linux-kernel - Re: [PATCH] sched/debug: avoid executing show

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Message-ID: <c4058896-947d-c802-ac57-bf03eb986378@linux.alibaba.com>
Date:   Wed, 3 Aug 2022 16:58:59 +0800
From:   Liu Song <liusong@...ux.alibaba.com>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        bristot@...hat.com, vschneid@...hat.com,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] sched/debug: avoid executing show_state and causing rcu
 stall warning


> * Liu Song <liusong@...ux.alibaba.com> wrote:
>
>> From: Liu Song <liusong@...ux.alibaba.com>
>>
>> If the number of CPUs is large, "sysrq_sched_debug_show" will execute for
>> a long time. Every time I execute "echo t > /proc/sysrq-trigger" on my
>> 128-core machine, the rcu stall warning will be triggered. Moreover,
>> sysrq_sched_debug_show does not need to be protected by rcu_read_lock,
>    ^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^^
>> and no rcu stall warning will appear after adjustment.
>>
> That doesn't mean it doesn't have to be protected by *any* lock - which
> your patch implements AFAICS.
>
> There's a couple of lines such as:
>
>          for_each_online_cpu(cpu) {

Hi,

Here I refer to the implementation of "sysrq_timer_list_show", and I 
don't see any lock.

Maybe there is a problem with the implementation of "sysrq_timer_list_show".


Thanks

>
> ... which need to be protected against CPU hotplug events.
>
> I haven't checked any of the deeper code to see what RCU or other
> protection it may need, but clearly you didn't either ...
>
> Thanks,
>
> 	Ingo