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

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Date:   Fri, 8 Feb 2019 16:03:43 -0800
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Waiman Long <longman@...hat.com>
Cc:     Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>,
        Will Deacon <will.deacon@....com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Linux List Kernel Mailing <linux-kernel@...r.kernel.org>,
        linux-alpha@...r.kernel.org, linux-arm-kernel@...ts.infradead.org,
        linux-hexagon@...r.kernel.org, linux-ia64@...r.kernel.org,
        linuxppc-dev@...ts.ozlabs.org,
        Linux-sh list <linux-sh@...r.kernel.org>,
        sparclinux@...r.kernel.org, linux-xtensa@...ux-xtensa.org,
        linux-arch <linux-arch@...r.kernel.org>,
        "the arch/x86 maintainers" <x86@...nel.org>,
        Arnd Bergmann <arnd@...db.de>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>,
        Davidlohr Bueso <dave@...olabs.net>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Tim Chen <tim.c.chen@...ux.intel.com>
Subject: Re: [PATCH-tip 00/22] locking/rwsem: Rework rwsem-xadd & enable new
 rwsem features

On Fri, Feb 8, 2019 at 12:31 PM Waiman Long <longman@...hat.com> 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?

                 Linus