Message-Id: <20120126181631.94c3c685.kamezawa.hiroyu@jp.fujitsu.com>
Date:	Thu, 26 Jan 2012 18:16:31 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	Ying Han <yinghan@...gle.com>
Cc:	Johannes Weiner <hannes@...xchg.org>,
	Hillf Danton <dhillf@...il.com>, linux-mm@...ck.org,
	Michal Hocko <mhocko@...e.cz>, Hugh Dickins <hughd@...gle.com>,
	Andrew Morton <akpm@...ux-foundation.org>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH] mm: vmscan: check mem cgroup over reclaimed

On Tue, 24 Jan 2012 15:33:11 -0800
Ying Han <yinghan@...gle.com> wrote:

> On Tue, Jan 24, 2012 at 1:08 AM, KAMEZAWA Hiroyuki
> <kamezawa.hiroyu@...fujitsu.com> wrote:
> > On Tue, 24 Jan 2012 09:33:47 +0100
> > Johannes Weiner <hannes@...xchg.org> wrote:
> >
> >> On Mon, Jan 23, 2012 at 08:30:42PM +0800, Hillf Danton wrote:
> >> > On Mon, Jan 23, 2012 at 6:47 PM, Johannes Weiner <hannes@...xchg.org> wrote:
> >> > > On Mon, Jan 23, 2012 at 09:55:07AM +0800, Hillf Danton wrote:
> >> > >> To avoid reduction in performance of reclaimee, checking overreclaim is added
> >> > >> after shrinking lru list, when pages are reclaimed from mem cgroup.
> >> > >>
> >> > >> If over reclaim occurs, shrinking remaining lru lists is skipped, and no more
> >> > >> reclaim for reclaim/compaction.
> >> > >>
> >> > >> Signed-off-by: Hillf Danton <dhillf@...il.com>
> >> > >> ---
> >> > >>
> >> > >> --- a/mm/vmscan.c     Mon Jan 23 00:23:10 2012
> >> > >> +++ b/mm/vmscan.c     Mon Jan 23 09:57:20 2012
> >> > >> @@ -2086,6 +2086,7 @@ static void shrink_mem_cgroup_zone(int p
> >> > >>       unsigned long nr_reclaimed, nr_scanned;
> >> > >>       unsigned long nr_to_reclaim = sc->nr_to_reclaim;
> >> > >>       struct blk_plug plug;
> >> > >> +     bool memcg_over_reclaimed = false;
> >> > >>
> >> > >>  restart:
> >> > >>       nr_reclaimed = 0;
> >> > >> @@ -2103,6 +2104,11 @@ restart:
> >> > >>
> >> > >>                               nr_reclaimed += shrink_list(lru, nr_to_scan,
> >> > >>                                                           mz, sc, priority);
> >> > >> +
> >> > >> +                             memcg_over_reclaimed = !scanning_global_lru(mz)
> >> > >> +                                     && (nr_reclaimed >= nr_to_reclaim);
> >> > >> +                             if (memcg_over_reclaimed)
> >> > >> +                                     goto out;
> >> > >
> >> > > Since this merge window, scanning_global_lru() is always false when
> >> > > the memory controller is enabled, i.e. most common configurations and
> >> > > distribution kernels.
> >> > >
> >> > > This will with quite likely have bad effects on zone balancing,
> >> > > pressure balancing between anon/file lru etc, while you haven't shown
> >> > > that any workloads actually benefit from this.
> >> > >
> >> > Hi Johannes
> >> >
> >> > Thanks for your comment, first.
> >> >
> >> > Impact on zone balance and lru-list balance is introduced actually, but I
> >> > dont think the patch is totally responsible for the balance mentioned,
> >> > because soft limit, embedded in mem cgroup, is setup by users according to
> >> > whatever tastes they have.
> >> >
> >> > Though there is room for the patch to be fine tuned in this direction or that,
> >> > over reclaim should not be neglected entirely, but be avoided as much as we
> >> > could, or users are enforced to set up soft limit with much care not to mess
> >> > up zone balance.
> >>
> >> Overreclaim is absolutely horrible with soft limits, but I think there
> >> are more direct reasons than checking nr_to_reclaim only after a full
> >> zone scan, for example, soft limit reclaim is invoked on zones that
> >> are totally fine.
> >>
> >
> >
> > IIUC..
> >  - Because zonelist is all visited by alloc_pages(), _all_ zones in zonelist
> >   are in memory shortage.
> >  - taking care of zone/node balancing.
> >
> > I know this 'full zone scan' affects latency of alloc_pages() if the number
> > of node is big.
> 
> >
> > IMHO, in case of direct-reclaim caused by memcg's limit, we should avoid
> > full zone scan because the reclaim is not caused by any memory shortage in zonelist.
> >

