linux-kernel - Re: [PATCH v2] sched,livepatch: Untangle cond

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Message-ID: <20250513140310.GA25639@noisy.programming.kicks-ass.net>
Date: Tue, 13 May 2025 16:03:10 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Miroslav Benes <mbenes@...e.cz>
Cc: Sebastian Andrzej Siewior <bigeasy@...utronix.de>,
	linux-kernel@...r.kernel.org, live-patching@...r.kernel.org,
	Josh Poimboeuf <jpoimboe@...hat.com>, mingo@...nel.com,
	juri.lelli@...hat.com, vincent.guittot@...aro.org,
	dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
	mgorman@...e.de, vschneid@...hat.com, jpoimboe@...nel.org,
	jikos@...nel.org, pmladek@...e.com, joe.lawrence@...hat.com,
	Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2] sched,livepatch: Untangle cond_resched() and
 live-patching

On Tue, May 13, 2025 at 03:34:50PM +0200, Miroslav Benes wrote:
> Hi,
> 
> thanks for the updated version.
> 
> On Fri, 9 May 2025, Sebastian Andrzej Siewior wrote:
> 
> > From: Peter Zijlstra <peterz@...radead.org>
> > 
> > With the goal of deprecating / removing VOLUNTARY preempt, live-patch
> > needs to stop relying on cond_resched() to make forward progress.
> > 
> > Instead, rely on schedule() with TASK_FREEZABLE set. Just like
> > live-patching, the freezer needs to be able to stop tasks in a safe /
> > known state.
> > 
> > Compile tested only.
> 
> livepatch selftests pass and I also ran some more.
>  
> > [bigeasy: use likely() in __klp_sched_try_switch() and update comments]
> > 
> > Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
> > Signed-off-by: Sebastian Andrzej Siewior <bigeasy@...utronix.de>
> 
> Acked-by: Miroslav Benes <mbenes@...e.cz>
> 
> A nit below if there is an another version, otherwise Petr might fix it 
> when merging.

Petr or Peter?

That is, who are we expecting to merge this :-)

Anyway, I've zapped the line in my copy.

> > @@ -365,27 +356,20 @@ static bool klp_try_switch_task(struct task_struct *task)
> >  
> >  void __klp_sched_try_switch(void)
> >  {
> > +	/*
> > +	 * This function is called from __schedule() while a context switch is
> > +	 * about to happen. Preemption is already disabled and klp_mutex
> > +	 * can't be acquired.
> > +	 * Disabled preemption is used to prevent racing with other callers of
> > +	 * klp_try_switch_task(). Thanks to task_call_func() they won't be
> > +	 * able to switch to this task while it's running.
> > +	 */
> > +	lockdep_assert_preemption_disabled();
> > +
> > +	/* Make sure current didn't get patched */
> >       if (likely(!klp_patch_pending(current)))
> >                return;
> 
> This last comment is not precise. If !klp_patch_pending(), there is 
> nothing to do. Fast way out. So if it was up to me, I would remove the 
> line all together.
> 
> Miroslav