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Date: Mon, 17 Jun 2013 17:08:01 -0700
From: Tim Chen <tim.c.chen@...ux.intel.com>
To: Davidlohr Bueso <davidlohr.bueso@...com>
Cc: Alex Shi <alex.shi@...el.com>, Ingo Molnar <mingo@...e.hu>,
Rik van Riel <riel@...hat.com>,
Peter Zijlstra <a.p.zijlstra@...llo.nl>,
Andrea Arcangeli <aarcange@...hat.com>,
Mel Gorman <mgorman@...e.de>, Andi Kleen <andi@...stfloor.org>,
Andrew Morton <akpm@...ux-foundation.org>,
Michel Lespinasse <walken@...gle.com>,
"Wilcox, Matthew R" <matthew.r.wilcox@...el.com>,
Dave Hansen <dave.hansen@...el.com>,
"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
"linux-mm@...ck.org" <linux-mm@...ck.org>
Subject: Re: Performance regression from switching lock to rw-sem for
anon-vma tree
On Mon, 2013-06-17 at 16:35 -0700, Davidlohr Bueso wrote:
> On Tue, 2013-06-18 at 07:20 +0800, Alex Shi wrote:
> > On 06/18/2013 12:22 AM, Davidlohr Bueso wrote:
> > > After a lot of benchmarking, I finally got the ideal results for aim7,
> > > so far: this patch + optimistic spinning with preemption disabled. Just
> > > like optimistic spinning, this patch by itself makes little to no
> > > difference, yet combined is where we actually outperform 3.10-rc5. In
> > > addition, I noticed extra throughput when disabling preemption in
> > > try_optimistic_spin().
> > >
> > > With i_mmap as a rwsem and these changes I could see performance
> > > benefits for alltests (+14.5%), custom (+17%), disk (+11%), high_systime
> > > (+5%), shared (+15%) and short (+4%), most of them after around 500
> > > users, for fewer users, it made little to no difference.
> >
> > A pretty good number. what's the cpu number in your machine? :)
>
> 8-socket, 80 cores (ht off)
>
>
David,
I wonder if you are interested to try the experimental patch below.
It tries to avoid unnecessary writes to the sem->count when we are
going to fail the down_write by executing rwsem_down_write_failed_s
instead of rwsem_down_write_failed. It should further reduce the
cache line bouncing. It didn't make a difference for my
workload. Wonder if it may help yours more in addition to the
other two patches. Right now the patch is an ugly hack. I'll merge
rwsem_down_write_failed_s and rwsem_down_write_failed into one
function if this approach actually helps things.
I'll clean these three patches after we have some idea of their
effectiveness.
Thanks.
Tim
Signed-off-by: Tim Chen <tim.c.chen@...ux.intel.com>
---
commit 04c8ad3f21861746d5b7fff55a6ef186a4dd0765
Author: Tim Chen <tim.c.chen@...ux.intel.com>
Date: Mon Jun 10 04:50:04 2013 -0700
Try skip write to rwsem->count when we have active lockers
diff --git a/include/linux/rwsem.h b/include/linux/rwsem.h
index 0616ffe..83f9184 100644
--- a/include/linux/rwsem.h
+++ b/include/linux/rwsem.h
@@ -33,6 +33,7 @@ struct rw_semaphore {
extern struct rw_semaphore *rwsem_down_read_failed(struct rw_semaphore *sem);
extern struct rw_semaphore *rwsem_down_write_failed(struct rw_semaphore *sem);
+extern struct rw_semaphore *rwsem_down_write_failed_s(struct rw_semaphore *sem);