This text is talking about memcg's direct reclaim scanning caused by 'limit'.


> > In case of global memory reclaim, kswapd doesn't use zonelist.
> >
> > So, only global-direct-reclaim is a problem here.
> > I think do-full-zone-scan will reduce the calls of try_to_free_pages()
> > in future and may reduce lock contention but adds a thread too much
> > penalty.
> 
> > In typical case, considering 4-node x86/64 NUMA, GFP_HIGHUSER_MOVABLE
> > allocation failure will reclaim 4*ZONE_NORMAL+ZONE_DMA32 = 160pages per scan.
> >
> > If 16-node, it will be 16*ZONE_NORMAL+ZONE_DMA32 = 544? pages per scan.
> >
> > 32pages may be too small but don't we need to have some threshold to quit
> > full-zone-scan ?
> 
> Sorry I am confused. Are we talking about doing full zonelist scanning
> within a memcg or doing anon/file lru balance within a zone? AFAIU, it
> is the later one.
> 
I'm sorry for confusing.

Above test is talking about global lru scanning, not memcg related.



> In this patch, we do early breakout (memcg_over_reclaimed) without
> finish scanning other lrus per-memcg-per-zone. I think the concern is
> what is the side effect of that ?
> 
> > Here, the topic is about softlimit reclaim. I think...
> >
> > 1. follow up for following comment(*) is required.
> > ==
> >                        nr_soft_scanned = 0;
> >                        nr_soft_reclaimed = mem_cgroup_soft_limit_reclaim(zone,
> >                                                sc->order, sc->gfp_mask,
> >                                                &nr_soft_scanned);
> >                        sc->nr_reclaimed += nr_soft_reclaimed;
> >                        sc->nr_scanned += nr_soft_scanned;
> >                        /* need some check for avoid more shrink_zone() */ <----(*)
> > ==
> >
> > 2. some threshold for avoinding full zone scan may be good.
> >   (But this may need deep discussion...)
> >
> > 3. About the patch, I think it will not break zone-balancing if (*) is
> >   handled in a good way.
> >
> >   This check is not good.
> >
> > +                               memcg_over_reclaimed = !scanning_global_lru(mz)
> > +                                       && (nr_reclaimed >= nr_to_reclaim);
> >
> >
> >  I like following
> >
> >  If (we-are-doing-softlimit-reclaim-for-global-direct-reclaim &&
> >      res_counter_soft_limit_excess(memcg->res))
> >       memcg_over_reclaimed = true;
> 
> This condition looks quite similar to what we've discussed on another
> thread, except that we do allow over-reclaim under softlimit after
> certain priority loop. (assume we have hard-to-reclaim memory on other
> cgroups above their softlimit)
> 

yes. I've cut this from that thread.


> There are some works needed to be done ( like reverting the rb-tree )
> on current soft limit implementation before we can even further to
> optimize it. It would be nice to settle the first part before
> everything else.
> 
Agreed.

I personally think Johannes' clean up should go first and removing
rb-tree before optimization is better.

Thanks,
-Kame

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