Re: [RFC PATCH] mm: swap: remove lru drain waiters

[Ahmed S. Darwish]

Hi Hillf,

For some reason, **all of your posts** from <hdanton@xxxxxxxx> do not
appear on lore.kernel.org.

Check, for example, https://lore.kernel.org/lkml/?q=hdanton%40sina.com,
where thread replies are there but not the actual posts.

Just wanted to let you know... Please continue below.

On Wed, Jun 03, 2020 at 10:21:45AM +0200, Sebastian Andrzej Siewior wrote:
> On 2020-06-01 22:37:34 [+0800], Hillf Danton wrote:
> >
> > After updating the lru drain sequence, new comers avoid waiting for
> > the current drainer, because he is flushing works on each online CPU,
> > by trying to lock the mutex; the drainer OTOH tries to do works for
> > those who fail to acquire the lock by checking the lru drain sequence
> > after releasing lock.
> >
> > See eef1a429f234 ("mm/swap.c: piggyback lru_add_drain_all() calls")
> > for reasons why we can skip waiting for the lock.
> >
> > The memory barriers around the sequence and the lock come together
> > to remove waiters without their drain works bandoned.
> >
> > Cc: Sebastian Andrzej Siewior <bigeasy@xxxxxxxxxxxxx>
> > Cc: Konstantin Khlebnikov <khlebnikov@xxxxxxxxxxxxxx>
> > Signed-off-by: Hillf Danton <hdanton@xxxxxxxx>
> > ---
> > This is inspired by one of the works from Sebastian.
>
> Not me, it was Ahmed.
>
> > --- a/mm/swap.c
> > +++ b/mm/swap.c
> > @@ -714,10 +714,11 @@ static void lru_add_drain_per_cpu(struct
> >   */
> >  void lru_add_drain_all(void)
> >  {
> > -	static seqcount_t seqcount = SEQCNT_ZERO(seqcount);
> > +	static unsigned int lru_drain_seq;
> >  	static DEFINE_MUTEX(lock);
> >  	static struct cpumask has_work;
> > -	int cpu, seq;
> > +	int cpu;
> > +	unsigned int seq;
> >
> >  	/*
> >  	 * Make sure nobody triggers this path before mm_percpu_wq is fully
> > @@ -726,18 +727,16 @@ void lru_add_drain_all(void)
> >  	if (WARN_ON(!mm_percpu_wq))
> >  		return;
> >
> > -	seq = raw_read_seqcount_latch(&seqcount);
> > +	lru_drain_seq++;
> > +	smp_mb();
> >
> > -	mutex_lock(&lock);
> > +more_work:
> >
> > -	/*
> > -	 * Piggyback on drain started and finished while we waited for lock:
> > -	 * all pages pended at the time of our enter were drained from vectors.
> > -	 */
> > -	if (__read_seqcount_retry(&seqcount, seq))
> > -		goto done;
> > +	if (!mutex_trylock(&lock))
> > +		return;
> >

The patch I've posted makes sure to preserve the existing draining
logic. It only fixes an erroneous usage of seqcount_t latching, plus a
memory barriers bugfix, found by John, and is to be included in v2:

    https://lkml.kernel.org/r/87y2pg9erj.fsf@xxxxxxxxxxxxxxxxxxxx

On the other hand, you're making the draining operation completely
asynchronous for a number of callers. This is such a huge change, and I
fail to see: 1) any rationale for it in the changelog, 2) whether it's
been verified that call-sites won't be affected.

Thanks,

--
Ahmed S. Darwish
Linutronix GmbH