linux-kernel - Re: [PATCH tip/core/rcu 3/4] rcu-tasks: Make ksoftirqd provide RCU Tasks quiescent states

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Date:   Thu, 13 May 2021 15:54:17 +0900
From:   Masami Hiramatsu <mhiramat@...nel.org>
To:     "Paul E. McKenney" <paulmck@...nel.org>
Cc:     rcu@...r.kernel.org, linux-kernel@...r.kernel.org,
        kernel-team@...com, mingo@...nel.org, jiangshanlai@...il.com,
        akpm@...ux-foundation.org, mathieu.desnoyers@...icios.com,
        josh@...htriplett.org, tglx@...utronix.de, peterz@...radead.org,
        rostedt@...dmis.org, dhowells@...hat.com, edumazet@...gle.com,
        fweisbec@...il.com, oleg@...hat.com, joel@...lfernandes.org,
        Toke Høiland-Jørgensen <toke@...hat.com>,
        Masami Hiramatsu <mhiramat@...nel.org>
Subject: Re: [PATCH tip/core/rcu 3/4] rcu-tasks: Make ksoftirqd provide RCU
 Tasks quiescent states

Hi Paul,

On Wed, 12 May 2021 11:27:46 -0700
"Paul E. McKenney" <paulmck@...nel.org> wrote:

> Heavy networking load can cause a CPU to execute continuously and
> indefinitely within ksoftirqd, in which case there will be no voluntary
> task switches and thus no RCU-tasks quiescent states.  This commit
> therefore causes the exiting rcu_softirq_qs() to provide an RCU-tasks
> quiescent state.
> 
> This of course means that __do_softirq() and its callers cannot be
> invoked from within a tracing trampoline.

I would like to confirm that you mean "tracing trampoline" here is
the code on the trampoline buffer, not the handler code which is
invoked from the trampoline buffer but it is protected by preempt_disable(),
am I understand correctly?

Thank you,

> 
> Reported-by: Toke Høiland-Jørgensen <toke@...hat.com>
> Tested-by: Toke Høiland-Jørgensen <toke@...hat.com>
> Signed-off-by: Paul E. McKenney <paulmck@...nel.org>
> Cc: Steven Rostedt <rostedt@...dmis.org>
> Cc: Masami Hiramatsu <mhiramat@...nel.org>
> ---
>  kernel/rcu/tree.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/kernel/rcu/tree.c b/kernel/rcu/tree.c
> index 8e78b2430c16..f4daa4e60b14 100644
> --- a/kernel/rcu/tree.c
> +++ b/kernel/rcu/tree.c
> @@ -242,6 +242,7 @@ void rcu_softirq_qs(void)
>  {
>  	rcu_qs();
>  	rcu_preempt_deferred_qs(current);
> +	rcu_tasks_qs(current, false);
>  }
>  
>  /*
> -- 
> 2.31.1.189.g2e36527f23
> 


-- 
Masami Hiramatsu <mhiramat@...nel.org>