linux-kernel - Re: [patch 1/4] mm: memcontrol: uncharge pages on swapout

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Message-ID: <20141022132038.GB17161@phnom.home.cmpxchg.org>
Date:	Wed, 22 Oct 2014 09:20:38 -0400
From:	Johannes Weiner <hannes@...xchg.org>
To:	Vladimir Davydov <vdavydov@...allels.com>
Cc:	Andrew Morton <akpm@...ux-foundation.org>,
	Hugh Dickins <hughd@...gle.com>, Michal Hocko <mhocko@...e.cz>,
	linux-mm@...ck.org, cgroups@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [patch 1/4] mm: memcontrol: uncharge pages on swapout

On Wed, Oct 22, 2014 at 12:33:53PM +0400, Vladimir Davydov wrote:
> On Tue, Oct 21, 2014 at 05:03:28PM -0400, Johannes Weiner wrote:
> > On Tue, Oct 21, 2014 at 04:52:52PM +0400, Vladimir Davydov wrote:
> > > On Mon, Oct 20, 2014 at 11:22:09AM -0400, Johannes Weiner wrote:
> > > > mem_cgroup_swapout() is called with exclusive access to the page at
> > > > the end of the page's lifetime.  Instead of clearing the PCG_MEMSW
> > > > flag and deferring the uncharge, just do it right away.  This allows
> > > > follow-up patches to simplify the uncharge code.
> > > > 
> > > > Signed-off-by: Johannes Weiner <hannes@...xchg.org>
> > > > ---
> > > >  mm/memcontrol.c | 17 +++++++++++++----
> > > >  1 file changed, 13 insertions(+), 4 deletions(-)
> > > > 
> > > > diff --git a/mm/memcontrol.c b/mm/memcontrol.c
> > > > index bea3fddb3372..7709f17347f3 100644
> > > > --- a/mm/memcontrol.c
> > > > +++ b/mm/memcontrol.c
> > > > @@ -5799,6 +5799,7 @@ static void __init enable_swap_cgroup(void)
> > > >   */
> > > >  void mem_cgroup_swapout(struct page *page, swp_entry_t entry)
> > > >  {
> > > > +	struct mem_cgroup *memcg;
> > > >  	struct page_cgroup *pc;
> > > >  	unsigned short oldid;
> > > >  
> > > > @@ -5815,13 +5816,21 @@ void mem_cgroup_swapout(struct page *page, swp_entry_t entry)
> > > >  		return;
> > > >  
> > > >  	VM_BUG_ON_PAGE(!(pc->flags & PCG_MEMSW), page);
> > > > +	memcg = pc->mem_cgroup;
> > > >  
> > > > -	oldid = swap_cgroup_record(entry, mem_cgroup_id(pc->mem_cgroup));
> > > > +	oldid = swap_cgroup_record(entry, mem_cgroup_id(memcg));
> > > >  	VM_BUG_ON_PAGE(oldid, page);
> > > > +	mem_cgroup_swap_statistics(memcg, true);
> > > >  
> > > > -	pc->flags &= ~PCG_MEMSW;
> > > > -	css_get(&pc->mem_cgroup->css);
> > > > -	mem_cgroup_swap_statistics(pc->mem_cgroup, true);
> > > > +	pc->flags = 0;
> > > > +
> > > > +	if (!mem_cgroup_is_root(memcg))
> > > > +		page_counter_uncharge(&memcg->memory, 1);
> > > 
> > > AFAIU it removes batched uncharge of swapped out pages, doesn't it? Will
> > > it affect performance?
> > 
> > During swapout and with lockless page counters?  I don't think so.
> 
> How is this different from page cache out? I mean, we can have a lot of
> pages in the swap cache that have already been swapped out, and are
> waiting to be unmapped, uncharged, and freed, just like usual page
> cache. Why do we use batching for file cache pages then?

The batching is mostly for munmap().  We do it for reclaim because
it's convenient, but I don't think an extra word per struct page to
batch one, sometimes a few, locked subtractions per swapped out page
is a reasonable trade-off.

> > > Besides, it looks asymmetric with respect to the page cache uncharge
> > > path, where we still defer uncharge to mem_cgroup_uncharge_list(), and I
> > > personally rather dislike this asymmetry.
> > 
> > The asymmetry is inherent in the fact that we mave memory and
> > memory+swap accounting, and here a memory charge is transferred out to
> > swap.  Before, the asymmetry was in mem_cgroup_uncharge_list() where
> > we separate out memory and memsw pages (which the next patch fixes).
> 
> I agree that memsw is inherently asymmetric, but IMO it isn't the case
> for swap *cache* vs page *cache*. We handle them similarly - removing
> from a mapping, uncharging, freeing. If one wants batching, why
> shouldn't the other?

It has to be worth it in practical terms.  You can argue symmetry
between swap cache and page cache, but swapping simply is a much
colder path than reclaiming page cache.  Our reclaim algorithm avoids
it like the plague.

> > So nothing changed, the ugliness was just moved around.  I actually
> > like it better now that it's part of the swap controller, because
> > that's where the nastiness actually comes from.  This will all go away
> > when we account swap separately.  Then, swapped pages can keep their
> > memory charge until mem_cgroup_uncharge() again and the swap charge
> > will be completely independent from it.  This reshuffling is just
> > necessary because it allows us to get rid of the per-page flag.
> 
> Do you mean that swap cache uncharge batching will be back soon?

Well, yes, once we switch from memsw to a separate swap couter, it
comes automatically.  Pages no longer carry two charges, and so the
uncharging of pages doesn't have to distinguish between swapped out
pages and other pages anymore.
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