From: Will Deacon <will.deacon@arm.com>
To: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
Waiman Long <Waiman.Long@hp.com>
Subject: Re: [PATCH] lockref: use cmpxchg64 explicitly for lockless updates
Date: Fri, 20 Sep 2013 11:08:07 +0100 [thread overview]
Message-ID: <20130920100806.GA5015@mudshark.cambridge.arm.com> (raw)
In-Reply-To: <CA+55aFxuc3yD5pH2w5uEr_VDsEANvEiGCWA6tu1cSFrAiaU4Ow@mail.gmail.com>
On Thu, Sep 19, 2013 at 07:11:32PM +0100, Linus Torvalds wrote:
> On Thu, Sep 19, 2013 at 1:06 PM, Will Deacon <will.deacon@arm.com> wrote:
> > The cmpxchg() function tends not to support 64-bit arguments on 32-bit
> > architectures. This could be either due to use of unsigned long arguments
> > (like on ARM) or lack of instruction support (cmpxchgq on x86). However,
> > these architectures may implement a specific cmpxchg64() function to
> > provide 64-bit cmpxchg support instead
>
> I'm certainly ok with this, but I wonder how much point there is to
> use the cmpxchg alternatives for 32-bit architectures at all...
>
> From a performance standpoint, lockref really is expected to mainly
> help with big machines. Only insane people would do big machines with
> 32-bit kernels these days.
Our definitions of "big" machines probably differ significantly, but it
would be interesting to see if this *does* make a difference on some of the
multi-cluster ARMv7 hardware. Unfortunately, my development boards are all
I/O bound, so I'll need to leave a strategically placed crate of
non-poisoned beer next to the server guys' office...
> Of course, it may be that cmpxchg is actually faster on some
> architectures, but at least on x86-32, cmpxchg8b is traditionally
> quite slow.
On ARMv7, our double-word exclusives shouldn't be slower than the word
exclusives (hell, everything apart from the machine registers will be >=
64-bit).
> In other words, I'd actually like to see some numbers if there are
> loads where this actually helps and matters...
That's fair enough; I just saw the new lockref stuff, thought "that's a cool
hack" then looked at playing with it on ARM. I'll go see what this AIM7
thing is all about...
Cheers,
Will
next prev parent reply other threads:[~2013-09-20 10:08 UTC|newest]
Thread overview: 7+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-09-19 18:06 [PATCH] lockref: use cmpxchg64 explicitly for lockless updates Will Deacon
2013-09-19 18:11 ` Linus Torvalds
2013-09-20 2:59 ` Waiman Long
2013-09-20 10:08 ` Will Deacon [this message]
2013-09-20 15:45 ` Will Deacon
2013-09-20 16:00 ` Linus Torvalds
2013-09-20 17:11 ` Will Deacon
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