On Thu, 2013-06-27 at 13:53 -0700, Tim Chen wrote:
> On Thu, 2013-06-27 at 10:36 +0200, Ingo Molnar wrote:
> > * Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx> wrote:
> >
> > > On Wed, 2013-06-26 at 14:36 -0700, Tim Chen wrote:
> > > > On Wed, 2013-06-26 at 11:51 +0200, Ingo Molnar wrote:
> > > > > * Tim Chen <tim.c.chen@xxxxxxxxxxxxxxx> wrote:
> > > > >
> > > > > > On Wed, 2013-06-19 at 09:53 -0700, Tim Chen wrote:
> > > > > > > On Wed, 2013-06-19 at 15:16 +0200, Ingo Molnar wrote:
> > > > > > >
> > > > > > > > > vmstat for mutex implementation:
> > > > > > > > > procs -----------memory---------- ---swap-- -----io---- --system-- -----cpu-----
> > > > > > > > > r b swpd free buff cache si so bi bo in cs us sy id wa st
> > > > > > > > > 38 0 0 130957920 47860 199956 0 0 0 56 236342 476975 14 72 14 0 0
> > > > > > > > > 41 0 0 130938560 47860 219900 0 0 0 0 236816 479676 14 72 14 0 0
> > > > > > > > >
> > > > > > > > > vmstat for rw-sem implementation (3.10-rc4)
> > > > > > > > > procs -----------memory---------- ---swap-- -----io---- --system-- -----cpu-----
> > > > > > > > > r b swpd free buff cache si so bi bo in cs us sy id wa st
> > > > > > > > > 40 0 0 130933984 43232 202584 0 0 0 0 321817 690741 13 71 16 0 0
> > > > > > > > > 39 0 0 130913904 43232 224812 0 0 0 0 322193 692949 13 71 16 0 0
> > > > > > > >
> > > > > > > > It appears the main difference is that the rwsem variant context-switches
> > > > > > > > about 36% more than the mutex version, right?
> > > > > > > >
> > > > > > > > I'm wondering how that's possible - the lock is mostly write-locked,
> > > > > > > > correct? So the lock-stealing from Davidlohr Bueso and Michel Lespinasse
> > > > > > > > ought to have brought roughly the same lock-stealing behavior as mutexes
> > > > > > > > do, right?
> > > > > > > >
> > > > > > > > So the next analytical step would be to figure out why rwsem lock-stealing
> > > > > > > > is not behaving in an equivalent fashion on this workload. Do readers come
> > > > > > > > in frequently enough to disrupt write-lock-stealing perhaps?
> > > > > >
> > > > > > Ingo,
> > > > > >
> > > > > > I did some instrumentation on the write lock failure path. I found that
> > > > > > for the exim workload, there are no readers blocking for the rwsem when
> > > > > > write locking failed. The lock stealing is successful for 9.1% of the
> > > > > > time and the rest of the write lock failure caused the writer to go to
> > > > > > sleep. About 1.4% of the writers sleep more than once. Majority of the
> > > > > > writers sleep once.
> > > > > >
> > > > > > It is weird that lock stealing is not successful more often.
> > > > >
> > > > > For this to be comparable to the mutex scalability numbers you'd have to
> > > > > compare wlock-stealing _and_ adaptive spinning for failed-wlock rwsems.
> > > > >
> > > > > Are both techniques applied in the kernel you are running your tests on?
> > > > >
> > > >
> > > > Ingo,
> > > >
> > > > The previous experiment was done on a kernel without spinning.
> > > > I've redone the testing on two kernel for a 15 sec stretch of the
> > > > workload run. One with the adaptive (or optimistic)
> > > > spinning and the other without. Both have the patches from Alex to avoid
> > > > cmpxchg induced cache bouncing.
> > > >
> > > > With the spinning, I sleep much less for lock acquisition (18.6% vs 91.58%).
> > > > However, I've got doubling of write lock acquisition getting
> > > > blocked. So that offset the gain from spinning which may be why
> > > > I didn't see gain for this particular workload.
> > > >
> > > > No Opt Spin Opt Spin
> > > > Writer acquisition blocked count 3448946 7359040
> > > > Blocked by reader 0.00% 0.55%
> > > > Lock acquired first attempt (lock stealing) 8.42% 16.92%
> > > > Lock acquired second attempt (1 sleep) 90.26% 17.60%
> > > > Lock acquired after more than 1 sleep 1.32% 1.00%
> > > > Lock acquired with optimistic spin N/A 64.48%
> > > >
> > >
> > > Adding also the mutex statistics for the 3.10-rc4 kernel with mutex
> > > implemenation of lock for anon_vma tree. Wonder if Ingo has any
> > > insight on why mutex performs better from these stats.
> > >
> > > Mutex acquisition blocked count 14380340
> > > Lock acquired in slowpath (no sleep) 0.06%
> > > Lock acquired in slowpath (1 sleep) 0.24%
> > > Lock acquired in slowpath more than 1 sleep 0.98%
> > > Lock acquired with optimistic spin 99.6%
> >
> > This is how I interpret the stats:
> >
> > It does appear that in the mutex case we manage to acquire via spinning
> > with a very high percentage - i.e. it essentialy behaves as a spinlock.
> >
> > That is actually good news in a way, because it makes it rather simple how
> > rwsems should behave in this case: since they have no substantial
> > read-locking aspect in this workload, the down_write()/up_write()s should
> > essentially behave like spinlocks as well, right?
