linux-kernel - Re: [PATCH v8 -tip 02/26] sched: Introduce sched_class::pick

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Message-ID: <4241e5ac-ecdf-8634-fa0d-dd6759e477e1@linux.microsoft.com>
Date:   Sat, 24 Oct 2020 07:10:26 -0400
From:   Vineeth Pillai <viremana@...ux.microsoft.com>
To:     "Li, Aubrey" <aubrey.li@...ux.intel.com>,
        Joel Fernandes <joel@...lfernandes.org>
Cc:     Nishanth Aravamudan <naravamudan@...italocean.com>,
        Julien Desfossez <jdesfossez@...italocean.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>,
        Aaron Lu <aaron.lwe@...il.com>,
        Aubrey Li <aubrey.intel@...il.com>,
        Thomas Glexiner <tglx@...utronix.de>,
        LKML <linux-kernel@...r.kernel.org>,
        Ingo Molnar <mingo@...nel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Frederic Weisbecker <fweisbec@...il.com>,
        Kees Cook <keescook@...omium.org>,
        Greg Kerr <kerrnel@...gle.com>, Phil Auld <pauld@...hat.com>,
        Valentin Schneider <valentin.schneider@....com>,
        Mel Gorman <mgorman@...hsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Paolo Bonzini <pbonzini@...hat.com>, vineeth@...byteword.org,
        Chen Yu <yu.c.chen@...el.com>,
        Christian Brauner <christian.brauner@...ntu.com>,
        Agata Gruza <agata.gruza@...el.com>,
        Antonio Gomez Iglesias <antonio.gomez.iglesias@...el.com>,
        graf@...zon.com, konrad.wilk@...cle.com,
        Dario Faggioli <dfaggioli@...e.com>,
        Paul Turner <pjt@...gle.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Patrick Bellasi <derkling@...gle.com>,
        benbjiang(蒋彪) <benbjiang@...cent.com>,
        Alexandre Chartre <alexandre.chartre@...cle.com>,
        James.Bottomley@...senpartnership.com, OWeisse@...ch.edu,
        Dhaval Giani <dhaval.giani@...cle.com>,
        Junaid Shahid <junaids@...gle.com>,
        Jesse Barnes <jsbarnes@...gle.com>,
        "Hyser,Chris" <chris.hyser@...cle.com>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        Tim Chen <tim.c.chen@...el.com>,
        "Ning, Hongyu" <hongyu.ning@...ux.intel.com>
Subject: Re: [PATCH v8 -tip 02/26] sched: Introduce sched_class::pick_task()

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