpmask, env->cpus) {
- if (cpumask_test_cpu(cpu, p->cpus_ptr)) {
+ if (cpu != env->src_cpu &&
+ cpumask_test_cpu(cpu, p->cpus_ptr)) {
env->flags |= LBF_DST_PINNED;
env->new_dst_cpu = cpu;
break;
--
2.27.0


2020-12-03 08:51:06

by Peter Zijlstra

[permalink] [raw]
Subject: Re: [PATCH] sched/fair: Don't migrate with src_cpu == dst_cpu

On Thu, Dec 03, 2020 at 12:04:49AM -0600, Scott Wood wrote:
> Besides being a waste of time to try to move tasks to where they already
> are, this avoids triggering the WARN_ON_ONCE(is_migration_disabled(p))
> in set_task_cpu().
>
> Signed-off-by: Scott Wood <[email protected]>
> ---
> Patch is against tip/master. Assertion was seen by running rteval on the
> RT tree.
>
> kernel/sched/fair.c | 3 ++-
> 1 file changed, 2 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index e7e21ac479a2..f443626164d4 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -7574,7 +7574,8 @@ int can_migrate_task(struct task_struct *p, struct lb_env *env)
>
> /* Prevent to re-select dst_cpu via env's CPUs: */
> for_each_cpu_and(cpu, env->dst_grpmask, env->cpus) {
> - if (cpumask_test_cpu(cpu, p->cpus_ptr)) {
> + if (cpu != env->src_cpu &&
> + cpumask_test_cpu(cpu, p->cpus_ptr)) {
> env->flags |= LBF_DST_PINNED;
> env->new_dst_cpu = cpu;
> break;

Do we have _any_ clue as to how we ended up in that situation? The above
sounds like it should be a WARN and we should avoid getting here in the
first place.

2020-12-07 10:27:26

by Crystal Wood

[permalink] [raw]
Subject: Re: [PATCH] sched/fair: Don't migrate with src_cpu == dst_cpu

On Thu, 2020-12-03 at 09:47 +0100, Peter Zijlstra wrote:
> On Thu, Dec 03, 2020 at 12:04:49AM -0600, Scott Wood wrote:
> > Besides being a waste of time to try to move tasks to where they already
> > are, this avoids triggering the WARN_ON_ONCE(is_migration_disabled(p))
> > in set_task_cpu().
> >
> > Signed-off-by: Scott Wood <[email protected]>
> > ---
> > Patch is against tip/master. Assertion was seen by running rteval on
> > the
> > RT tree.
> >
> > kernel/sched/fair.c | 3 ++-
> > 1 file changed, 2 insertions(+), 1 deletion(-)
> >
> > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> > index e7e21ac479a2..f443626164d4 100644
> > --- a/kernel/sched/fair.c
> > +++ b/kernel/sched/fair.c
> > @@ -7574,7 +7574,8 @@ int can_migrate_task(struct task_struct *p, struct
> > lb_env *env)
> >
> > /* Prevent to re-select dst_cpu via env's CPUs: */
> > for_each_cpu_and(cpu, env->dst_grpmask, env->cpus) {
> > - if (cpumask_test_cpu(cpu, p->cpus_ptr)) {
> > + if (cpu != env->src_cpu &&
> > + cpumask_test_cpu(cpu, p->cpus_ptr)) {
> > env->flags |= LBF_DST_PINNED;
> > env->new_dst_cpu = cpu;
> > break;
>
> Do we have _any_ clue as to how we ended up in that situation? The above
> sounds like it should be a WARN and we should avoid getting here in the
> first place.

My initial impression was that there simply wasn't anything stopping it from
happening, but digging deeper it looks like it's specific to NUMA domains
with overlapping CPUs.

-Scott