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Sender: Ingo Molnar <mingo.kernel.org@gmail.com>
Date:   Wed, 3 Aug 2022 11:10:17 +0200
From:   Ingo Molnar <mingo@kernel.org>
To:     Liu Song <liusong@linux.alibaba.com>
Cc:     mingo@redhat.com, peterz@infradead.org, juri.lelli@redhat.com,
        vincent.guittot@linaro.org, dietmar.eggemann@arm.com,
        rostedt@goodmis.org, bsegall@google.com, mgorman@suse.de,
        bristot@redhat.com, vschneid@redhat.com,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH] sched/debug: avoid executing show_state and causing rcu
 stall warning
Message-ID: <Yuo7eYzQnzqx4u7F@gmail.com>
References: <1659489525-82994-1-git-send-email-liusong@linux.alibaba.com>
 <Yuo0U2aWUZRLBAsA@gmail.com>
 <c4058896-947d-c802-ac57-bf03eb986378@linux.alibaba.com>
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* Liu Song <liusong@linux.alibaba.com> wrote:

> 
> > * Liu Song <liusong@linux.alibaba.com> wrote:
> > 
> > > From: Liu Song <liusong@linux.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