The patch titled
posix-timers: RCU conversion
has been added to the -mm tree. Its filename is
posix-timers-rcu-conversion.patch
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Subject: posix-timers: RCU conversion
From: Eric Dumazet <eric.dumazet@xxxxxxxxx>
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@xxxxxxxx>
Signed-off-by: Eric Dumazet <eric.dumazet@xxxxxxxxx>
Tested-by: Ben Nagy <ben@xxxxxxxx>
Cc: Avi Kivity <avi@xxxxxxxxxx>
Cc: Thomas Gleixner <tglx@xxxxxxxxxxxxx>
Cc: John Stultz <johnstul@xxxxxxxxxx>
Cc: Richard Cochran <richard.cochran@xxxxxxxxxx>
Cc: Paul E. McKenney <paulmck@xxxxxxxxxxxxxxxxxx>
Signed-off-by: Andrew Morton <akpm@xxxxxxxxxxxxxxxxxxxx>
---
include/linux/posix-timers.h | 1 +
kernel/posix-timers.c | 25 ++++++++++++++-----------
2 files changed, 15 insertions(+), 11 deletions(-)
diff -puN include/linux/posix-timers.h~posix-timers-rcu-conversion include/linux/posix-timers.h
--- a/include/linux/posix-timers.h~posix-timers-rcu-conversion
+++ a/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 -puN kernel/posix-timers.c~posix-timers-rcu-conversion kernel/posix-timers.c
--- a/kernel/posix-timers.c~posix-timers-rcu-conversion
+++ a/kernel/posix-timers.c
@@ -491,6 +491,13 @@ static struct k_itimer * alloc_posix_tim
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
}
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;
}
_
Patches currently in -mm which might be from eric.dumazet@xxxxxxxxx are
linux-next.patch
vfs-avoid-large-kmallocs-for-the-fdtable.patch
posix-timers-rcu-conversion.patch
net-convert-%p-usage-to-%pk.patch
percpu_counter-change-return-value-and-add-comments.patch
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