linux-kernel - Re: [PATCH v3 1/3] rcu/exp: Use NMI to get the backtrace of cpu_curr(other

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Date:   Wed, 3 Aug 2022 17:07:00 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>
Cc:     Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Juri Lelli <juri.lelli@...hat.com>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
        Daniel Bristot de Oliveira <bristot@...hat.com>,
        Valentin Schneider <vschneid@...hat.com>,
        linux-kernel@...r.kernel.org,
        Frederic Weisbecker <frederic@...nel.org>,
        Neeraj Upadhyay <quic_neeraju@...cinc.com>,
        Josh Triplett <josh@...htriplett.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Joel Fernandes <joel@...lfernandes.org>, rcu@...r.kernel.org
Subject: Re: [PATCH v3 1/3] rcu/exp: Use NMI to get the backtrace of
 cpu_curr(other_cpu) first

On Tue, Aug 02, 2022 at 10:06:00AM +0800, Leizhen (ThunderTown) wrote:
> 
> 
> On 2022/8/2 7:14, Paul E. McKenney wrote:
> > On Sat, Jul 30, 2022 at 06:23:28PM +0800, Zhen Lei wrote:
> >> The backtrace of cpu_curr(other_cpu) is unwinded based on the 'fp' saved
> >> during its last switch-out. For the most part, it's out of date. So try
> >> to use NMI to get the backtrace first, just like those functions in
> >> "tree_stall.h" did. Such as rcu_dump_cpu_stacks().
> >>
> >> Signed-off-by: Zhen Lei <thunder.leizhen@...wei.com>
> > 
> > Much better, thank you!
> > 
> >> ---
> >>  kernel/rcu/tree_exp.h | 3 ++-
> >>  1 file changed, 2 insertions(+), 1 deletion(-)
> >>
> >> diff --git a/kernel/rcu/tree_exp.h b/kernel/rcu/tree_exp.h
> >> index 0f70f62039a9090..21381697de23f0b 100644
> >> --- a/kernel/rcu/tree_exp.h
> >> +++ b/kernel/rcu/tree_exp.h
> >> @@ -665,7 +665,8 @@ static void synchronize_rcu_expedited_wait(void)
> >>  				mask = leaf_node_cpu_bit(rnp, cpu);
> >>  				if (!(READ_ONCE(rnp->expmask) & mask))
> >>  					continue;
> >> -				dump_cpu_task(cpu);
> >> +				if (!trigger_single_cpu_backtrace(cpu))
> >> +					dump_cpu_task(cpu);
> > 
> > But why not just leave this unchanged, rather than adding the call to
> > trigger_single_cpu_backtrace() in this patch and then removing it in
> > the next patch?
> 
> To make the patch clear and easy to describe. Otherwise, I need to
> give an additional description of it in the next patch, because I
> searched all dump_cpu_task(). This seems to make the next patch
> less simple.
> 
> Some of the patch sets I've seen have been done step by step like
> this. But I can't find it now.
> 
> On the other hand, this patch is a small fix. Earlier versions may
> only backport it, not the next cleanup patch.

You do have the option of doing a Cc to stable to control the backporting,
if that is a potential issue for you.

On the commit log, just say that the one use case already avoided doing
the trigger_single_cpu_backtrace(), and thus did not need to be updated.

So please resend the series, but without the undo/redo.  There would
thus be two patches rather than three, but there are plenty of other
things that need fixing anyway.

							Thanx, Paul

> >>  			}
> >>  		}
> >>  		jiffies_stall = 3 * rcu_exp_jiffies_till_stall_check() + 3;
> >> -- 
> >> 2.25.1
> >>
> > .
> > 
> 
> -- 
> Regards,
>   Zhen Lei