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Subject: Re: [PATCH v8 -tip 02/26] sched: Introduce sched_class::pick_task()
To:     Joel Fernandes <joel@joelfernandes.org>
Cc:     Nishanth Aravamudan <naravamudan@digitalocean.com>,
        Julien Desfossez <jdesfossez@digitalocean.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Tim Chen <tim.c.chen@linux.intel.com>,
        Vineeth Pillai <viremana@linux.microsoft.com>,
        Aaron Lu <aaron.lwe@gmail.com>,
        Aubrey Li <aubrey.intel@gmail.com>,
        Thomas Glexiner <tglx@linutronix.de>,
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        Ingo Molnar <mingo@kernel.org>,
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        Mel Gorman <mgorman@techsingularity.net>,
        Pawan Gupta <pawan.kumar.gupta@linux.intel.com>,
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        Steven Rostedt <rostedt@goodmis.org>,
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 <20201023214702.GA3603399@google.com>
From:   "Li, Aubrey" <aubrey.li@linux.intel.com>
Message-ID: <d9a70e58-f424-3957-9957-b89071480005@linux.intel.com>
Date:   Sat, 24 Oct 2020 10:48:27 +0800
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On 2020/10/24 5:47, Joel Fernandes wrote:
> On Fri, Oct 23, 2020 at 01:25:38PM +0800, Li, Aubrey wrote:
>>>>> @@ -2517,6 +2528,7 @@ const struct sched_class dl_sched_class
>>>>>
>>>>>  #ifdef CONFIG_SMP
>>>>>       .balance                = balance_dl,
>>>>> +     .pick_task              = pick_task_dl,
>>>>>       .select_task_rq         = select_task_rq_dl,
>>>>>       .migrate_task_rq        = migrate_task_rq_dl,
>>>>>       .set_cpus_allowed       = set_cpus_allowed_dl,
>>>>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>>>>> index dbd9368a959d..bd6aed63f5e3 100644
>>>>> --- a/kernel/sched/fair.c
>>>>> +++ b/kernel/sched/fair.c
>>>>> @@ -4450,7 +4450,7 @@ 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) {
>>>>> @@ -6976,6 +6976,35 @@ static void check_preempt_wakeup(struct rq *rq, struct task_struct *p, int wake_
>>>>>               set_last_buddy(se);
>>>>>  }
>>>>>
>>>>> +#ifdef CONFIG_SMP
>>>>> +static struct task_struct *pick_task_fair(struct rq *rq)
>>>>> +{
>>>>> +     struct cfs_rq *cfs_rq = &rq->cfs;
>>>>> +     struct sched_entity *se;
>>>>> +
>>>>> +     if (!cfs_rq->nr_running)
>>>>> +             return NULL;
>>>>> +
>>>>> +     do {
>>>>> +             struct sched_entity *curr = cfs_rq->curr;
>>>>> +
>>>>> +             se = pick_next_entity(cfs_rq, NULL);
>>>>> +
>>>>> +             if (curr) {
>>>>> +                     if (se && curr->on_rq)
>>>>> +                             update_curr(cfs_rq);
>>>>> +
>>>>> +                     if (!se || entity_before(curr, se))
>>>>> +                             se = curr;
>>>>> +             }
>>>>> +
>>>>> +             cfs_rq = group_cfs_rq(se);
>>>>> +     } while (cfs_rq);
>>>>> ++
>>>>> +     return task_of(se);
>>>>> +}
>>>>> +#endif
>>>>
>>>> One of my machines hangs when I run uperf with only one message:
>>>> [  719.034962] BUG: kernel NULL pointer dereference, address: 0000000000000050
>>>>
>>>> Then I replicated the problem on my another machine(no serial console),
>>>> here is the stack by manual copy.
>>>>
>>>> Call Trace:
>>>>  pick_next_entity+0xb0/0x160
>>>>  pick_task_fair+0x4b/0x90
>>>>  __schedule+0x59b/0x12f0
>>>>  schedule_idle+0x1e/0x40
>>>>  do_idle+0x193/0x2d0
>>>>  cpu_startup_entry+0x19/0x20
>>>>  start_secondary+0x110/0x150
>>>>  secondary_startup_64_no_verify+0xa6/0xab
>>>
>>> Interesting. Wondering if we screwed something up in the rebase.
>>>
>>> Questions:
>>> 1. Does the issue happen if you just apply only up until this patch,
>>> or the entire series?
>>
>> I applied the entire series and just find a related patch to report the
>> issue.
> 
> Ok.
> 
>>> 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.

Thanks,
-Aubrey