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

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Date:   Mon, 1 Aug 2022 16:14:15 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Zhen Lei <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 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 For the most part, it's out of date.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?

							Thanx, Paul

>  			}
>  		}
>  		jiffies_stall = 3 * rcu_exp_jiffies_till_stall_check() + 3;
> -- 
> 2.25.1
>