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

[Ingo Molnar]

* Michel Lespinasse <walken@xxxxxxxxxx> wrote:

> On Wed, Jan 30, 2013 at 1:14 AM, Yuanhan Liu
> <yuanhan.liu@xxxxxxxxxxxxxxx> 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) {
> 
> 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 - but I'm crossing my fingers and 
> thinking that it'll most likely turn out fine.

My gut feeling, from having implemented lock stealing in a 
number of locking primitives in the past, is that writer 
lock-stealing will be a measurable win for mmap_sem as well.

Thanks,

	Ingo
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