linux-kernel - Re: [PATCH v1 02/25] mm/swap: Don't abuse the seqcount latching API

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Date:   Mon, 25 May 2020 17:24:01 +0200
From:   "Ahmed S. Darwish" <a.darwish@...utronix.de>
To:     Peter Zijlstra <peterz@...radead.org>
Cc:     Ingo Molnar <mingo@...hat.com>, Will Deacon <will@...nel.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        "Paul E. McKenney" <paulmck@...nel.org>,
        "Sebastian A. Siewior" <bigeasy@...utronix.de>,
        Steven Rostedt <rostedt@...dmis.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
        linux-mm@...ck.org
Subject: Re: [PATCH v1 02/25] mm/swap: Don't abuse the seqcount latching API

Peter Zijlstra <peterz@...radead.org> wrote:
> On Tue, May 19, 2020 at 11:45:24PM +0200, Ahmed S. Darwish wrote:
> > @@ -713,10 +713,20 @@ static void lru_add_drain_per_cpu(struct work_struct *dummy)
> >   */
> >  void lru_add_drain_all(void)
> >  {
>

Re-adding cut-out comment for context:

	/*
	 * lru_drain_gen - Current generation of pages that could be in vectors
	 *
	 * (A) Definition: lru_drain_gen = x implies that all generations
	 *     0 < n <= x are already scheduled for draining.
	 *
	 * This is an optimization for the highly-contended use case where a
	 * user space workload keeps constantly generating a flow of pages
	 * for each CPU.
	 */
> > +	static unsigned int lru_drain_gen;
> >  	static struct cpumask has_work;
> > +	static DEFINE_MUTEX(lock);
> > +	int cpu, this_gen;
> >
> >  	/*
> >  	 * Make sure nobody triggers this path before mm_percpu_wq is fully
> > @@ -725,21 +735,48 @@ void lru_add_drain_all(void)
> >  	if (WARN_ON(!mm_percpu_wq))
> >  		return;
> >
>

Re-adding cut-out comment for context:

	/*
	 * (B) Cache the LRU draining generation number
	 *
	 * smp_rmb() ensures that the counter is loaded before the mutex is
	 * taken. It pairs with the smp_wmb() inside the mutex critical section
	 * at (D).
	 */
> > +	this_gen = READ_ONCE(lru_drain_gen);
> > +	smp_rmb();
>
> 	this_gen = smp_load_acquire(&lru_drain_gen);

ACK. will do.

> >
> >  	mutex_lock(&lock);
> >
> >  	/*
> > +	 * (C) Exit the draining operation if a newer generation, from another
> > +	 * lru_add_drain_all(), was already scheduled for draining. Check (A).
> >  	 */
> > +	if (unlikely(this_gen != lru_drain_gen))
> >  		goto done;
> >
>

Re-adding cut-out comment for context:

	/*
	 * (D) Increment generation number
	 *
	 * Pairs with READ_ONCE() and smp_rmb() at (B), outside of the critical
	 * section.
	 *
	 * This pairing must be done here, before the for_each_online_cpu loop
	 * below which drains the page vectors.
	 *
	 * Let x, y, and z represent some system CPU numbers, where x < y < z.
	 * Assume CPU #z is is in the middle of the for_each_online_cpu loop
	 * below and has already reached CPU #y's per-cpu data. CPU #x comes
	 * along, adds some pages to its per-cpu vectors, then calls
	 * lru_add_drain_all().
	 *
	 * If the paired smp_wmb() below is done at any later step, e.g. after
	 * the loop, CPU #x will just exit at (C) and miss flushing out all of
	 * its added pages.
	 */
> > +	WRITE_ONCE(lru_drain_gen, lru_drain_gen + 1);
> > +	smp_wmb();
>
> You can leave this smp_wmb() out and rely on the smp_mb() implied by
> queue_work_on()'s test_and_set_bit().
>

Won't this be too implicit?

Isn't it possible that, over the years, queue_work_on() impementation
changes and the test_and_set_bit()/smp_mb() gets removed?

If that happens, this commit will get *silently* broken and the local
CPU pages won't be drained.

> >  	cpumask_clear(&has_work);
> > -
> >  	for_each_online_cpu(cpu) {
> >  		struct work_struct *work = &per_cpu(lru_add_drain_work, cpu);
> >
>
> While you're here, do:
>
> 	s/cpumask_set_cpu/__&/
>

ACK.

Thanks,

--
Ahmed S. Darwish
Linutronix GmbH