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SubjectRe: [PATCH] posix-timers: RCU conversion
On Sun, Apr 03, 2011 at 06:54:20PM +0200, Eric Dumazet wrote:
> Andrew, I submitted this patch some time ago and got no feedback from
> Thomas, John, Richard or Paul.
>
> https://lkml.org/lkml/2011/3/22/29
>
> Could you please take it in mm ?
>
> Thanks
>
> [PATCH] 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

Good stuff!

Reviewed-by: Paul E. McKenney <paulmck@linux.vnet.ibm.com>

> Reported-by: Ben Nagy <ben@iagu.net>
> Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
> Tested-by: Ben Nagy <ben@iagu.net>
> Cc: Avi Kivity <avi@redhat.com>
> Cc: Thomas Gleixner <tglx@linutronix.de>
> Cc: John Stultz <johnstul@us.ibm.com>
> Cc: Richard Cochran <richard.cochran@omicron.at>
> Cc: Paul E. McKenney <paulmck@linux.vnet.ibm.com>
> ---
> include/linux/posix-timers.h | 1 +
> kernel/posix-timers.c | 25 ++++++++++++++-----------
> 2 files changed, 15 insertions(+), 11 deletions(-)
>
> diff --git a/include/linux/posix-timers.h b/include/linux/posix-timers.h
> index d51243a..5dc27ca 100644
> --- a/include/linux/posix-timers.h
> +++ b/include/linux/posix-timers.h
> @@ -81,6 +81,7 @@ struct k_itimer {
> unsigned long expires;
> } mmtimer;
> } it;
> + struct rcu_head rcu;
> };
>
> struct k_clock {
> diff --git a/kernel/posix-timers.c b/kernel/posix-timers.c
> index 4c01249..acb9be9 100644
> --- a/kernel/posix-timers.c
> +++ b/kernel/posix-timers.c
> @@ -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, 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->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;
> }
>
>
> --
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