Re: Kswapd in 3.2.0-rc5 is a CPU hog

From: Dave Chinner
Date: Fri Dec 23 2011 - 15:45:11 EST

On Fri, Dec 23, 2011 at 03:04:02PM +0400, nowhere wrote:
> Ð ÐÑ., 23/12/2011 Ð 21:20 +1100, Dave Chinner ÐÐÑÐÑ:
> > On Fri, Dec 23, 2011 at 01:01:20PM +0400, nowhere wrote:
> > > Ð ÐÑ., 22/12/2011 Ð 09:55 +1100, Dave Chinner ÐÐÑÐÑ:
> > > > 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.
>
> ./trace-cmd record -e vmscan/*
>
> Here is the report of trace-cmd while dd'ing
> https://80.237.6.56/report-dd.xz

Ok, it's not a shrink_slab() problem - it's just being called ~100uS
by kswapd. The pattern is:

- reclaim 94 (batches of 32,32,30) pages from iinactive list
of zone 1, node 0, prio 12
- call shrink_slab
- scan all caches
- all shrinkers return 0 saying nothing to shrink
- 40us gap
- reclaim 10-30 pages from inactive list of zone 2, node 0, prio 12
- call shrink_slab
- scan all caches
- all shrinkers return 0 saying nothing to shrink
- 40us gap
- isolate 9 pages from LRU zone ?, node ?, none isolated, none freed
- isolate 22 pages from LRU zone ?, node ?, none isolated, none freed
- call shrink_slab
- scan all caches
- all shrinkers return 0 saying nothing to shrink
40us gap

And it just repeats over and over again. After a while, nid=0,zone=1
drops out of the traces, so reclaim only comes in batches of 10-30
pages from zone 2 between each shrink_slab() call.

The trace starts at 111209.881s, with 944776 pages on the LRUs. It
finishes at 111216.1 with kswapd going to sleep on node 0 with
930067 pages on the LRU. So 7 seconds to free 15,000 pages (call it
2,000 pages/s) which is awfully slow....

vmscan gurus - time for you to step in now...

Cheers,

Dave.
--
Dave Chinner
david@xxxxxxxxxxxxx
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