Re: System CPU increasing on idle 2.6.36

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On Fri, Jan 07, 2011 at 10:05:39AM -0800, Mark Moseley wrote:

> NOTE: NFS/Kernel guys: I've left the 2.6.36.2 box still thrashing
> about in case there's something you'd like me to look at.
> 
> Ok, both my 2.6.36.2 kernel box and my 2.6.35->2.6.36 bisect box (was
> bisected to ce7db282a3830f57f5b05ec48288c23a5c4d66d5 -- this is my
> first time doing bisect, so I'll preemptively apologize for doing

That's a merge commit, so that doesn't sound right (it doesn't contain
changes by itself).  How were you running the bisect?  Were you
definitely seeing a good case since v2.6.35?  Could you post your
"git bisect log" so I can see if I can follow it?

> anything silly) both went berserk within 15 mins of each other, after
> an uptime of around 63 hours for 2.6.36.2 and 65 hours for the
> bisected box. The 2.6.36.2 one is still running with all the various
> flush-0:xx threads spinning wildly. The bisected box just keeled over
> and died, but is back up now. The only kernel messages logged are just
> of the "task kworker/4:1:359 blocked for more than 120 seconds"
> variety, all with _raw_spin_lock_irq at the top of the stack trace.
> 
> Looking at slabinfo, specifically, this: stats=$(grep nfs_inode_cache
> /proc/slabinfo | awk '{ print $2, $3 }')
> the active_objs and num_objs both increase to over a million (these
> boxes are delivering mail to NFS-mounted mailboxes, so that's
> perfectly reasonable). On both boxes, looking at sar, things start to
> go awry around 10am today EST. At that time on the 2.6.36.2 box, the
> NFS numbers look like this:
> 
> Fri Jan  7 09:58:00 2011: 1079433 1079650
> Fri Jan  7 09:59:00 2011: 1079632 1080300
> Fri Jan  7 10:00:00 2011: 1080196 1080300
> Fri Jan  7 10:01:01 2011: 1080599 1080716
> Fri Jan  7 10:02:01 2011: 1081074 1081288
> 
> on the bisected, like this:
> 
> Fri Jan  7 09:59:34 2011: 1162786 1165320
> Fri Jan  7 10:00:34 2011: 1163301 1165320
> Fri Jan  7 10:01:34 2011: 1164369 1165450
> Fri Jan  7 10:02:35 2011: 1164179 1165450
> Fri Jan  7 10:03:35 2011: 1165795 1166958
> 
> When the bisected box finally died, the last numbers were:
> 
> Fri Jan  7 10:40:33 2011: 1177156 1177202
> Fri Jan  7 10:42:21 2011: 1177157 1177306
> Fri Jan  7 10:44:55 2011: 1177201 1177324
> Fri Jan  7 10:45:55 2011: 1177746 1177826

Yeah, I was seeing load proportional to these, but I don't think their
growth is anything wrong.  It just seems to be something that is taking
up CPU from flush proceses as they grow, and something which is causing a
thundering herd of reclaim with lots of spinlock contention.

Hmmm.. I tried to see if drop_caches would clear nfs_inode_cache and thus
the CPU usage.  The top of slabtop before doing anything:

  OBJS ACTIVE  USE OBJ SIZE  SLABS OBJ/SLAB CACHE SIZE NAME
1256343 1221799  97%    0.95K  38071       33   1218272K nfs_inode_cache

# echo 2 > /proc/sys/vm/drop_caches

this took about 40 seconds to execute, and perf top showed this for
pretty much the whole time:

             samples  pcnt function                 DSO
             _______ _____ ________________________ ___________

