linux-kernel - Re: [PATCH RFC 1/1] mm, page_alloc: fix incorrect zone

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Date:   Tue, 20 Dec 2016 15:35:02 +0100
From:   Michal Hocko <mhocko@...nel.org>
To:     Mel Gorman <mgorman@...hsingularity.net>
Cc:     Jia He <hejianet@...il.com>, linux-mm@...ck.org,
        linux-kernel@...r.kernel.org,
        Andrew Morton <akpm@...ux-foundation.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Johannes Weiner <hannes@...xchg.org>,
        Joonsoo Kim <iamjoonsoo.kim@....com>,
        Taku Izumi <izumi.taku@...fujitsu.com>
Subject: Re: [PATCH RFC 1/1] mm, page_alloc: fix incorrect zone_statistics
 data

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

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.

-- 
Michal Hocko
SUSE Labs