Re: [Bug #14141] order 2 page allocation failures in iwlagn

[Jens Axboe]

On Mon, Oct 19 2009, Pekka Enberg wrote:
> (Adding Jens to CC.)
> 
> On Wednesday 14 October 2009, Frans Pop wrote:
> > > > There still has not been a mm-change identified that makes
> > > > fragmentation significantly worse.
> 
> On Mon, 2009-10-19 at 01:33 +0200, Frans Pop wrote:
> > > My bisection shows a very clear point, even if not an individual commit,
> > > in the 'akpm' merge where SKB errors suddenly become *much* more
> > > frequent and easy to trigger.
> > > I'm sorry to say this, but the fact that nothing has been identified yet
> > > is IMO the result of a lack of effort, not because there is no such
> > > change.
> > 
> > I was wrong. It turns out that I was creating the variations in the test 
> > results around the akpm merge myself by tiny changes in the way I ran the 
> > tests. It took another round of about 30 compilations and tests purely in 
> > this range to show that, but those same tests also made me aware of other 
> > patterns I should look at.
> > 
> > Until a few days ago I was concentrating on "do I see SKB allocation errors 
> > or not". Since then I've also been looking more consciously at when they 
> > happen, at disk access patterns and at desktop freeze patterns.
> > 
> > I think I did mention before that this whole issue is rather subtle :-/
> > So, my apologies for finguering the wrong area for so long, but it looked 
> > solid given the info available at the time.
> > 
> > On Thursday 15 October 2009, Mel Gorman wrote:
> > > Outside the range of commits suspected of causing problems was the
> > > following. It's extremely low probability
> > >
> > > Commit 8aa7e84 Fix congestion_wait() sync/async vs read/write confusion
> > >         This patch alters the call to congestion_wait() in the page
> > >         allocator. Frankly, I don't get the change but it might worth
> > >         checking if replacing BLK_RW_ASYNC with WRITE on top of 2.6.31
> > >         makes any difference
> > 
> > This is the real culprit. Mel: thanks very much for looking beyond the 
> > area I identified. Your overview of mm changes was exactly what I needed 
> > and really helped a lot during my later tests.
> > 
> > This commit definitely causes most of the problems; confirmed by reverting 
> > it on top of 2.6.31 (also requires reverting 373c0a7e, which is a later 
> > build fix).
> 
> Mel/Jens, any ideas why commit 8aa7e84 makes us run out of high order
> pages? Should we be using BLK_RW_SYNC in mm/page_alloc.c instead of
> BLK_RW_ASYNC?

No, I think that is definitely broken since the page freeing should be
using async writes. If the commit in question is making the difference
and the below does indeed fix it, I think that's primarliy due to timing
issues and the general brokeness of the congestion bits. With the below
change, you essentially guarenteed to be waiting 20ms every time and
it's quite likely that that is enough to change the picture.

So I'd like elsewhere for the real problem, it's not likely to be caused
by the sync vs async bits themselves.

-- 
Jens Axboe

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