             1188.00 45.1% _raw_spin_lock           [kernel]
              797.00 30.2% rpcauth_cache_shrinker   [kernel]
              132.00  5.0% shrink_slab              [kernel]
               60.00  2.3% queue_io                 [kernel]
               59.00  2.2% _cond_resched            [kernel]
               44.00  1.7% nfs_writepages           [kernel]
               35.00  1.3% writeback_single_inode   [kernel]
               34.00  1.3% writeback_sb_inodes      [kernel]
               23.00  0.9% do_writepages            [kernel]
               20.00  0.8% bit_waitqueue            [kernel]
               15.00  0.6% redirty_tail             [kernel]
               11.00  0.4% __iget                   [kernel]
               11.00  0.4% __GI_strstr              libc-2.7.so

slabtop then showed:

  OBJS ACTIVE  USE OBJ SIZE  SLABS OBJ/SLAB CACHE SIZE NAME
1256211 531720  42%    0.95K  38067       33   1218144K nfs_inode_cache

and this doesn't change further with drop_caches=3 or 1.  It's strange
that most of them were freed, but not all...  We do actually have
RPC_CREDCACHE_DEFAULT_HASHBITS set to 10 at build time, which might
explain a bigger credcache and time to free that.  Did you change your
auth_hashtable_size at all?  Anyway, even with all of that, system CPU
doesn't change.

Ok, weird, Running "sync" actually caused "vmstat 1" to go from 25% CPU
spikes every 5 seconds with nfs_writepages in the "perf top" every 5
seconds to being idle!  /proc/vmstat showed only nr_dirty 8 and
nr_writeback 0 before I ran "sync".  Either that, or it was just good
timing.

