Re: [PATCH] rwsem-spinlock: let rwsem write lock stealable

Yuanhan Liu
2013-01-31T12:23:05Z

On Thu, Jan 31, 2013 at 11:45:41AM +0100, Ingo Molnar wrote:
>
> * Yuanhan Liu <yuanhan.liu@linux.intel.com> wrote:
>
> > > > output with this patch:
> > > > -----------------------
> > > > cpu 00: 0 0 ... 1 1 2 1 1 1 2 1 1 1 .... 1 3
> > > > cpu 01: 0 0 ... 1 1 1 1 1 1 2 1 1 1 .... 1 3
> > > > cpu 02: 0 0 ... 2 2 3 2 0 2 1 2 1 1 .... 1 1
> > > > cpu 03: 0 0 ... 2 2 3 2 1 2 1 2 1 1 .... 1 1
> > > > cpu 04: 0 1 ... 2 0 0 1 0 1 3 1 1 1 .... 1 1
> > > > cpu 05: 0 1 ... 2 0 1 1 0 1 2 1 1 1 .... 1 1
> > > > cpu 06: 0 0 ... 2 1 1 2 0 1 2 1 1 1 .... 2 1
> > > > cpu 07: 0 0 ... 2 1 1 2 0 1 2 1 1 1 .... 2 1
> > > > cpu 08: 0 0 ... 1 1 1 1 1 1 1 1 1 1 .... 0 0
> > > > cpu 09: 0 0 ... 1 1 1 1 1 1 1 1 1 1 .... 0 0
> > > > cpu 10: 0 0 ... 1 1 1 0 0 1 1 1 1 1 .... 0 0
> > > > cpu 11: 0 0 ... 1 1 1 0 0 1 1 1 1 2 .... 1 0
> > > > cpu 12: 0 0 ... 1 1 1 0 1 1 0 0 0 1 .... 2 1
> > > > cpu 13: 0 0 ... 1 1 1 0 1 1 1 0 1 2 .... 2 0
> > > > cpu 14: 0 0 ... 2 0 0 0 0 1 1 1 1 1 .... 2 2
> > > > cpu 15: 0 0 ... 2 0 0 1 0 1 1 1 1 1 .... 2 2
> > > > ------------------------------------------------------------------------
> > > > Where you can see that CPU is much busier with this patch.
> > >
> > > That looks really good - quite similar to how it behaved
> > > with mutexes, right?
> >
> > Yes :)
> >
> > And the result is almost same with mutex lock when MUTEX_SPIN_ON_OWNER
> > is disabled, and that's the reason you will see massive processes(about
> > 100) queued on each CPU in my last report:
> > https://lkml.org/lkml/2013/1/29/84
>
> Just curious: how does MUTEX_SPIN_ON_OWNER versus
> !MUTEX_SPIN_ON_OWNER compare, for this particular,
> massively-contended anon-vma locks benchmark?

In above testcase, MUTEX_SPIN_ON_OWNER is slightly doing better job(like
3% ~ 4%) than !MUTEX_SPIN_ON_OWNER.

>
> > > Does this recover most of the performance regression?
> >
> > Yes, there is only a 10% gap here then. I guess that's because

Sorry, to be accurate, it's about 14% gap; when MUTEX_SPIN_ON_OWNER is
enabled.

> > I used the general rwsem lock
> > implementation(lib/rwsem-spinlock.c), but not the XADD
> > one(lib/rwsem.c). I guess the gap may be a little smaller if
> > we do the same thing to lib/rwsem.c.
>
> Is part of the gap due to MUTEX_SPIN_ON_OWNER perhaps?

Nope, !MUTEX_SPIN_ON_OWNER does introduce a little performance drop just
as above stated.

So, to make it clear, here is the list:

lock case performance drop compared to mutex lock
----------------------------------------------------------------------------
mutex lock w/o MUTEX_SPIN_ON_OWNER 3.x%
rwsem-spinlock with write stealing 14.x%
rwsem-spinlock >100%

>
> I'm surprised that rwsem-spinlock versus rwsem.c would show a
> 10% performance difference -

Yes, it may not. And there is only about 0.9% performance difference in
above test between rwsem-spinlock and XADD rwsem. The difference maybe
enlarged when both has write lock stealing enabled, which will be known
only after we do same thing to lib/rwsem.c.

Thanks.

--yliu

> assuming you have lock
> debugging/tracing disabled in the .config.
>
> ( Once the performance regression is fixed, another thing to
> check would be to reduce anon-vma lock contention. )
>
> Thanks,
>
> Ingo