linux-kernel - Re: [for-next][PATCH 6/8] ftrace: Prevent RCU stall on PREEMPT

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Date:   Wed, 14 Dec 2022 11:17:42 -0500
From:   Steven Rostedt <rostedt@...dmis.org>
To:     "Guilherme G. Piccoli" <gpiccoli@...lia.com>
Cc:     linux-kernel@...r.kernel.org,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Mark Rutland <mark.rutland@....com>
Subject: Re: [for-next][PATCH 6/8] ftrace: Prevent RCU stall on
 PREEMPT_VOLUNTARY kernels

On Wed, 14 Dec 2022 11:15:44 -0500
Steven Rostedt <rostedt@...dmis.org> wrote:

> On Wed, 14 Dec 2022 12:53:04 -0300
> "Guilherme G. Piccoli" <gpiccoli@...lia.com> wrote:
> 
> > On 14/12/2022 11:01, Steven Rostedt wrote:  
> > > From: "gpiccoli@...lia.com" <gpiccoli@...lia.com>    
> > 
> > Hi Steve, would be possible to use my full name here, like: Guilherme G.
> > Piccoli <gpiccoli@...lia.com> ?  
> 
> Sure, I haven't pushed to the next repo yet.
> 
> I found the reason it didn't go into my patchwork, and did a resend via a
> "redirect", and that must have changed the from address :-/
> 

Here's the new update:

-- Steve


>From d0b24b4e91fcb8408c4979888547f86be514e337 Mon Sep 17 00:00:00 2001
From: "Guilherme G. Piccoli" <gpiccoli@...lia.com>
Date: Tue, 15 Nov 2022 17:48:47 -0300
Subject: [PATCH] ftrace: Prevent RCU stall on PREEMPT_VOLUNTARY kernels

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.

Link: https://lkml.kernel.org/r/20221115204847.593616-1-gpiccoli@igalia.com

Cc: Mark Rutland <mark.rutland@....com>
Suggested-by: Steven Rostedt <rostedt@...dmis.org>
Acked-by: Paul E. McKenney <paulmck@...nel.org> # from RCU CPU stall warning perspective
Signed-off-by: Guilherme G. Piccoli <gpiccoli@...lia.com>
Acked-by: Masami Hiramatsu (Google) <mhiramat@...nel.org>
Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
---
 kernel/trace/ftrace.c | 1 +
 1 file changed, 1 insertion(+)

diff --git a/kernel/trace/ftrace.c b/kernel/trace/ftrace.c
index d04552c0c275..b8e374a372e5 100644
--- a/kernel/trace/ftrace.c
+++ b/kernel/trace/ftrace.c
@@ -4204,6 +4204,7 @@ 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);
-- 
2.35.1