From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1753054Ab1AZLQ2 (ORCPT ); Wed, 26 Jan 2011 06:16:28 -0500 Received: from mail-wy0-f174.google.com ([74.125.82.174]:59782 "EHLO mail-wy0-f174.google.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1751807Ab1AZLQ1 (ORCPT ); Wed, 26 Jan 2011 06:16:27 -0500 DomainKey-Signature: a=rsa-sha1; c=nofws; d=gmail.com; s=gamma; h=subject:from:to:cc:in-reply-to:references:content-type:date :message-id:mime-version:x-mailer:content-transfer-encoding; b=vA0vZf32BV0DJkTl4TC8B2VzLE6nURby0zsy6czi6PGXIqyuR4NOCv22lLnw/Eacs8 GN0OdFHf16Sz/bBo12eeIZi/5AtIp9vvJkgk6Ht4V+NjLIpllurmuvz/IYqWIxgFJ8C3 U7h3yD2+D+lKSqLYnQWZeaosyZlzHbgM26Gq4= Subject: Re: sys_epoll_wait high CPU load in 2.6.37 From: Eric Dumazet To: Simon Kirby Cc: linux-kernel@vger.kernel.org, Shawn Bohrer , Davide Libenzi , Andrew Morton In-Reply-To: <1296026298.2633.19.camel@edumazet-laptop> References: <20110126000932.GA23089@hostway.ca> <1296026298.2633.19.camel@edumazet-laptop> Content-Type: text/plain; charset="UTF-8" Date: Wed, 26 Jan 2011 12:16:17 +0100 Message-ID: <1296040578.2899.59.camel@edumazet-laptop> Mime-Version: 1.0 X-Mailer: Evolution 2.30.3 Content-Transfer-Encoding: 8bit Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Le mercredi 26 janvier 2011 à 08:18 +0100, Eric Dumazet a écrit : > Le mardi 25 janvier 2011 à 16:09 -0800, Simon Kirby a écrit : > > 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- > > Yes, 95aac7b1cd is the problem, but anvil should use a 0 (no) timeout > instead of 2147483647 ms : epoll_wait() doesnt have to arm a timer in > this case, it is a bit faster. > > Slowness comes from timespec_add_ns() : This one assumed small 'ns' argument, since it wants to avoid a divide instruction. static __always_inline void timespec_add_ns(struct timespec *a, u64 ns) { a->tv_sec += __iter_div_u64_rem(a->tv_nsec + ns, NSEC_PER_SEC, &ns); a->tv_nsec = ns; } We should do this differently for epoll usage ;) Please try following patch : [PATCH] epoll: epoll_wait() should be careful in timespec_add_ns use commit 95aac7b1cd224f (epoll: make epoll_wait() use the hrtimer range feature) added a performance regression because it used timespec_add_ns() with potential very large 'ns' values. Reported-by: Simon Kirby Signed-off-by: Eric Dumazet CC: Shawn Bohrer CC: Davide Libenzi CC: Andrew Morton --- fs/eventpoll.c | 4 +++- 1 file changed, 3 insertions(+), 1 deletion(-) diff --git a/fs/eventpoll.c b/fs/eventpoll.c index cc8a9b7..7ec0890 100644 --- a/fs/eventpoll.c +++ b/fs/eventpoll.c @@ -1126,7 +1126,9 @@ static int ep_poll(struct eventpoll *ep, struct epoll_event __user *events, if (timeout > 0) { ktime_get_ts(&end_time); - timespec_add_ns(&end_time, (u64)timeout * NSEC_PER_MSEC); + end_time.tv_sec += timeout / MSEC_PER_SEC; + timeout %= MSEC_PER_SEC; + timespec_add_ns(&end_time, timeout * NSEC_PER_MSEC); slack = select_estimate_accuracy(&end_time); to = &expires; *to = timespec_to_ktime(end_time);