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Subject: Re: [RFC][PATCH 00/16] sched: Core scheduling
From:   Subhra Mazumdar <subhra.mazumdar@oracle.com>
To:     Aubrey Li <aubrey.intel@gmail.com>
Cc:     Mel Gorman <mgorman@techsingularity.net>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@kernel.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Paul Turner <pjt@google.com>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Fr?d?ric Weisbecker <fweisbec@gmail.com>,
        Kees Cook <keescook@chromium.org>,
        Greg Kerr <kerrnel@google.com>
References: <20190218165620.383905466@infradead.org>
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 <20190222124544.GY9565@techsingularity.net>
 <14a9adf7-9b50-1dfa-0c35-d04e976081c2@oracle.com>
 <CAERHkru9DzvszvdGHWp8PhEAqeUc-wVR1Gey-KtQ6aMRLx68bQ@mail.gmail.com>
 <19d5d492-a4c1-b3c8-cae4-da2fdfcb872b@oracle.com>
Message-ID: <ac10b8fd-da1a-2cc2-afee-4e2b9b4278ee@oracle.com>
Date:   Mon, 11 Mar 2019 16:33:22 -0700
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On 3/11/19 11:34 AM, Subhra Mazumdar wrote:
>
> On 3/10/19 9:23 PM, Aubrey Li wrote:
>> On Sat, Mar 9, 2019 at 3:50 AM Subhra Mazumdar
>> <subhra.mazumdar@oracle.com> wrote:
>>> expected. Most of the performance recovery happens in patch 15 which,
>>> unfortunately, is also the one that introduces the hard lockup.
>>>
>> After applied Subhra's patch, the following is triggered by enabling
>> core sched when a cgroup is
>> under heavy load.
>>
> It seems you are facing some other deadlock where printk is involved. 
> Can you
> drop the last patch (patch 16 sched: Debug bits...) and try?
>
> Thanks,
> Subhra
>
Never Mind, I am seeing the same lockdep deadlock output even w/o patch 
16. Btw
the NULL fix had something missing, following works.

--------->8------------

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 1d0dac4..27cbc64 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -4131,7 +4131,7 @@ pick_next_entity(struct cfs_rq *cfs_rq, struct 
sched_entity *curr)
          * Avoid running the skip buddy, if running something else can
          * be done without getting too unfair.
*/
-       if (cfs_rq->skip == se) {
+       if (cfs_rq->skip && cfs_rq->skip == se) {
                 struct sched_entity *second;

                 if (se == curr) {
@@ -4149,13 +4149,15 @@ pick_next_entity(struct cfs_rq *cfs_rq, struct 
sched_entity *curr)
/*
          * Prefer last buddy, try to return the CPU to a preempted task.
*/
-       if (cfs_rq->last && wakeup_preempt_entity(cfs_rq->last, left) < 1)
+       if (left && cfs_rq->last && wakeup_preempt_entity(cfs_rq->last, 
left)
+           < 1)
                 se = cfs_rq->last;

/*
          * Someone really wants this to run. If it's not unfair, run it.
*/
-       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)
                 se = cfs_rq->next;

         clear_buddies(cfs_rq, se);
@@ -6958,6 +6960,9 @@ pick_task_fair(struct rq *rq)

                 se = pick_next_entity(cfs_rq, NULL);

+               if (!(se || curr))
+                       return NULL;
+
                 if (curr) {
                         if (se && curr->on_rq)
update_curr(cfs_rq);