procs -----------memory---------- ---swap-- -----io---- -system-- ----cpu----
 r  b   swpd   free   buff  cache   si   so    bi    bo   in   cs us sy id wa
 0  0      0 6767892   9696  41088    0    0     5   180   15   70 62  7 29  1
 0  0      0 6767892   9696  41088    0    0     0     0 1124 1978  0  0 100  0
 0  0      0 6767892   9696  41088    0    0     0     0 1105 1967  0  0 100  0
 0  0      0 6767892   9696  41088    0    0     0     0 1180 1984  0  2 98  0
 2  0      0 6767892   9696  41088    0    0     0     0 2102 1968  0 22 78  0
 0  0      0 6767768   9704  41084    0    0     0    52 1504 2022  0  8 91  1
 0  0      0 6767768   9704  41084    0    0     0     0 1059 1965  0  0 100  0
 0  0      0 6767768   9704  41088    0    0     0     0 1077 1982  0  0 100  0
 0  0      0 6767768   9704  41088    0    0     0     0 1166 1972  1  2 97  0
 0  0      0 6767768   9704  41088    0    0     0     0 1353 1990  0  6 94  0
 0  0      0 6767644   9704  41100    0    0     0     0 2531 2050  0 27 72  0
 0  0      0 6767644   9704  41100    0    0     0     0 1065 1974  0  0 100  0
 1  0      0 6767644   9704  41100    0    0     0     0 1090 1967  0  0 100  0
 0  0      0 6767644   9704  41100    0    0     0     0 1175 1971  0  2 98  0
 0  0      0 6767644   9704  41100    0    0     0     0 1321 1967  0  6 94  0
 1  0      0 6767644   9712  41100    0    0     0    52 1995 1995  0 18 81  1
 0  0      0 6767644   9712  41100    0    0     0     0 1342 1964  0  6 94  0
 0  0      0 6767644   9712  41100    0    0     0     0 1064 1974  0  0 100  0
 0  0      0 6767644   9712  41100    0    0     0     0 1163 1968  0  2 98  0
 0  0      0 6767644   9712  41100    0    0     0     0 1390 1976  1  6 94  0
 0  0      0 6767768   9712  41112    0    0     0     0 1282 2018  0  1 99  0
 0  0      0 6767768   9712  41112    0    0     0     0 2166 1972  0 24 76  0
 0  0      0 6786260   9712  41112    0    0     0     0 1092 1989  0  0 100  0
 0  0      0 6786260   9712  41112    0    0     0     0 1039 2003  0  2 98  0
 0  0      0 6774588   9712  41112    0    0     0     0 1524 2015  3  9 89  0
 0  1      0 6770124   9716  41112    0    0     0    48 1315 1990  3  0 87 11
 1  0      0 6769628   9720  41112    0    0     0     4 2196 1966  0 20 79  0
 0  0      0 6768364   9724  41108    0    0    36     0 1403 2031  1  5 94  1
 0  0      0 6768388   9724  41144    0    0     0     0 1185 1969  0  2 98  0
 0  0      0 6768388   9724  41144    0    0     0     0 1351 1975  0  6 94  0
 0  0      0 6768140   9724  41156    0    0     0     0 1261 2020  0  1 99  0
 0  0      0 6768140   9724  41156    0    0     0     0 1053 1973  0  0 100  0
 1  0      0 6768140   9724  41156    0    0     0     0 2194 1971  1 26 73  0
 0  0      0 6768140   9732  41148    0    0     0    52 1283 2088  0  4 96  0
 2  0      0 6768140   9732  41156    0    0     0     0 1184 1965  0  2 98  0
 0  0      0 6786368   9732  41156    0    0     0     0 1297 2000  0  4 97  0
 0  0      0 6786384   9732  41156    0    0     0     0  998 1949  0  0 100  0
(stop vmstat 1, run sync, restart vmstat 1)
 1  0      0 6765304   9868  41736    0    0     0     0 1239 1967  0  0 100  0
 0  0      0 6765304   9868  41736    0    0     0     0 1174 2035  0  0 100  0
 0  0      0 6765304   9868  41736    0    0     0     0 1084 1976  0  0 100  0
 1  0      0 6765172   9868  41736    0    0     0     0 1100 1975  0  0 100  0
 0  0      0 6765304   9868  41760    0    0     0     0 1501 1997  1  1 98  0
 0  0      0 6765304   9868  41760    0    0     0     0 1092 1964  0  0 100  0
 1  0      0 6765304   9868  41760    0    0     0     0 1096 1963  0  0 100  0
 0  0      0 6765304   9868  41760    0    0     0     0 1083 1967  0  0 100  0
 0  0      0 6765304   9876  41752    0    0     0    76 1101 1980  0  0 99  1
 0  0      0 6765304   9876  41760    0    0     0     4 1199 2097  0  2 98  0
 0  0      0 6765304   9876  41760    0    0     0     0 1055 1964  0  0 100  0
 0  0      0 6765304   9876  41760    0    0     0     0 1064 1977  0  0 100  0
 0  0      0 6765304   9876  41760    0    0     0     0 1073 1968  0  0 100  0
 0  0      0 6765304   9876  41760    0    0     0     0 1080 1976  0  0 100  0
 0  0      0 6765156   9884  41772    0    0     0    20 1329 2034  1  1 98  1
 0  0      0 6765180   9884  41780    0    0     0     0 1097 1971  0  1 99  0
 0  0      0 6765180   9884  41780    0    0     0     0 1073 1960  0  0 100  0
 0  0      0 6765180   9884  41780    0    0     0     0 1079 1976  0  0 100  0
 0  0      0 6765172   9884  41780    0    0     0     0 1120 1980  0  0 100  0
 0  0      0 6765180   9892  41772    0    0     0    60 1097 1982  0  0 100  0
 0  0      0 6765180   9892  41780    0    0     0     0 1067 1969  0  0 100  0
 0  0      0 6765180   9892  41780    0    0     0     0 1063 1973  0  0 100  0
 0  0      0 6765180   9892  41780    0    0     0     0 1064 1969  0  1 100  0

Hmmm.  I'll watch it and see if it climbs back up again.  That would mean
that for now, echo 2 > /proc/sys/vm/drop_caches ; sync is a workaround
for the problem we're seeing with system CPU slowly creeping up since
2.6.36.  I have no idea if this affects the sudden flush process spinlock
contention and attempted writeback without much progress during normal
load, but it certainly changes things here.

http://0x.ca/sim/ref/2.6.37/cpu_after_drop_caches_2_and_sync.png

Simon-
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