extern struct rw_semaphore *rwsem_wake(struct rw_semaphore *);
extern struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem);
diff --git a/kernel/rwsem.c b/kernel/rwsem.c
index cfff143..188f6ea 100644
--- a/kernel/rwsem.c
+++ b/kernel/rwsem.c
@@ -42,12 +42,22 @@ EXPORT_SYMBOL(down_read_trylock);
/*
* lock for writing
*/
+
+static void ___down_write(struct rw_semaphore *sem)
+{
+ if (sem->count & RWSEM_ACTIVE_MASK) {
+ rwsem_down_write_failed_s(sem);
+ return;
+ }
+ __down_write(sem);
+}
+
void __sched down_write(struct rw_semaphore *sem)
{
might_sleep();
rwsem_acquire(&sem->dep_map, 0, 0, _RET_IP_);
- LOCK_CONTENDED(sem, __down_write_trylock, __down_write);
+ LOCK_CONTENDED(sem, __down_write_trylock, ___down_write);
}
EXPORT_SYMBOL(down_write);
diff --git a/lib/rwsem.c b/lib/rwsem.c
index 19c5fa9..25143b5 100644
--- a/lib/rwsem.c
+++ b/lib/rwsem.c
@@ -248,6 +248,63 @@ struct rw_semaphore __sched *rwsem_down_write_failed(struct rw_semaphore *sem)
return sem;
}
+struct rw_semaphore __sched *rwsem_down_write_failed_s(struct rw_semaphore *sem)
+{
+ long count, adjustment = 0;
+ struct rwsem_waiter waiter;
+ struct task_struct *tsk = current;
+
+ /* set up my own style of waitqueue */
+ waiter.task = tsk;
+ waiter.type = RWSEM_WAITING_FOR_WRITE;
+
+ raw_spin_lock_irq(&sem->wait_lock);
+ if (list_empty(&sem->wait_list))
+ adjustment += RWSEM_WAITING_BIAS;
+ list_add_tail(&waiter.list, &sem->wait_list);
+
+ /* If there were already threads queued before us and there are no
+ * active writers, the lock must be read owned; so we try to wake
+ * any read locks that were queued ahead of us. */
+ if (adjustment == 0) {
+ if (sem->count > RWSEM_WAITING_BIAS)
+ sem = __rwsem_do_wake(sem, RWSEM_WAKE_READERS);
+ } else
+ count = rwsem_atomic_update(adjustment, sem);
+
+ /* wait until we successfully acquire the lock */
+ set_task_state(tsk, TASK_UNINTERRUPTIBLE);
+ while (true) {
+ if (!(sem->count & RWSEM_ACTIVE_MASK)) {
+ /* Try acquiring the write lock. */
+ count = RWSEM_ACTIVE_WRITE_BIAS;
+ if (!list_is_singular(&sem->wait_list))
+ count += RWSEM_WAITING_BIAS;
+
+ if (sem->count == RWSEM_WAITING_BIAS &&
+ cmpxchg(&sem->count, RWSEM_WAITING_BIAS, count) ==
+ RWSEM_WAITING_BIAS)
+ break;
+ }
+
+ raw_spin_unlock_irq(&sem->wait_lock);
+
+ /* Block until there are no active lockers. */
+ do {
+ schedule();
+ set_task_state(tsk, TASK_UNINTERRUPTIBLE);
+ } while ((count = sem->count) & RWSEM_ACTIVE_MASK);
+
+ raw_spin_lock_irq(&sem->wait_lock);
+ }
+
+ list_del(&waiter.list);
+ raw_spin_unlock_irq(&sem->wait_lock);
+ tsk->state = TASK_RUNNING;
+
+ return sem;
+}
+
/*
* handle waking up a waiter on the semaphore
* - up_read/up_write has decremented the active part of count if we come here
@@ -289,5 +346,6 @@ struct rw_semaphore *rwsem_downgrade_wake(struct rw_semaphore *sem)
EXPORT_SYMBOL(rwsem_down_read_failed);
EXPORT_SYMBOL(rwsem_down_write_failed);
+EXPORT_SYMBOL(rwsem_down_write_failed_s);
EXPORT_SYMBOL(rwsem_wake);
EXPORT_SYMBOL(rwsem_downgrade_wake);
--
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