linux-kernel - Re: [RFC][PATCH 03/16] sched: Wrap rq::lock access

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Date:   Fri, 22 Mar 2019 16:44:22 -0700
From:   Tim Chen <tim.c.chen@...ux.intel.com>
To:     Subhra Mazumdar <subhra.mazumdar@...cle.com>,
        Julien Desfossez <jdesfossez@...italocean.com>,
        Peter Zijlstra <peterz@...radead.org>, mingo@...nel.org,
        tglx@...utronix.de, pjt@...gle.com, torvalds@...ux-foundation.org
Cc:     linux-kernel@...r.kernel.org, fweisbec@...il.com,
        keescook@...omium.org, kerrnel@...gle.com,
        Vineeth Pillai <vpillai@...italocean.com>,
        Nishanth Aravamudan <naravamudan@...italocean.com>,
        Pawan Gupta <pawan.kumar.gupta@...ux.intel.com>,
        Aubrey <aubrey.li@...el.com>
Subject: Re: [RFC][PATCH 03/16] sched: Wrap rq::lock access

On 3/22/19 4:28 PM, Tim Chen wrote:
> On 3/19/19 7:29 PM, Subhra Mazumdar wrote:
>>
>> On 3/18/19 8:41 AM, Julien Desfossez wrote:
>>> The case where we try to acquire the lock on 2 runqueues belonging to 2
>>> different cores requires the rq_lockp wrapper as well otherwise we
>>> frequently deadlock in there.
>>>
>>> This fixes the crash reported in
>>> 1552577311-8218-1-git-send-email-jdesfossez@...italocean.com
>>>
>>> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
>>> index 76fee56..71bb71f 100644
>>> --- a/kernel/sched/sched.h
>>> +++ b/kernel/sched/sched.h
>>> @@ -2078,7 +2078,7 @@ static inline void double_rq_lock(struct rq *rq1, struct rq *rq2)
>>>           raw_spin_lock(rq_lockp(rq1));
>>>           __acquire(rq2->lock);    /* Fake it out ;) */
>>>       } else {
>>> -        if (rq1 < rq2) {
>>> +        if (rq_lockp(rq1) < rq_lockp(rq2)) {
>>>               raw_spin_lock(rq_lockp(rq1));
>>>               raw_spin_lock_nested(rq_lockp(rq2), SINGLE_DEPTH_NESTING);
>>>           } else {
> 
> 
> Pawan was seeing occasional crashes and lock up that's avoided by doing the following.
> We're trying to dig a little more tracing to see why pick_next_entity is returning
> NULL.
> 

We found the root cause was a missing chunk when we port Subhra's fix of pick_next_entity

         * 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) 

That fixes the problem of pick_next_entity returning NULL.  sorry for the noise.

Tim