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

Yuanhan Liu
2013-01-31T12:39:35Z

On Thu, Jan 31, 2013 at 03:57:51AM -0800, Michel Lespinasse wrote:
> On Wed, Jan 30, 2013 at 1:14 AM, 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.
>
> I noticed that you haven't modified __down_write_trylock() - for
> consistency with __down_write() you should replace
> if (sem->activity == 0 && list_empty(&sem->wait_list)) {
> with
> if (sem->activity == 0) {

Yes, my bad for missing that. Thanks a lot for pointing it out. Will fix it.
>
> Other than that, I like the idea. I was originally uncomfortable with
> doing lock stealing for the rwsem, but I think doing it for writers
> only as you propose should be fine. Readers wait for any queued
> writers, and in exchange they are guaranteed to get the lock once
> they've blocked.

> You *still* want to check for regressions that this
> change might cause - not with anon_vma as this was a mutex not long
> ago, but possibly with mmap_sem

Yes. Well, at least it passed Fengguang's 0-DAY test, which did lots
of tests on almost all ARCHs. Well, you reminds me that I just enabled
RWSEM_GENERIC_SPINLOCK for x86 ARCH, thus I need to enable
RWSEM_GENERIC_SPINLOCK to all ARCHs and do test again.

BTW, mind to tell a nice test case for mmap_sem?

> - but I'm crossing my fingers and
> thinking that it'll most likely turn out fine.

Thanks!

>
> I may be able to help with the non-spinlock version of this as I still
> remember how this works.

That would be great! Especially I will have vacation soon for Chinese
New Year.

--yliu