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

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Message-ID: <20230530083553.6322240f@rorschach.local.home>
Date:   Tue, 30 May 2023 08:35:53 -0400
From:   Steven Rostedt <rostedt@...dmis.org>
To:     Peter Zijlstra <peterz@...radead.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>
Subject: Re: [PATCH] x86/alternatives: Add cond_resched() to
 text_poke_bp_batch()

On Tue, 30 May 2023 14:01:48 +0200
Peter Zijlstra <peterz@...radead.org> wrote:

> On Sun, May 28, 2023 at 08:46:52AM -0400, Steven Rostedt wrote:
> > 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();  
> 
> But but but... you can only have TP_VEC_MAX pokes queued, which is 256
> on normal setups.
> 
> Please explain how this leads to problems and why you need _3_
> reschedule points here.

Maybe this was me being overly paranoid (and thinking, it doesn't hurt).

I could try it with just adding one (to make sure it happens every
flush), and see if it doesn't trigger the softlock up.

-- Steve