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posix-timers: RCU conversion

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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>
This commit is contained in:
Eric Dumazet 2011-05-24 11:12:58 +02:00 committed by Thomas Gleixner
parent d762f43831
commit 8af088710d
2 changed files with 15 additions and 11 deletions
Show stats Download patch file Download diff file Expand all files Collapse all files

1
include/linux/posix-timers.h
View file

@ -82,6 +82,7 @@ struct k_itimer {
unsigned long expires;
} mmtimer;
struct alarm alarmtimer;
struct rcu_head rcu;
} it;
};

25
kernel/posix-timers.c
View file

@ -491,6 +491,13 @@ static struct k_itimer * alloc_posix_timer(void)
return tmr;
}
static void k_itimer_rcu_free(struct rcu_head *head)
{
struct k_itimer *tmr = container_of(head, struct k_itimer, it.rcu);
kmem_cache_free(posix_timers_cache, tmr);
}
#define IT_ID_SET 1
#define IT_ID_NOT_SET 0
static void release_posix_timer(struct k_itimer *tmr, int it_id_set)
@ -503,7 +510,7 @@ static void release_posix_timer(struct k_itimer *tmr, int it_id_set)
}
put_pid(tmr->it_pid);
sigqueue_free(tmr->sigq);
kmem_cache_free(posix_timers_cache, tmr);
call_rcu(&tmr->it.rcu, k_itimer_rcu_free);
}
static struct k_clock *clockid_to_kclock(const clockid_t id)
@ -631,22 +638,18 @@ SYSCALL_DEFINE3(timer_create, const clockid_t, which_clock,
static struct k_itimer *__lock_timer(timer_t timer_id, unsigned long *flags)
{
struct k_itimer *timr;
/*
* Watch out here. We do a irqsave on the idr_lock and pass the
* flags part over to the timer lock. Must not let interrupts in
* while we are moving the lock.
*/
spin_lock_irqsave(&idr_lock, *flags);
rcu_read_lock();
timr = idr_find(&posix_timers_id, (int)timer_id);
if (timr) {
spin_lock(&timr->it_lock);
spin_lock_irqsave(&timr->it_lock, *flags);
if (timr->it_signal == current->signal) {
spin_unlock(&idr_lock);
rcu_read_unlock();
return timr;
}
spin_unlock(&timr->it_lock);
spin_unlock_irqrestore(&timr->it_lock, *flags);
}
spin_unlock_irqrestore(&idr_lock, *flags);
rcu_read_unlock();
return NULL;
}