From: Mel Gorman <mgorman@techsingularity.net>
To: Michal Hocko <mhocko@kernel.org>
Cc: Jia He <hejianet@gmail.com>,
linux-mm@kvack.org, linux-kernel@vger.kernel.org,
Andrew Morton <akpm@linux-foundation.org>,
Vlastimil Babka <vbabka@suse.cz>,
Johannes Weiner <hannes@cmpxchg.org>,
Joonsoo Kim <iamjoonsoo.kim@lge.com>,
Taku Izumi <izumi.taku@jp.fujitsu.com>
Subject: Re: [PATCH RFC 1/1] mm, page_alloc: fix incorrect zone_statistics data
Date: Tue, 20 Dec 2016 14:54:35 +0000 [thread overview]
Message-ID: <20161220145435.c3htqyfhpjt5uma7@techsingularity.net> (raw)
In-Reply-To: <20161220143501.GI3769@dhcp22.suse.cz>
On Tue, Dec 20, 2016 at 03:35:02PM +0100, Michal Hocko wrote:
> On Tue 20-12-16 14:28:45, Mel Gorman wrote:
> > On Tue, Dec 20, 2016 at 02:26:43PM +0100, Michal Hocko wrote:
> > > On Tue 20-12-16 13:10:40, Mel Gorman wrote:
> > > > On Tue, Dec 20, 2016 at 10:18:14AM +0100, Michal Hocko wrote:
> > > > > On Mon 12-12-16 13:59:07, Jia He wrote:
> > > > > > In commit b9f00e147f27 ("mm, page_alloc: reduce branches in
> > > > > > zone_statistics"), it reconstructed codes to reduce the branch miss rate.
> > > > > > Compared with the original logic, it assumed if !(flag & __GFP_OTHER_NODE)
> > > > > > z->node would not be equal to preferred_zone->node. That seems to be
> > > > > > incorrect.
> > > > >
> > > > > I am sorry but I have hard time following the changelog. It is clear
> > > > > that you are trying to fix a missed NUMA_{HIT,OTHER} accounting
> > > > > but it is not really clear when such thing happens. You are adding
> > > > > preferred_zone->node check. preferred_zone is the first zone in the
> > > > > requested zonelist. So for the most allocations it is a node from the
> > > > > local node. But if something request an explicit numa node (without
> > > > > __GFP_OTHER_NODE which would be the majority I suspect) then we could
> > > > > indeed end up accounting that as a NUMA_MISS, NUMA_FOREIGN so the
> > > > > referenced patch indeed caused an unintended change of accounting AFAIU.
> > > > >
> > > >
> > > > This is a similar concern to what I had. If the preferred zone, which is
> > > > the first valid usable zone, is not a "hit" for the statistics then I
> > > > don't know what "hit" is meant to mean.
> > >
> > > But the first valid usable zone is defined based on the requested numa
> > > node. Unless the requested node is memoryless then we should have a hit,
> > > no?
> > >
> >
> > Should be. If the local node is memoryless then there would be a difference
> > between hit and whether it's local or not but that to me is a little
> > useless. A local vs remote page allocated has a specific meaning and
> > consequence. It's hard to see how hit can be meaningfully interpreted if
> > there are memoryless nodes. I don't have a strong objection to the patch
> > so I didn't nak it, I'm just not convinced it matters.
>
> So what do you think about
> http://lkml.kernel.org/r/20161220091814.GC3769@dhcp22.suse.cz
>
This doesn't appear to resolve for me and I've 30 minutes left before
being offline for 4 days so didn't go digging.
> I think that we should get rid of __GFP_OTHER_NODE thingy. It is just
> one off thing and the gfp space it rather precious.
>
However, broadly speaking, I'd be ok with getting rid of
__GFP_OTHER_NODE altogether and making it truely only about local vs
remote hits because those are the ones that matter in terms of
performance. If a user has memoryless nodes or policies that allow local
CPUs but forbid local memory and they need to debug an issue, they're
going to need tracepoints anyway. Hit/miss/other is not sufficient for
most interesting problems involving local or remote memory usage.
--
Mel Gorman
SUSE Labs
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next prev parent reply other threads:[~2016-12-20 14:54 UTC|newest]
Thread overview: 23+ messages / expand[flat|nested] mbox.gz Atom feed top
2016-12-12 5:59 [PATCH RFC 0/1] mm, page_alloc: fix incorrect zone_statistics data Jia He
2016-12-12 5:59 ` [PATCH RFC 1/1] " Jia He
2016-12-20 9:18 ` Michal Hocko
2016-12-20 13:10 ` Mel Gorman
2016-12-20 13:26 ` Michal Hocko
2016-12-20 14:28 ` Mel Gorman
2016-12-20 14:35 ` Michal Hocko
2016-12-20 14:49 ` Vlastimil Babka
2016-12-20 14:54 ` Mel Gorman [this message]
2016-12-21 7:57 ` Michal Hocko
2016-12-21 8:06 ` [PATCH 1/2] mm: fix remote numa hits statistics Michal Hocko
2016-12-21 8:06 ` [PATCH 2/2] mm: get rid of __GFP_OTHER_NODE Michal Hocko
2017-01-02 14:18 ` Vlastimil Babka
2016-12-29 11:46 ` [PATCH 1/2] mm: fix remote numa hits statistics Mel Gorman
2016-12-29 12:28 ` Michal Hocko
2017-01-02 14:16 ` Vlastimil Babka
2017-01-02 14:46 ` Michal Hocko
2017-01-02 15:07 ` Vlastimil Babka
2016-12-20 14:42 ` [PATCH RFC 1/1] mm, page_alloc: fix incorrect zone_statistics data Mel Gorman
2016-12-20 15:13 ` Vlastimil Babka
2016-12-21 3:01 ` hejianet
2016-12-20 12:31 ` Mel Gorman
2016-12-21 3:07 ` hejianet
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