linux-kernel - Re: [LKP] [lkp] [xfs] 68a9f5e700: aim7.jobs-per-min -13.6% regression

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Date:	Thu, 11 Aug 2016 09:55:33 -0700
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Christoph Hellwig <hch@....de>
Cc:	"Huang, Ying" <ying.huang@...el.com>,
	Dave Chinner <david@...morbit.com>,
	LKML <linux-kernel@...r.kernel.org>,
	Bob Peterson <rpeterso@...hat.com>,
	Wu Fengguang <fengguang.wu@...el.com>, LKP <lkp@...org>
Subject: Re: [LKP] [lkp] [xfs] 68a9f5e700: aim7.jobs-per-min -13.6% regression

On Thu, Aug 11, 2016 at 8:57 AM, Christoph Hellwig <hch@....de> wrote:
>
> The one liner below (not tested yet) to simply remove it should fix that
> up.  I also noticed we have a spurious pagefault_disable/enable, I
> need to dig into the history of that first, though.

Hopefully the pagefault_disable/enable doesn't matter for this case.

Can we get this one-liner tested with the kernel robot for comparison?
I really think a messed-up LRU list could cause bad IO patterns, and
end up keeping dirty pages around that should be streaming out to disk
and re-used, so causing memory pressure etc for no good reason.

I think the mapping->tree_lock issue that Dave sees is interesting
too, but the kswapd activity (and the extra locking it causes) could
also be a symptom of the same thing - memory pressure due to just
putting pages in the active file that simply shouldn't be there.

So the trivial oneliner _might_ just explain things. It would be
really nice if the regression turns out to be due to something so
easily fixed.

                Linus