linux-kernel - Re: [RFC][PATCH 12/13] sched/deadline: Introduce deadline servers

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Date:   Thu, 8 Aug 2019 09:52:14 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Juri Lelli <juri.lelli@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>, mingo@...nel.org,
        linux-kernel@...r.kernel.org, luca.abeni@...tannapisa.it,
        bristot@...hat.com, balsini@...roid.com, dvyukov@...gle.com,
        tglx@...utronix.de, vpillai@...italocean.com, rostedt@...dmis.org
Subject: Re: [RFC][PATCH 12/13] sched/deadline: Introduce deadline servers

On 8/8/19 8:52 AM, Juri Lelli wrote:
> Hi Dietmar,
> 
> On 07/08/19 18:31, Dietmar Eggemann wrote:
>> On 7/26/19 4:54 PM, Peter Zijlstra wrote:
>>>
>>>
>>> Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
>>
>> [...]
>>
>>> @@ -889,6 +891,8 @@ static void update_curr(struct cfs_rq *c
>>>  		trace_sched_stat_runtime(curtask, delta_exec, curr->vruntime);
>>>  		cgroup_account_cputime(curtask, delta_exec);
>>>  		account_group_exec_runtime(curtask, delta_exec);
>>> +		if (curtask->server)
>>> +			dl_server_update(curtask->server, delta_exec);
>>>  	}
>>
>> I get a lockdep_assert_held(&rq->lock) related warning in start_dl_timer()
>> when running the full stack.
>>
>> ...
>> [    0.530216] root domain span: 0-5 (max cpu_capacity = 1024)
>> [    0.538655] devtmpfs: initialized
>> [    0.556485] update_curr: rq mismatch rq[0] != rq[4]
>> [    0.561519] update_curr: rq mismatch rq[0] != rq[4]
>> [    0.566497] update_curr: rq mismatch rq[0] != rq[4]
>> [    0.571443] update_curr: rq mismatch rq[0] != rq[4]
>> [    0.576762] update_curr: rq mismatch rq[2] != rq[4]
>> [    0.581674] update_curr: rq mismatch rq[2] != rq[4]
>> [    0.586569] ------------[ cut here ]------------
>> [    0.591220] WARNING: CPU: 2 PID: 2 at kernel/sched/deadline.c:916 start_dl_timer+0x160/0x178
>> [    0.599686] Modules linked in:
>> [    0.602756] CPU: 2 PID: 2 Comm: kthreadd Tainted: G        W         5.3.0-rc3-00013-ga33cf033cc99-dirty #64
>> [    0.612620] Hardware name: ARM Juno development board (r0) (DT)
>> [    0.618560] pstate: 60000085 (nZCv daIf -PAN -UAO)
>> [    0.623369] pc : start_dl_timer+0x160/0x178
>> [    0.627572] lr : start_dl_timer+0x160/0x178
>> [    0.631768] sp : ffff000010013cb0
>> ...
>> [    0.715075] Call trace:
>> [    0.717531]  start_dl_timer+0x160/0x178
>> [    0.721382]  update_curr_dl_se+0x108/0x208
>> [    0.725494]  dl_server_update+0x2c/0x38
>> [    0.729348]  update_curr+0x1b4/0x3b8
>> [    0.732934]  task_tick_fair+0x74/0xa88
>> [    0.736698]  scheduler_tick+0x94/0x110
>> [    0.740461]  update_process_times+0x48/0x60
>> ...
>>
>> Seems to be related to the fact that the rq can change:
>>
>> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
>> index e4c14851a34c..5e3130a200ec 100644
>> --- a/kernel/sched/fair.c
>> +++ b/kernel/sched/fair.c
>> @@ -891,8 +891,17 @@ static void update_curr(struct cfs_rq *cfs_rq)
>>                 trace_sched_stat_runtime(curtask, delta_exec, curr->vruntime);
>>                 cgroup_account_cputime(curtask, delta_exec);
>>                 account_group_exec_runtime(curtask, delta_exec);
>> -               if (curtask->server)
>> +               if (curtask->server) {
>> +                       struct rq *rq = rq_of(cfs_rq);
>> +                       struct rq *rq2 = curtask->server->rq;
>> +
>> +                       if (rq != rq2) {
>> +                               printk("update_curr: rq mismatch rq[%d] != rq[%d]\n",
>> +                                      cpu_of(rq), cpu_of(rq2));
>> +                       }
>> +
>>                         dl_server_update(curtask->server, delta_exec);
>> +               }
>>         }
>>
>> ...
> 
> Yeah, I actually noticed the same. Some debugging seems to point to
> early boot spawning of kthreads. I can reliably for example attribute
> this mismatch to ksoftirqd(s). It looks like they can avoid the
> dl_server assignment in pick_next_task_dl() and this breaks things.
> Still need to figure out why this happens and how to fix it, though.

Yeah, can confirm this:

...
[0.556941] update_curr: rq mismatch rq[0] != rq[4] curr=[kthreadd, 39]
[0.563722] update_curr: rq mismatch rq[0] != rq[4] curr=[netns, 39]
[0.570179] update_curr: rq mismatch rq[0] != rq[4] curr=[netns, 39]
[0.576606] update_curr: rq mismatch rq[0] != rq[4] curr=[netns, 39]
[0.583708] update_curr: rq mismatch rq[2] != rq[1] curr=[kworker/2:1, 40]
[0.590793] update_curr: rq mismatch rq[2] != rq[1] curr=[kworker/2:1, 40]
[0.615096] update_curr: rq mismatch rq[2] != rq[1] curr=[kworker/2:1, 40]
...
[0.626644] WARNING: CPU: 2 PID: 40 at kernel/sched/deadline.c:916 start_dl_timer+0x160/0x178
...