linux-kernel - Re: [PATCH] x86/alternatives: Add cond_resched() to text_poke_bp

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Message-Id: <20230529115246.a61734ce4e6d7644e2faec72@kernel.org>
Date:   Mon, 29 May 2023 11:52:46 +0900
From:   Masami Hiramatsu (Google) <mhiramat@...nel.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     LKML <linux-kernel@...r.kernel.org>, x86@...nel.org,
        Masami Hiramatsu <mhiramat@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
        Peter Zijlstra <peterz@...radead.org>
Subject: Re: [PATCH] x86/alternatives: Add cond_resched() to
 text_poke_bp_batch()

On Sun, 28 May 2023 08:46:52 -0400
Steven Rostedt <rostedt@...dmis.org> wrote:

> From: "Steven Rostedt (Google)" <rostedt@...dmis.org>
> 
> Debugging in the kernel has started slowing down the kernel by a
> noticeable amount. The ftrace start up tests are triggering the softlockup
> watchdog on some boxes. This is caused by the start up tests that enable
> function and function graph tracing several times. Sprinkling
> cond_resched() just in the start up test code was not enough to stop the
> softlockup from triggering. It would sometimes trigger in the
> text_poke_bp_batch() code.
> 
> The text_poke_bp_batch() is run in schedulable context. Add
> cond_resched() between each phase (adding the int3, updating the code, and
> removing the int3). This keeps the softlockup from triggering in the start
> up tests.
> 
> Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
> ---
>  arch/x86/kernel/alternative.c | 13 ++++++++++++-
>  1 file changed, 12 insertions(+), 1 deletion(-)
> 
> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> index f615e0cb6d93..e024eddd457f 100644
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -1953,6 +1953,14 @@ static void text_poke_bp_batch(struct text_poke_loc *tp, unsigned int nr_entries
>  	 */
>  	atomic_set_release(&bp_desc.refs, 1);
>  
> +	/*
> +	 * Function tracing can enable thousands of places that need to be
> +	 * updated. This can take quite some time, and with full kernel debugging
> +	 * enabled, this could cause the softlockup watchdog to trigger.
> +	 * Add cond_resched() calls to each phase.
> +	 */
> +	cond_resched();

Hmm, why don't you put this between the first step (put int3) and the
second step (put other bytes)? I guess those would takes more time.

Thank you,

> +
>  	/*
>  	 * Corresponding read barrier in int3 notifier for making sure the
>  	 * nr_entries and handler are correctly ordered wrt. patching.
> @@ -2030,6 +2038,7 @@ static void text_poke_bp_batch(struct text_poke_loc *tp, unsigned int nr_entries
>  		 * better safe than sorry (plus there's not only Intel).
>  		 */
>  		text_poke_sync();
> +		cond_resched();
>  	}
>  
>  	/*
> @@ -2049,8 +2058,10 @@ static void text_poke_bp_batch(struct text_poke_loc *tp, unsigned int nr_entries
>  		do_sync++;
>  	}
>  
> -	if (do_sync)
> +	if (do_sync) {
>  		text_poke_sync();
> +		cond_resched();
> +	}
>  
>  	/*
>  	 * Remove and wait for refs to be zero.
> -- 
> 2.39.2
> 


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