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Subject: Re: [PATCH v8 -tip 02/26] sched: Introduce sched_class::pick_task()
To:     "Li, Aubrey" <aubrey.li@linux.intel.com>,
        Joel Fernandes <joel@joelfernandes.org>
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From:   Vineeth Pillai <viremana@linux.microsoft.com>
Message-ID: <4241e5ac-ecdf-8634-fa0d-dd6759e477e1@linux.microsoft.com>
Date:   Sat, 24 Oct 2020 07:10:26 -0400
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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