Re: [PATCH-tip 00/22] locking/rwsem: Rework rwsem-xadd & enable new rwsem features

From: Linus Torvalds
Date: Fri Feb 08 2019 - 19:04:07 EST

On Fri, Feb 8, 2019 at 12:31 PM Waiman Long <longman@xxxxxxxxxx> wrote:
> > (b) what's the new fastpath case
> The only change in the fastpath is the use of cmpxchg for writer lock.

.. since a big deal here was about using the generic atomic accessor
functions, I really was looking forward to seeing the *actual* fast
path code generation.

In other words, right now I have very little visibility in how it
actually affects the code. Looking at the patches themselves doesn't
make it obvious. I was hoping for the overview to really explain the
whole "before and after" situation, and it didn't. Not at the high
level, and not at a low level. And no performance numbers in the
overview either.

And yes, I see the numbers in the patches, but what I really hoped for
was some real load numbers. In particular, I would have loved to see
numbers from th ekernel test robot "will-it-scale.per_thread_ops"
case, which is the one that had a 65% regression due to the lack of
reader spinning.

So I was kind of hoping to hear whether that regression is basically
entirely gone with this patch series, or if we still have a regression
due to the extra downgrade, or what?