| Message ID | 1575483700-22153-1-git-send-email-vincent.guittot@linaro.org |
|---|---|
| State | New |
| Headers | show |
| Series | sched/fair: fix find_idlest_group() to handle CPU affinity | expand |
On 04/12/2019 18:21, Vincent Guittot wrote: > Because of CPU affinity, the local group can be skipped which breaks the > assumption that statistics are always collected for local group. With > uninitialized local_sgs, the comparison is meaningless and the behavior > unpredictable. This can even end up to use local pointer which is to > NULL in this case. > > If the local group has been skipped because of CPU affinity, we return > the idlest group. > I stared at find_idlest_group() before the rework out of curiosity and AFAICT the "never visit local group" thing was there already. However, we would only use the load and spare capacity of that group, and the relevant variables where initialized to ULONG_MAX and 0 respectively. This would lead us to return 'idlest' (or 'most_spare_sg', but it's the same as 'idlest' now). So IMO this is just restoring the previous behaviour, which is what we want methinks. Reviewed-by: Valentin Schneider <valentin.schneider@arm.com>
On 12/04/19 19:21, Vincent Guittot wrote: > Because of CPU affinity, the local group can be skipped which breaks the > assumption that statistics are always collected for local group. With > uninitialized local_sgs, the comparison is meaningless and the behavior > unpredictable. This can even end up to use local pointer which is to > NULL in this case. For the record; I think this is safe and I can't see how the local_sgs can be used uninitialized, but experience shows that if this happened once it's likely to happen again when things change. So I think it'd be safer to always initialize local_sgs to something sensible and avoid future trouble. I don't see any cost to initializing it. My 2p :-) The change is good for me as-is otherwise. Cheers -- Qais Yousef > > If the local group has been skipped because of CPU affinity, we return > the idlest group. > > Fixes: 57abff067a08 ("sched/fair: Rework find_idlest_group()") > Reported-by: John Stultz <john.stultz@linaro.org> > Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org> > Tested-by: John Stultz <john.stultz@linaro.org> > --- > kernel/sched/fair.c | 4 ++++ > 1 file changed, 4 insertions(+) > > diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c > index 08a233e..146b6c8 100644 > --- a/kernel/sched/fair.c > +++ b/kernel/sched/fair.c > @@ -8417,6 +8417,10 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p, > if (!idlest) > return NULL; > > + /* The local group has been skipped because of CPU affinity */ > + if (!local) > + return idlest; > + > /* > * If the local group is idler than the selected idlest group > * don't try and push the task. > -- > 2.7.4 >
On Wed, Dec 4, 2019 at 10:21 AM Vincent Guittot <vincent.guittot@linaro.org> wrote: > > Because of CPU affinity, the local group can be skipped which breaks the > assumption that statistics are always collected for local group. With > uninitialized local_sgs, the comparison is meaningless and the behavior > unpredictable. This can even end up to use local pointer which is to > NULL in this case. > > If the local group has been skipped because of CPU affinity, we return > the idlest group. > > Fixes: 57abff067a08 ("sched/fair: Rework find_idlest_group()") > Reported-by: John Stultz <john.stultz@linaro.org> > Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org> > Tested-by: John Stultz <john.stultz@linaro.org> Just wanted to follow up on this, as its seemed to have missed -rc2? thanks -john
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c index 08a233e..146b6c8 100644 --- a/kernel/sched/fair.c +++ b/kernel/sched/fair.c @@ -8417,6 +8417,10 @@ find_idlest_group(struct sched_domain *sd, struct task_struct *p, if (!idlest) return NULL; + /* The local group has been skipped because of CPU affinity */ + if (!local) + return idlest; + /* * If the local group is idler than the selected idlest group * don't try and push the task.
Because of CPU affinity, the local group can be skipped which breaks the assumption that statistics are always collected for local group. With uninitialized local_sgs, the comparison is meaningless and the behavior unpredictable. This can even end up to use local pointer which is to NULL in this case. If the local group has been skipped because of CPU affinity, we return the idlest group. Fixes: 57abff067a08 ("sched/fair: Rework find_idlest_group()") Reported-by: John Stultz <john.stultz@linaro.org> Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org> Tested-by: John Stultz <john.stultz@linaro.org> --- kernel/sched/fair.c | 4 ++++ 1 file changed, 4 insertions(+) -- 2.7.4