linux-kernel - sys_epoll_wait high CPU load in 2.6.37

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Date:	Tue, 25 Jan 2011 16:09:32 -0800
From:	Simon Kirby <sim@...tway.ca>
To:	linux-kernel@...r.kernel.org,
	Shawn Bohrer <shawn.bohrer@...il.com>,
	Eric Dumazet <eric.dumazet@...il.com>
Subject: sys_epoll_wait high CPU load in 2.6.37

Hello!

Since upgrading 2.6.36 -> 2.6.37, dovecot's "anvil" process seems to end
up taking a lot more time in "top", and "perf top" shows output like this
(system-wide):

  samples  pcnt function                      DSO
  _______ _____ _____________________________ __________________________

  2405.00 68.8% sys_epoll_wait                [kernel.kallsyms]
    33.00  0.9% mail_cache_lookup_iter_next   libdovecot-storage.so.0.0.0
    30.00  0.9% _raw_spin_lock                [kernel.kallsyms]
    ...etc...

It only wakes up 5-10 times per second or so (on this box), and does
stuff like this:

epoll_wait(12, {{EPOLLIN, {u32=19417616, u64=19417616}}}, 25, 2147483647) = 1
read(29, "PENALTY-GET\t192.168.31.10\n"..., 738) = 26
write(29, "0 0\n"..., 4)                = 4
epoll_wait(12, {{EPOLLIN, {u32=19395632, u64=19395632}}}, 25, 2147483647) = 1
read(18, "LOOKUP\tpop3/192.168.31.10/tshield"..., 668) = 58
write(18, "0\n"..., 2)                  = 2
epoll_wait(12, {{EPOLLIN, {u32=19373072, u64=19373072}}}, 25, 2147483647) = 1
read(7, "CONNECT\t3490\tpop3/192.168.31.10/t"..., 254) = 64
epoll_wait(12, {{EPOLLIN, {u32=19373072, u64=19373072}}}, 25, 2147483647) = 1
read(7, "DISCONNECT\t3482\tpop3/192.168.31.1"..., 190) = 62

Anything obvious here?  anvil talks over UNIX sockets to the rest of
dovecot, and uses epoll_wait.  So, suspect commits might be:

95aac7b1cd224f568fb83937044cd303ff11b029
5456f09aaf88731e16dbcea7522cb330b6846415
or other bits from
 git log v2.6.36..v2.6.37 net/unix/af_unix.c fs/eventpoll.c

I suspect it has something to do with that "infinite value" check removal
in that first commit.  It doesn't show up easily on a test box, but I can
try reverting 95aac7b1cd in production if it's not obvious.

Simon-
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