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

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Date:   Tue, 2 Aug 2022 10:06:00 +0800
From:   "Leizhen (ThunderTown)" <thunder.leizhen@...wei.com>
To:     <paulmck@...nel.org>
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 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 For the most part, it's out of date.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.

> 
> 							Thanx, Paul
> 
>>  			}
>>  		}
>>  		jiffies_stall = 3 * rcu_exp_jiffies_till_stall_check() + 3;
>> -- 
>> 2.25.1
>>
> .
> 

-- 
Regards,
  Zhen Lei