Re: [PATCH v8 -tip 02/26] sched: Introduce sched_class::pick_task()

[Vineeth Pillai]

Hi Aubrey,


On 10/23/20 10:48 PM, Li, Aubrey wrote:
> > > > 2. Do you see the issue in v7? Not much if at all has changed in this
> > > > part of the code from v7 -> v8 but could be something in the newer
> > > > kernel.
> > > IIRC, I can run uperf successfully on v7.
> > > I'm on tip/master 2d3e8c9424c9 (origin/master) "Merge branch 'linus'."
> > > Please let me know if this is a problem, or you have a repo I can pull
> > > for testing.
> > Here is a repo with v8 series on top of v5.9 release:
> > https://git.kernel.org/pub/scm/linux/kernel/git/jfern/linux.git/log/?h=coresched-v5.9
> I didn't see NULL pointer dereference BUG of this repo, will post performance
> data later.
There has been a change in tip in pick_next_entity which caused
removal of a coresched related change to fix the crash. Could
you please try this patch on top of the posted v8 and see if this
works?

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 93a3b874077d..4cae5ac48b60 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4428,12 +4428,14 @@ pick_next_entity(struct cfs_rq *cfs_rq, struct sched_entity *curr)
                        se = second;
        }

-       if (cfs_rq->next && wakeup_preempt_entity(cfs_rq->next, left) < 1) {
+       if (left && cfs_rq->next &&
+                       wakeup_preempt_entity(cfs_rq->next, left) < 1) {
                /*
                 * Someone really wants this to run. If it's not unfair, run it.
                 */
                se = cfs_rq->next;
-       } else if (cfs_rq->last && wakeup_preempt_entity(cfs_rq->last, left) < 1) {
+       } else if (left && cfs_rq->last &&
+                       wakeup_preempt_entity(cfs_rq->last, left) < 1) {
                /*
                 * Prefer last buddy, try to return the CPU to a preempted task.


There reason for left being NULL needs to be investigated. This was
there from v1 and we did not yet get to it. I shall try to debug later
this week.

Kindly test this patch and let us know if it fixes the crash.

Thanks,
Vineeth