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Subject[RFC] posix-timers: RCU conversion
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Le lundi 21 mars 2011 à 23:57 +0545, Ben Nagy a écrit :
> On Mon, Mar 21, 2011 at 10:57 PM, Eric Dumazet <eric.dumazet@gmail.com> wrote:
> > Le lundi 21 mars 2011 à 19:02 +0200, Avi Kivity a écrit :
> >
> >> Any ideas on how to fix it? We could pre-allocate IDs and batch them in
> >> per-cpu caches, but it seems like a lot of work.
> >>
> >
> > Hmm, I dont know what syscalls kvm do, but even a timer_gettime() has to
> > lock this idr_lock.
> >
> > Sounds crazy, and should be fixed in kernel code, not kvm ;)
>
> Hi,
>
> I'll need to work out a way I can make the perf.data available
> (~100M), but here's the summary
>
> http://paste.ubuntu.com/583425/
>
> And here's the summary of the summary
>
> # Overhead Command Shared Object
> Symbol
> # ........ ............... ....................
> ..........................................
> #
> 71.86% kvm [kernel.kallsyms] [k] __ticket_spin_lock
> |
> --- __ticket_spin_lock
> |
> |--54.19%-- default_spin_lock_flags
> | _raw_spin_lock_irqsave
> | |
> | --54.14%-- __lock_timer
> | |
> | |--31.92%-- sys_timer_gettime
> | | system_call_fastpath
> | |
> | --22.22%-- sys_timer_settime
> | system_call_fastpath
> |
> |--15.66%-- _raw_spin_lock
> [...]
>
> Which I guess seems to match what Eric just said.
>
> I'll post a link to the full data tomorrow. Many thanks for the help
> so far. If it's a kernel scaling limit then I guess we just wait until
> the kernel gets better. :S I'll check it out with a linux guest
> tomorrow anyway.

Here is a quick & dirty patch to lower idr_lock use. You could try it
eventually ;)

Thanks

[RFC] 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.

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

Switching to RCU should be quite easy, IDR being already RCU ready.

idr_lock should be locked only before an insert/delete, not a find.

Reported-by: Ben Nagy <ben@iagu.net>
Signed-off-by: Eric Dumazet <eric.dumazet@gmail.com>
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>
---
include/linux/posix-timers.h | 1 +
kernel/posix-timers.c | 26 ++++++++++++++------------
2 files changed, 15 insertions(+), 12 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..e2a823a 100644
--- a/kernel/posix-timers.c
+++ b/kernel/posix-timers.c
@@ -491,6 +491,11 @@ static struct k_itimer * alloc_posix_timer(void)
return tmr;
}

+static void k_itimer_rcu_free(struct rcu_head *head)
+{
+ kmem_cache_free(posix_timers_cache, container_of(head, struct k_itimer, rcu));
+}
+
#define IT_ID_SET 1
#define IT_ID_NOT_SET 0
static void release_posix_timer(struct k_itimer *tmr, int it_id_set)
@@ -499,11 +504,12 @@ static void release_posix_timer(struct k_itimer *tmr, int it_id_set)
unsigned long flags;
spin_lock_irqsave(&idr_lock, flags);
idr_remove(&posix_timers_id, tmr->it_id);
+ tmr->it_id = -1;
spin_unlock_irqrestore(&idr_lock, flags);
}
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 +637,18 @@ 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);
- if (timr->it_signal == current->signal) {
- spin_unlock(&idr_lock);
+ spin_lock_irqsave(&timr->it_lock, *flags);
+ if (timr->it_id == (int)timer_id && timr->it_signal == current->signal) {
+ 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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