linux-kernel - Re: sys_epoll_wait high CPU load in 2.6.37

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Date:	Wed, 26 Jan 2011 12:16:17 +0100
From:	Eric Dumazet <eric.dumazet@...il.com>
To:	Simon Kirby <sim@...tway.ca>
Cc:	linux-kernel@...r.kernel.org,
	Shawn Bohrer <shawn.bohrer@...il.com>,
	Davide Libenzi <davidel@...ilserver.org>,
	Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: sys_epoll_wait high CPU load in 2.6.37

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 <sim@...tway.ca>
Signed-off-by: Eric Dumazet <eric.dumazet@...il.com>
CC: Shawn Bohrer <shawn.bohrer@...il.com>
CC: Davide Libenzi <davidel@...ilserver.org>
CC: Andrew Morton <akpm@...ux-foundation.org>
---
 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);


--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/