linux-kernel - Re: [PATCH] mm/swap: piggyback lru_add_drain

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Date:   Fri, 4 Oct 2019 14:27:27 +0200
From:   Michal Hocko <mhocko@...nel.org>
To:     Matthew Wilcox <willy@...radead.org>
Cc:     Konstantin Khlebnikov <khlebnikov@...dex-team.ru>,
        linux-mm@...ck.org, Andrew Morton <akpm@...ux-foundation.org>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH] mm/swap: piggyback lru_add_drain_all() calls

On Fri 04-10-19 05:10:17, 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?

Yeah, this makes sense. From correctness point of view it should be ok
because no caller can expect that per-cpu pvecs are empty on return.
This might have some runtime effects that some paths might retry more -
e.g. offlining path drains pcp pvces before migrating the range away, if
there are pages still waiting for a worker to drain them then the
migration would fail and we would retry. But this not a correctness
issue.

-- 
Michal Hocko
SUSE Labs