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

Ingo Molnar
2013-01-31T10:46:01Z

* 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?

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

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