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

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Date:   Wed, 3 Aug 2022 10:42:06 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Ingo Molnar <mingo@...nel.org>
Cc:     Steven Rostedt <rostedt@...dmis.org>,
        Liu Song <liusong@...ux.alibaba.com>, mingo@...hat.com,
        peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org, dietmar.eggemann@....com,
        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

On Wed, Aug 03, 2022 at 07:14:52PM +0200, Ingo Molnar wrote:
> 
> * Paul E. McKenney <paulmck@...nel.org> wrote:
> 
> > There is an rcu_sysrq_start() and rcu_sysrq_end() to suppress this.  
> > These are invoked by __handle_sysrq().  The value of 
> > rcu_cpu_stall_suppress should be non-zero during the sysrq execution, and 
> > this should prevent RCU CPU stall warnings from being printed.
> > 
> > That said, the code currently does not support overlapping calls to the 
> > various functions that suppress RCU CPU stall warnings.  Except that the 
> > only other use in current mainline is rcu_panic(), which never 
> > unsuppresses.
> > 
> > So could you please check the value of rcu_cpu_stall_suppress? Just in 
> > case some other form of suppression was added somewhere that I missed?
> 
> So instead of supressing the (justified!) RCU stall messages, I'd much 
> rather we apply only the minimal locking necessary for this debug printout.
> 
> That should also solve the stall warnings as a side effect.

I am certainly with you in spirit!  If I recall correctly, the issue
that led to the current state was that there was no way to walk the
task list locklessly except under an RCU read-side critical section.
Yes, you can use get_task_struct(), but that only prevents that task
structure from being freed, not from being removed from the list.

Here is hoping that there is a better way to nail down a task while
RCU-pausing a task-list traversal.  Thoughts?

							Thanx, Paul