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

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Date:   Wed, 3 Aug 2022 11:10:17 +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:

> 
> > * 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".

But we are talking about sysrq_sched_debug_show(), which your patch tries 
to relax the RCU locking of.

Thanks,

	Ingo