posix-timers: RCU conversion
authorEric Dumazet <eric.dumazet@gmail.com>
Tue, 24 May 2011 09:12:58 +0000 (11:12 +0200)
committerThomas Gleixner <tglx@linutronix.de>
Tue, 24 May 2011 10:10:51 +0000 (12:10 +0200)
commit8af088710d1eb3c980e0ef3779c8d47f3f217b48
treee122a2e65684f0a40d263ba73afe3d54a2c5993a
parentd762f4383100c2a87b1a3f2d678cd3b5425655b4
posix-timers: RCU conversion

Ben Nagy reported a scalability problem with KVM/QEMU that hit very hard
a single spinlock (idr_lock) in posix-timers code, on its 48 core
machine.

Even on a 16 cpu machine (2x4x2), a single test can show 98% of cpu time
used in ticket_spin_lock, from lock_timer

Ref: http://www.spinics.net/lists/kvm/msg51526.html

Switching to RCU is quite easy, IDR being already RCU ready. idr_lock
should be locked only for an insert/delete, not a lookup.

Benchmark on a 2x4x2 machine, 16 processes calling timer_gettime().

Before :

real    1m18.669s
user    0m1.346s
sys     1m17.180s

After :

real    0m3.296s
user    0m1.366s
sys     0m1.926s

Reported-by: Ben Nagy <ben@iagu.net>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
Tested-by: Ben Nagy <ben@iagu.net>
Cc: Oleg Nesterov <oleg@redhat.com>
Cc: Avi Kivity <avi@redhat.com>
Cc: John Stultz <johnstul@us.ibm.com>
Cc: Richard Cochran <richard.cochran@omicron.at>
Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
Cc: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
include/linux/posix-timers.h
kernel/posix-timers.c