Re: [PATCH RFC 1/1] mm, page_alloc: fix incorrect zone_statistics data

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 14:28:45 +0000	[thread overview]
Message-ID: <20161220142845.drbedcibjcggdxk7@techsingularity.net> (raw)
In-Reply-To: <20161220132643.GG3769@dhcp22.suse.cz>

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.

-- 
Mel Gorman
SUSE Labs