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

Yuanhan Liu
2013-01-31T10:08:37Z

On Thu, Jan 31, 2013 at 10:39:31AM +0100, Ingo Molnar wrote:
>
> * Yuanhan Liu <yuanhan.liu@linux.intel.com> wrote:
>
> > We(Linux Kernel Performance project) found a regression introduced by
> > commit 5a50508, which just convert all mutex lock to rwsem write lock.
> > The semantics is same, but the results is quite huge in some cases.
> > After investigation, we found the root cause: mutex support lock
> > stealing. Here is the link for the detailed regression report:
> > https://lkml.org/lkml/2013/1/29/84
> >
> > Ingo suggests to add write lock stealing to rwsem as well:
> > "I think we should allow lock-steal between rwsem writers - that
> > will not hurt fairness as most rwsem fairness concerns relate to
> > reader vs. writer fairness"
> >
> > I then tried it with rwsem-spinlock first as I found it much easier to
> > implement it than lib/rwsem.c. And here I sent out this patch first for
> > comments. I'd try lib/rwsem.c later once the change to rwsem-spinlock
> > is OK to you guys.
> >
> > With this patch, we got a double performance increase in one test box
> > with following aim7 workfile:
> > FILESIZE: 1M
> > POOLSIZE: 10M
> > 10 fork_test
> >
> > some /usr/bin/time output w/o patch some /usr/bin/time_output with patch
> > ----------------------------------------------------------------------------
> > Percent of CPU this job got: 369% Percent of CPU this job got: 537%
> > Voluntary context switches: 640595016 Voluntary context switches: 157915561
> > ----------------------------------------------------------------------------
> > You will see we got a 45% increase of CPU usage and saves about 3/4
> > voluntary context switches.
> >
> >
> > Here is the .nr_running filed for all CPUs from /proc/sched_debug.
> >
> > output w/o this patch:
> > ----------------------
> > cpu 00: 0 0 ... 0 0 0 0 0 0 0 1 0 1 .... 0 0
> > cpu 01: 0 0 ... 1 0 0 0 0 0 1 1 0 1 .... 0 0
> > cpu 02: 0 0 ... 1 1 0 0 0 1 0 0 1 0 .... 1 1
> > cpu 03: 0 0 ... 0 1 0 0 0 1 1 0 1 1 .... 0 0
> > cpu 04: 0 1 ... 0 0 2 1 1 2 1 0 1 0 .... 1 0
> > cpu 05: 0 1 ... 0 0 2 1 1 2 1 1 1 1 .... 0 0
> > cpu 06: 0 0 ... 2 0 0 1 0 0 1 0 0 0 .... 0 0
> > cpu 07: 0 0 ... 2 0 0 0 1 0 1 1 0 0 .... 1 0
> > cpu 08: 0 0 ... 1 0 0 0 1 0 0 1 0 0 .... 0 1
> > cpu 09: 0 0 ... 1 0 0 0 1 0 0 1 0 0 .... 0 1
> > cpu 10: 0 0 ... 0 0 0 2 0 0 1 0 1 1 .... 1 2
> > cpu 11: 0 0 ... 0 0 0 2 2 0 1 0 1 0 .... 1 2
> > cpu 12: 0 0 ... 2 0 0 0 1 1 3 1 1 1 .... 1 0
> > cpu 13: 0 0 ... 2 0 0 0 1 1 3 1 1 0 .... 1 1
> > cpu 14: 0 0 ... 0 0 0 2 0 0 1 1 0 0 .... 1 0
> > cpu 15: 0 0 ... 1 0 0 2 0 0 1 1 0 0 .... 0 0
> >
> > 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

>
> 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.

Thanks.

--yliu