linux-kernel - Re: regression caused by cgroups optimization in 3.17-rc2

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Message-ID: <20140905092841.GD26243@dhcp22.suse.cz>
Date:	Fri, 5 Sep 2014 11:28:41 +0200
From:	Michal Hocko <mhocko@...e.cz>
To:	Dave Hansen <dave.hansen@...el.com>
Cc:	Dave Hansen <dave@...1.net>, Johannes Weiner <hannes@...xchg.org>,
	Hugh Dickins <hughd@...gle.com>, Tejun Heo <tj@...nel.org>,
	Linux-MM <linux-mm@...ck.org>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Linus Torvalds <torvalds@...ux-foundation.org>,
	Vladimir Davydov <vdavydov@...allels.com>,
	LKML <linux-kernel@...r.kernel.org>
Subject: Re: regression caused by cgroups optimization in 3.17-rc2

On Thu 04-09-14 15:53:46, Dave Hansen wrote:
> On 09/04/2014 01:27 PM, Dave Hansen wrote:
> > On 09/04/2014 07:27 AM, Michal Hocko wrote:
> >> Ouch. free_pages_and_swap_cache completely kills the uncharge batching
> >> because it reduces it to PAGEVEC_SIZE batches.
> >>
> >> I think we really do not need PAGEVEC_SIZE batching anymore. We are
> >> already batching on tlb_gather layer. That one is limited so I think
> >> the below should be safe but I have to think about this some more. There
> >> is a risk of prolonged lru_lock wait times but the number of pages is
> >> limited to 10k and the heavy work is done outside of the lock. If this
> >> is really a problem then we can tear LRU part and the actual
> >> freeing/uncharging into a separate functions in this path.
> >>
> >> Could you test with this half baked patch, please? I didn't get to test
> >> it myself unfortunately.
> > 
> > 3.16 settled out at about 11.5M faults/sec before the regression.  This
> > patch gets it back up to about 10.5M, which is good.  The top spinlock
> > contention in the kernel is still from the resource counter code via
> > mem_cgroup_commit_charge(), though.
> > 
> > I'm running Johannes' patch now.
> 
> This looks pretty good.  The area where it plateaus (above 80 threads
> where hyperthreading kicks in) might be a bit slower than it was in
> 3.16, but that could easily be from other things.

Good news indeed. But I think it would be safer to apply Johannes'
revert for now. Both changes are still worth having anyway because they
have potential to improve memcg case.

> > https://www.sr71.net/~dave/intel/bb.html?1=3.16.0-rc4-g67b9d76/&2=3.17.0-rc3-g57b252f
> 
> Feel free to add my Tested-by:

Thanks a lot! I have posted another patch which reduces the batching for
LRU handling because this would be too risky. So I haven't added your
Tested-by yet.
 

-- 
Michal Hocko
SUSE Labs
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