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

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Date:   Wed, 3 Aug 2022 10:39:47 +0200
From:   Ingo Molnar <mingo@...nel.org>
To:     Liu Song <liusong@...ux.alibaba.com>
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) {

... 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