From: Tim Chen <tim.c.chen@linux.intel.com>
To: Ingo Molnar <mingo@kernel.org>
Cc: Ingo Molnar <mingo@elte.hu>,
Andrea Arcangeli <aarcange@redhat.com>,
Mel Gorman <mgorman@suse.de>, "Shi, Alex" <alex.shi@intel.com>,
Andi Kleen <andi@firstfloor.org>,
Andrew Morton <akpm@linux-foundation.org>,
Michel Lespinasse <walken@google.com>,
Davidlohr Bueso <davidlohr.bueso@hp.com>,
"Wilcox, Matthew R" <matthew.r.wilcox@intel.com>,
Dave Hansen <dave.hansen@intel.com>,
Peter Zijlstra <a.p.zijlstra@chello.nl>,
Rik van Riel <riel@redhat.com>,
linux-kernel@vger.kernel.org, linux-mm <linux-mm@kvack.org>
Subject: Re: Performance regression from switching lock to rw-sem for anon-vma tree
Date: Fri, 28 Jun 2013 14:04:21 -0700 [thread overview]
Message-ID: <1372453461.22432.216.camel@schen9-DESK> (raw)
In-Reply-To: <20130628093809.GB29205@gmail.com>
On Fri, 2013-06-28 at 11:38 +0200, Ingo Molnar wrote:
> * Tim Chen <tim.c.chen@linux.intel.com> wrote:
>
> > 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%
>
> So lock stealing+spinning now acquires the lock successfully ~90% of the
> time, the remaining sleeps are:
>
> > Lock acquired second attempt (1 sleep) ...... 9.32%
>
> And the reason these sleeps are mostly due to:
>
> > Optimistic spin abort 2 ..... 9.22%
>
> Right?
>
> So this particular #2 abort point is:
>
> | preempt_disable();
> | for (;;) {
> | owner = ACCESS_ONCE(sem->owner);
> | if (owner && !rwsem_spin_on_owner(sem, owner))
> | break; <--------------------------- abort (2)
>
> Next step would be to investigate why we decide to not spin there, why
> does rwsem_spin_on_owner() fail?
>
> If I got all the patches right, rwsem_spin_on_owner() is this:
>
> +static noinline
> +int rwsem_spin_on_owner(struct rw_semaphore *lock, struct task_struct *owner)
> +{
> + rcu_read_lock();
> + while (owner_running(lock, owner)) {
> + if (need_resched())
> + break;
> +
> + arch_mutex_cpu_relax();
> + }
> + rcu_read_unlock();
> +
> + /*
> + * We break out the loop above on need_resched() and when the
> + * owner changed, which is a sign for heavy contention. Return
> + * success only when lock->owner is NULL.
> + */
> + return lock->owner == NULL;
> +}
>
> where owner_running() is similar to the mutex spinning code: it in the end
> checks owner->on_cpu - like the mutex code.
>
> If my analysis is correct so far then it might be useful to add two more
> stats: did rwsem_spin_on_owner() fail because lock->owner == NULL [owner
> released the rwsem], or because owner_running() failed [owner went to
> sleep]?
Ingo,
I tabulated the cases where rwsem_spin_on_owner returns false and causes
us to stop spinning.
97.12% was due to lock's owner switching to another writer
0.01% was due to the owner of the lock sleeping
2.87% was due to need_resched()
I made a change to allow us to continue to spin even when lock's
owner switch to another writer. I did get the lock to be acquired
now mostly (98%) via optimistic spin and lock stealing, but my
benchmark's throughput actually got reduced by 30% (too many cycles
spent on useless spinning?). The lock statistics are below:
Writer acquisition blocked count 7538864
Blocked by reader 0.37%
Lock acquired first attempt (lock stealing) 18.45%
Lock acquired second attempt (1 sleep) 1.69%
Lock acquired after more than 1 sleep 0.25%
Lock acquired with optimistic spin 79.62%
Optimistic spin failure (abort point 1) 1.37%
Optimistic spin failure (abort point 2) 0.32%
Optimistic spin failure (abort point 3) 0.23%
(Opt spin abort point 2 breakdown) owner sleep 0.00%
(Opt spin abort point 2 breakdown) need_resched 0.32%
Thanks.
Tim
next prev parent reply other threads:[~2013-06-28 21:04 UTC|newest]
Thread overview: 46+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-06-13 23:26 Performance regression from switching lock to rw-sem for anon-vma tree Tim Chen
2013-06-19 13:16 ` Ingo Molnar
2013-06-19 16:53 ` Tim Chen
2013-06-26 0:19 ` Tim Chen
2013-06-26 9:51 ` Ingo Molnar
2013-06-26 21:36 ` Tim Chen
2013-06-27 0:25 ` Tim Chen
2013-06-27 8:36 ` Ingo Molnar
2013-06-27 20:53 ` Tim Chen
2013-06-27 23:31 ` Tim Chen
2013-06-28 9:38 ` Ingo Molnar
2013-06-28 21:04 ` Tim Chen [this message]
2013-06-29 7:12 ` Ingo Molnar
2013-07-01 20:28 ` Tim Chen
2013-07-02 6:45 ` Ingo Molnar
2013-07-16 17:53 ` Tim Chen
2013-07-23 9:45 ` Ingo Molnar
2013-07-23 9:51 ` Peter Zijlstra
2013-07-23 9:53 ` Ingo Molnar
2013-07-30 0:13 ` Tim Chen
2013-07-30 19:24 ` Ingo Molnar
2013-08-05 22:08 ` Tim Chen
2013-07-30 19:59 ` Davidlohr Bueso
2013-07-30 20:34 ` Tim Chen
2013-07-30 21:45 ` Davidlohr Bueso
2013-08-06 23:55 ` Davidlohr Bueso
2013-08-07 0:56 ` Tim Chen
2013-08-12 18:52 ` Ingo Molnar
2013-08-12 20:10 ` Tim Chen
2013-06-28 9:20 ` Ingo Molnar
[not found] <1371165333.27102.568.camel@schen9-DESK>
[not found] ` <1371167015.1754.14.camel@buesod1.americas.hpqcorp.net>
2013-06-14 16:09 ` Tim Chen
2013-06-14 22:31 ` Davidlohr Bueso
2013-06-14 22:44 ` Tim Chen
2013-06-14 22:47 ` Michel Lespinasse
2013-06-17 22:27 ` Tim Chen
2013-06-16 9:50 ` Alex Shi
2013-06-17 16:22 ` Davidlohr Bueso
2013-06-17 18:45 ` Tim Chen
2013-06-17 19:05 ` Davidlohr Bueso
2013-06-17 22:28 ` Tim Chen
2013-06-17 23:18 ` Alex Shi
2013-06-17 23:20 ` Alex Shi
2013-06-17 23:35 ` Davidlohr Bueso
2013-06-18 0:08 ` Tim Chen
2013-06-19 23:11 ` Davidlohr Bueso
2013-06-19 23:24 ` Tim Chen
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