Re: [PATCH] rwsem: reduce spinlock contention in wakeup code path

[Ingo Molnar <mingo@kernel.org>]

* Peter Zijlstra <peterz@infradead.org> wrote:

> On Tue, Oct 01, 2013 at 09:28:15AM +0200, Ingo Molnar wrote:
>
> > That I mostly agree with, except that without a serious usecase do we 
> > have a guarantee that bugs in fancies queueing in rwsems gets ironed 
> > out?
> 
> Methinks mmap_sem is still a big enough lock to work out a locking 
> primitive :-)

I mean the AIM7 usecase probably falls away - we need to find another one 
that shows the inefficiencies.

> In fact, try something like this from userspace:
> 
> n-threads:
> 
>   pthread_mutex_lock(&mutex);
>   foo = mmap();
>   pthread_mutex_lock(&mutex);
> 
>   /* work */
> 
>   pthread_mutex_unlock(&mutex);
>   munma(foo);
>   pthread_mutex_unlock(&mutex);
> 
> vs
> 
> n-threads:
> 
>   foo = mmap();
>   /* work */
>   munmap(foo);
> 
> 
> I've had reports that the former was significantly faster than the
> latter.

That looks like a legitimate pattern that ought to trigger in many apps. 
Would be nice to turn this into a:

	perf bench mm thread-create

testcase or so.

Thanks,

	Ingo