Date:	Fri, 23 Dec 2011 21:20:27 +1100
From:	Dave Chinner <david@...morbit.com>
To:	nowhere <nowhere@...kenden.ath.cx>
Cc:	Michal Hocko <mhocko@...e.cz>, linux-kernel@...r.kernel.org,
	linux-mm@...ck.org
Subject: Re: Kswapd in 3.2.0-rc5 is a CPU hog

On Fri, Dec 23, 2011 at 01:01:20PM +0400, nowhere wrote:
> В Чт., 22/12/2011 в 09:55 +1100, Dave Chinner пишет:
> > On Wed, Dec 21, 2011 at 10:52:49AM +0100, Michal Hocko wrote:
> > > [Let's CC linux-mm]
> > > 
> > > On Wed 21-12-11 07:10:36, Nikolay S. wrote:
> > > > Hello,
> > > > 
> > > > I'm using 3.2-rc5 on a machine, which atm does almost nothing except
> > > > file system operations and network i/o (i.e. file server). And there is
> > > > a problem with kswapd.
> > > 
> > > What kind of filesystem do you use?
> > > 
> > > > 
> > > > I'm playing with dd:
> > > > dd if=/some/big/file of=/dev/null bs=8M
> > > > 
> > > > I.e. I'm filling page cache.
> > > > 
> > > > So when the machine is just rebooted, kswapd during this operation is
> > > > almost idle, just 5-8 percent according to top.
> > > > 
> > > > After ~5 days of uptime (5 days,  2:10), the same operation demands ~70%
> > > > for kswapd:
> > > > 
> > > >   PID USER      S %CPU %MEM    TIME+  SWAP COMMAND
> > > >   420 root      R   70  0.0  22:09.60    0 kswapd0
> > > > 17717 nowhere   D   27  0.2   0:01.81  10m dd
> > > > 
> > > > In fact, kswapd cpu usage on this operation steadily increases over
> > > > time.
> > > > 
> > > > Also read performance degrades over time. After reboot:
> > > > dd if=/some/big/file of=/dev/null bs=8M
> > > > 1019+1 records in
> > > > 1019+1 records out
> > > > 8553494018 bytes (8.6 GB) copied, 16.211 s, 528 MB/s
> > > > 
> > > > After ~5 days uptime:
> > > > dd if=/some/big/file of=/dev/null bs=8M
> > > > 1019+1 records in
> > > > 1019+1 records out
> > > > 8553494018 bytes (8.6 GB) copied, 29.0507 s, 294 MB/s
> > > > 
> > > > Whereas raw disk sequential read performance stays the same:
> > > > dd if=/some/big/file of=/dev/null bs=8M iflag=direct
> > > > 1019+1 records in
> > > > 1019+1 records out
> > > > 8553494018 bytes (8.6 GB) copied, 14.7286 s, 581 MB/s
> > > > 
> > > > Also after dropping caches, situation somehow improves, but not to the
> > > > state of freshly restarted system:
> > > >   PID USER      S %CPU %MEM    TIME+  SWAP COMMAND
> > > >   420 root      S   39  0.0  23:31.17    0 kswapd0
> > > > 19829 nowhere   D   24  0.2   0:02.72 7764 dd
> > > > 
> > > > perf shows:
> > > > 
> > > >     31.24%  kswapd0  [kernel.kallsyms]  [k] _raw_spin_lock
> > > >     26.19%  kswapd0  [kernel.kallsyms]  [k] shrink_slab
> > > >     16.28%  kswapd0  [kernel.kallsyms]  [k] prune_super
> > > >      6.55%  kswapd0  [kernel.kallsyms]  [k] grab_super_passive
> > > >      5.35%  kswapd0  [kernel.kallsyms]  [k] down_read_trylock
> > > >      4.03%  kswapd0  [kernel.kallsyms]  [k] up_read
> > > >      2.31%  kswapd0  [kernel.kallsyms]  [k] put_super
> > > >      1.81%  kswapd0  [kernel.kallsyms]  [k] drop_super
> > > >      0.99%  kswapd0  [kernel.kallsyms]  [k] __put_super
> > > >      0.25%  kswapd0  [kernel.kallsyms]  [k] __isolate_lru_page
> > > >      0.23%  kswapd0  [kernel.kallsyms]  [k] free_pcppages_bulk
> > > >      0.19%  kswapd0  [r8169]            [k] rtl8169_interrupt
> > > >      0.15%  kswapd0  [kernel.kallsyms]  [k] twa_interrupt
> > > 
> > > Quite a lot of time spent shrinking slab (dcache I guess) and a lot of
> > > spin lock contention.
> > 
> > That's just scanning superblocks, not apparently doing anything
> > useful like shrinking dentries or inodes attached to each sb. i.e.
> > the shrinkers are being called an awful lot and basically have
> > nothing to do. I'd be suspecting a problem higher up in the stack to
> > do with how shrink_slab is operating or being called.
> > 
> > I'd suggest gathering event traces for mm_shrink_slab_start/
> > mm_shrink_slab_end to try to see how the shrinkers are being
> > driven...
> > 
> > Cheers,
> > 
> > Dave.
> 
> I have recompiled kernel with tracers, and today the problem is visible
> again. So here is the trace for mm_shrink_slab_start (it is HUGE):
> 
>          kswapd0   421 [000] 103976.627873: mm_shrink_slab_start: prune_super+0x0 0xffff88011b00d300: objects to shrink 12 gfp_flags GFP_KERNELGFP_NOTRACK pgs_scanned 32 lru_pgs 942483 cache items 1500 delt
>          kswapd0   421 [000] 103976.627882: mm_shrink_slab_start: prune_super+0x0 0xffff88011a20ab00: objects to shrink 267 gfp_flags GFP_KERNELGFP_NOTRACK pgs_scanned 32 lru_pgs 942483 cache items 5300 del

And possibly useless in this form. I need to see the
mm_shrink_slab_start/mm_shrink_slab_end events interleaved so I can
see exactly how much work each shrinker call is doing, and the start
events are truncated so not all the info I need is present.

Perhaps you should install trace-cmd.

$ trace-cmd record -e mm_shrink_slab*
(wait 30s, then ^C)
$ trace-cmd report > shrink.trace

And then compress and attach the trace file or put up on the web
somewhere for me ot download if it's too large for email...

As it is, there's ~940k pages in the LRU, and shrink_slab is being
called after 32, 95, 8, 8, 32 and 32 pages on the LRU have been
scanned. That seems like the shrinkers are being called rather too
often.

The end traces indicate the shrinker caches aren't able to free
anything. So it looks like the vmscan code has got itself in a
situation where it is not scanning many pages between shrinker
callouts, and the shrinkers scan but can't make any progress. Looks
like a vmscan balancing problem right now, not anything to do with
the shrinker code. A better trace will confirm that.

FWIW, if you use trace-cmd, it might be worthwhile collecting all the
vmscan trace events too, as that might help the VM folk understand
the problem without needing to ask you for more info.

Cheers,

Dave.
-- 
Dave Chinner
david@...morbit.com
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