>
> Yes, it makes sense.
>
> >
> > Yet in the rwsem-spinning case the stats show that we only acquire the
> > lock via spinning in 65% of the cases, plus we lock-steal in 16.9% of the
> > cases:
> >
> > Because lock stealing is essentially a single-spin spinning as well:
> >
> > > > Lock acquired first attempt (lock stealing) ...... 16.92%
> >
> > So rwsems in this case behave like spinlocks in 65%+16.9% == 81.9% of the
> > time.
> >
> > What remains is the sleeping component:
> >
> > > > Lock acquired second attempt (1 sleep) ...... 17.60%
> >
> > Yet the 17.6% sleep percentage is still much higher than the 1% in the
> > mutex case. Why doesn't spinning work - do we time out of spinning
> > differently?
>
> I have some stats for the 18.6% cases (including 1% more than
> 1 sleep cases) that go to sleep and failed optimistic spinning.
> There are 3 abort points in the rwsem_optimistic_spin code:
>
> 1. 11.8% is due to abort point #1, where we don't find an owner and
> assumed that probably a reader owned lock as we've just tried
> to acquire lock previously for lock stealing. I think I will need
> to actually check the sem->count to make sure we have reader owned lock
> before aborting spin.
I tried some tweaking that checks sem->count for read owned lock.
Even though it reduces the percentage of acquisitions that
need sleeping by 8.14% (from 18.6% to 10.46%), it increases the writer
acquisition blocked count by 11%. This change still doesn't boost
throughput and has a tiny regression for the workload.
Opt Spin Opt Spin
(with tweak)
Writer acquisition blocked count 7359040 8168006
Blocked by reader 0.55% 0.52%
Lock acquired first attempt (lock stealing) 16.92% 19.70%
Lock acquired second attempt (1 sleep) 17.60% 9.32%
Lock acquired after more than 1 sleep 1.00% 1.14%
Lock acquired with optimistic spin 64.48% 69.84%
Optimistic spin abort 1 11.77% 1.14%
Optimistic spin abort 2 6.81% 9.22%
Optimistic spin abort 3 0.02% 0.10%
--- a/lib/rwsem.c
+++ b/lib/rwsem.c
@@ -221,16 +221,21 @@ static inline bool rwsem_can_spin_on_owner(struct rw_semaphore *sem)
{
int retval;
struct task_struct *owner;
+ long count;
rcu_read_lock();
owner = ACCESS_ONCE(sem->owner);
- /* Spin only if active writer running */
+ /* Don't spin if active writer not running or reader holds lock */
if (owner)
retval = owner->on_cpu;
- else
- retval = false;
-
+ else {
+ count = ACCESS_ONCE(sem->count);
+ if (count > RWSEM_WAITING_BIAS)
+ retval = false;
+ else
+ retval = true;
+ }
rcu_read_unlock();
/*
Thanks.
Tim
> 2. 6.8% is due to abort point #2, where the mutex owner switches
> to another writer or we need rescheduling.
>
> 3. Minuscule amount due to abort point #3, where we don't have
> a owner of the lock but need rescheduling
>
> int rwsem_optimistic_spin(struct rw_semaphore *sem)
> {
> struct task_struct *owner;
> int ret = 0;
>
> /* sem->wait_lock should not be held when doing optimistic spinning */
> if (!rwsem_can_spin_on_owner(sem))
> return ret; <------------------------------- abort (1)
>
> preempt_disable();
> for (;;) {
> owner = ACCESS_ONCE(sem->owner);
> if (owner && !rwsem_spin_on_owner(sem, owner))
> break; <--------------------------- abort (2)
>
> /* wait_lock will be acquired if write_lock is obtained */
> if (rwsem_try_write_lock(sem->count, true, sem)) {
> ret = 1;
> break;
> }
>
> /*
> * When there's no owner, we might have preempted between the
> * owner acquiring the lock and setting the owner field. If
> * we're an RT task that will live-lock because we won't let
> * the owner complete.
> */
> if (!owner && (need_resched() || rt_task(current)))
> break; <---------------------------- abort (3)
>
> /*
> * The cpu_relax() call is a compiler barrier which forces
> * everything in this loop to be re-loaded. We don't need
> * memory barriers as we'll eventually observe the right
> * values at the cost of a few extra spins.
> */
> arch_mutex_cpu_relax();
>
> }
>
> preempt_enable();
> return ret;
>
> See the other thread for complete patch of rwsem optimistic spin code:
> https://lkml.org/lkml/2013/6/26/692
>
> Any suggestions on tweaking this is appreciated.
>
> > Is there some other aspect that defeats optimistic spinning and forces the
> > slowpath and creates sleeping, scheduling and thus extra overhead?
> >
> There are other aspects that are different from mutex in my optimistic
> spinning for rwsem:
>
> 1. Mutex spinning has MCS lock.
> I have disabled MCS lock in mutex and get same profile and
> performance for my tests. So this is probably not a reason for
> performance difference.
>
> 2. Preemption was disabled at the beginning of mutex acquisition.
> I have tried moving the preemption disable of rwsem from
> the optimistic spin to the top of rwsem_down_write_failed.
> However, I didn't see a change in performance.
>
>
> > For example after a failed lock-stealing, do we still try optimistic
> > spinning to write-acquire the rwsem, or go into the slowpath and thus
> > trigger excessive context-switches?
>
> I do try optimistic spinning after a failed lock stealing. However,
> not after we have gone to sleep.
>
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