Donate to e Foundation | Murena handsets with /e/OS | Own a part of Murena! Learn more

Commit 8af08871 authored by Eric Dumazet's avatar Eric Dumazet Committed by Thomas Gleixner
Browse files

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: default avatarBen Nagy <ben@iagu.net>
Signed-off-by: default avatarEric Dumazet <eric.dumazet@gmail.com>
Tested-by: default avatarBen 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: default avatarThomas Gleixner <tglx@linutronix.de>
parent d762f438

include/linux/posix-timers.h

+1 −0
Original line number Diff line number Diff line
@@ -82,6 +82,7 @@ struct k_itimer { 
			unsigned long expires;

		} mmtimer;

		struct alarm alarmtimer;

		struct rcu_head rcu;

	} it;

};

kernel/posix-timers.c

+14 −11
Original line number Diff line number Diff line
@@ -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 @@ out: 
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;

}