linux-kernel - Re: [PATCH v2] x86/alternatives: Add cond_resched() to text_poke_bp

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Message-Id: <20230601134806.a1c9eab11d0bbdbb1379f6c4@kernel.org>
Date:   Thu, 1 Jun 2023 13:48:06 +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>,
        Peter Zijlstra <peterz@...radead.org>,
        Borislav Petkov <bp@...en8.de>
Subject: Re: [PATCH v2] x86/alternatives: Add cond_resched() to
 text_poke_bp_batch()

On Wed, 31 May 2023 09:24:19 -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.
> 
> When function tracing enables all functions, it will call
> text_poke_queue() to queue the places that need to be patched. Every
> 256 entries will do a "flush" that calls text_poke_bp_batch() to do the
> update of the 256 locations. As this is in a scheduleable context,
> calling cond_resched() at the start of text_poke_bp_batch() will ensure
> that other tasks could get a chance to run while the patching is
> happening. This keeps the softlockup from triggering in the start up
> tests.
> 

Looks good to me.

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

Thank you!

> Signed-off-by: Steven Rostedt (Google) <rostedt@...dmis.org>
> ---
> Changes since v1: https://lkml.kernel.org/r/20230528084652.5f3b48f0@rorschach.local.home
> 
>  - Just call cond_resched() once in text_poke_bp_batch() and not for
>    each phase, as it only needs to be called once every 256 entries.
> 
>  arch/x86/kernel/alternative.c | 10 ++++++++++
>  1 file changed, 10 insertions(+)
> 
> diff --git a/arch/x86/kernel/alternative.c b/arch/x86/kernel/alternative.c
> index f615e0cb6d93..412ad66cd240 100644
> --- a/arch/x86/kernel/alternative.c
> +++ b/arch/x86/kernel/alternative.c
> @@ -1953,6 +1953,16 @@ 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.
> +	 * This function gets called every 256 entries added to be patched.
> +	 * Call cond_resched() here to make sure that other tasks can get scheduled
> +	 * while processing all the functions being patched.
> +	 */
> +	cond_resched();
> +
>  	/*
>  	 * Corresponding read barrier in int3 notifier for making sure the
>  	 * nr_entries and handler are correctly ordered wrt. patching.
> -- 
> 2.39.2
> 


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