Re: [PATCH] mm/swap: piggyback lru_add_drain_all() calls

[Konstantin Khlebnikov]

On 04/10/2019 15.10, Matthew Wilcox wrote:
> On Fri, Oct 04, 2019 at 01:11:06PM +0300, Konstantin Khlebnikov wrote:
> > This is very slow operation. There is no reason to do it again if somebody
> > else already drained all per-cpu vectors after we waited for lock.
> > +	seq = raw_read_seqcount_latch(&seqcount);
> > +
> >   	mutex_lock(&lock);
> > +
> > +	/* Piggyback on drain done by somebody else. */
> > +	if (__read_seqcount_retry(&seqcount, seq))
> > +		goto done;
> > +
> > +	raw_write_seqcount_latch(&seqcount);
> > +
>
> Do we really need the seqcount to do this?  Wouldn't a mutex_trylock()
> have the same effect?

No, this is completely different semantics.

Operation could be safely skipped only if somebody else started and
finished drain after current task called this function.