linux-kernel - Re: WARNING: CPU: 112 PID: 2041 at kernel/sched/sched.h:1453

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Date:   Fri, 30 Jul 2021 14:22:18 +0200
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Bruno Goncalves <bgoncalv@...hat.com>
Cc:     CKI Project <cki-project@...hat.com>, linux-kernel@...r.kernel.org,
        nathan@...nel.org, Memory Management <mm-qe@...hat.com>,
        linux-arm-kernel@...ts.infradead.org
Subject: Re: WARNING: CPU: 112 PID: 2041 at kernel/sched/sched.h:1453

On 29/07/2021 16:38, Dietmar Eggemann wrote:
> On 29/07/2021 14:36, Bruno Goncalves wrote:
>> On Wed, Jul 28, 2021 at 5:55 PM Dietmar Eggemann
>> <dietmar.eggemann@....com> wrote:
>>>
>>> On 28/07/2021 15:11, Bruno Goncalves wrote:
> 
> [...]
> 
>>> Can't reproduce it on my Juno (arm64) (slow-switching (scpi-cpufreq
>>> driver)).
>>
>> We seem to be able to reproduce this only on Ampere Altra machines,
>> specifically on mtjade and mtsnow cpus.
>>
>> # cpupower frequency-info
>> analyzing CPU 0:
>>   driver: cppc_cpufreq
>>   CPUs which run at the same hardware frequency: 0
>>   CPUs which need to have their frequency coordinated by software: 0
>>   maximum transition latency:  Cannot determine or is not supported.
>>   hardware limits: 1000 MHz - 2.80 GHz
>>   available cpufreq governors: conservative ondemand userspace
>> powersave performance schedutil
>>   current policy: frequency should be within 2.00 GHz and 2.80 GHz.
>>                   The governor "schedutil" may decide which speed to use
>>                   within this range.
>>   current CPU frequency: 1.55 GHz (asserted by call to hardware)
>>
>> # ps -eTo comm,pid,pri,class | grep sugov
>> sugov:0            1082 140 DLN
>> sugov:1            1085 140 DLN
>> ...
>> sugov:78           1319 140 DLN
>> sugov:79           1320 140 DLN
> 
> Thanks! In the meantime I got access to an Ampere Altra so I can try
> 5.14.0-rc1 later today.

The task causing this seem to be the new `cppc_fie` DL task introduced
by commit 1eb5dde674f5 "cpufreq: CPPC: Add support for frequency
invariance" in v5.14-rc1.

With `CONFIG_ACPI_CPPC_CPUFREQ_FIE=y` and schedutil cpufreq governor on
slow-switching system:

DL task curr=`sugov:X` makes p=`cppc_fie` migrate and since it is in
`non_contending` state, migrate_task_rq_dl() calls 

  sub_running_bw()->__sub_running_bw()->cpufreq_update_util()->
  rq_clock()->assert_clock_updated()

on p. 

Can you try this snippet? It should fix it.

--8<--

From: Dietmar Eggemann <dietmar.eggemann@....com>
Date: Fri, 30 Jul 2021 14:03:40 +0200
Subject: [PATCH] sched/deadline: Fix missing clock update in
 migrate_task_rq_dl()

Signed-off-by: Dietmar Eggemann <dietmar.eggemann@....com>
---
 kernel/sched/deadline.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index aaacd6cfd42f..4920f498492f 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1733,6 +1733,7 @@ static void migrate_task_rq_dl(struct task_struct *p, int new_cpu __maybe_unused
 	 */
 	raw_spin_rq_lock(rq);
 	if (p->dl.dl_non_contending) {
+		update_rq_clock(rq);
 		sub_running_bw(&p->dl, &rq->dl);
 		p->dl.dl_non_contending = 0;
 		/*
-- 
2.25.1