Re: [PATCH] ftrace: Prevent RCU stall on PREEMPT_VOLUNTARY kernels

From: Google
Date: Wed Nov 16 2022 - 01:37:50 EST


On Tue, 15 Nov 2022 17:48:47 -0300
"Guilherme G. Piccoli" <gpiccoli@xxxxxxxxxx> 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@xxxxxxxxxxx>
> Acked-by: Paul E. McKenney <paulmck@xxxxxxxxxx> # from RCU CPU stall warning perspective
> Cc: Masami Hiramatsu <mhiramat@xxxxxxxxxx>
> Cc: Mark Rutland <mark.rutland@xxxxxxx>
> Signed-off-by: Guilherme G. Piccoli <gpiccoli@xxxxxxxxxx>

This looks good to me.

Acked-by: Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>

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@xxxxxxxxxx/
>
>
> 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@xxxxxxxxxx>