linux-kernel - Re: [PATCH] ftrace: Prevent RCU stall on PREEMPT

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Date:   Wed, 16 Nov 2022 15:36:20 +0900
From:   Masami Hiramatsu (Google) <mhiramat@...nel.org>
To:     "Guilherme G. Piccoli" <gpiccoli@...lia.com>
Cc:     linux-kernel@...r.kernel.org, rostedt@...dmis.org,
        rcu@...r.kernel.org, kernel-dev@...lia.com, kernel@...ccoli.net,
        "Paul E . McKenney" <paulmck@...nel.org>,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [PATCH] ftrace: Prevent RCU stall on PREEMPT_VOLUNTARY kernels

On Tue, 15 Nov 2022 17:48:47 -0300
"Guilherme G. Piccoli" <gpiccoli@...lia.com> wrote:

> The function match_records() may take a while due to a large
> number of string comparisons, so when in PREEMPT_VOLUNTARY
> kernels we could face RCU stalls due to that.
> 
> Add a cond_resched() to prevent that.
> 
> Suggested-by: Steven Rostedt <rostedt@...dmis.org>
> Acked-by: Paul E. McKenney <paulmck@...nel.org> # from RCU CPU stall warning perspective
> Cc: Masami Hiramatsu <mhiramat@...nel.org>
> Cc: Mark Rutland <mark.rutland@....com>
> Signed-off-by: Guilherme G. Piccoli <gpiccoli@...lia.com>

This looks good to me.

Acked-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>

Thanks!

> ---
> 
> Hi Steve / Paul, thanks for the discussions on the first thread [0],
> much appreciated! Here is the "official" version.
> 
> Steve: lemme know if it's good for you, and in case you prefer to
> send it yourself (since you proposed it on IRC), fine by me!
> 
> Paul: kept your ACK (thanks for that BTW) even though I changed the
> place of cond_resched() to align with Steve's preference. Lemme know
> in case you want to drop this ACK.
> 
> Cheers,
> 
> Guilherme
> 
> 
> [0] https://lore.kernel.org/lkml/1ef5fe19-a82f-835e-fda5-455e9c2b94b4@igalia.com/
> 
> 
>  kernel/trace/ftrace.c | 1 +
>  1 file changed, 1 insertion(+)
> 
> diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
> index 7dc023641bf1..80639bdb85f6 100644
> --- a/kernel/trace/ftrace.c
> +++ b/kernel/trace/ftrace.c
> @@ -4192,6 +4192,7 @@ match_records(struct ftrace_hash *hash, char *func, int len, char *mod)
>  			}
>  			found = 1;
>  		}
> +		cond_resched();
>  	} while_for_each_ftrace_rec();
>   out_unlock:
>  	mutex_unlock(&ftrace_lock);
> -- 
> 2.38.0
> 


-- 
Masami Hiramatsu (Google) <mhiramat@...nel.org>