From: Linus Torvalds <torvalds@linux-foundation.org>
To: Waiman Long <longman@redhat.com>
Cc: linux-arch <linux-arch@vger.kernel.org>,
linux-xtensa@linux-xtensa.org,
Davidlohr Bueso <dave@stgolabs.net>,
linux-ia64@vger.kernel.org, Tim Chen <tim.c.chen@linux.intel.com>,
Arnd Bergmann <arnd@arndb.de>,
Linux-sh list <linux-sh@vger.kernel.org>,
Peter Zijlstra <peterz@infradead.org>,
linux-hexagon@vger.kernel.org,
the arch/x86 maintainers <x86@kernel.org>,
Will Deacon <will.deacon@arm.com>,
Linux List Kernel Mailing <linux-kernel@vger.kernel.org>,
Ingo Molnar <mingo@redhat.com>, Borislav Petkov <bp@alien8.de>,
"H. Peter Anvin" <hpa@zytor.com>,
"linux-alpha@vger.kernel.org" <linux-alpha@vger.kernel.org>,
sparclinux@vger.kernel.org, Thomas Gleixner <tglx@linutronix.de>,
linuxppc-dev@lists.ozlabs.org,
Andrew Morton <akpm@linux-foundation.org>,
"linux-alpha@vger.kernel.org"
<linux-arm-kernel@lists.infradead.org>
Subject: Re: [PATCH v3 2/2] locking/rwsem: Optimize down_read_trylock()
Date: Thu, 14 Feb 2019 10:09:44 -0800 [thread overview]
Message-ID: <CAHk-=wi03J1jurebJNHPxGCVss7ARAN+2JuVthEGoEx=qkT4dQ@mail.gmail.com> (raw)
In-Reply-To: <CAHk-=whKmuranma_HUOKBbDJHGmdWZr9MYW-+cmGzsOiJ2N1Sg@mail.gmail.com>
On Thu, Feb 14, 2019 at 9:51 AM Linus Torvalds
<torvalds@linux-foundation.org> wrote:
>
> The arm64 numbers scaled horribly even before, and that's because
> there is too much ping-pong, and it's probably because there is no
> "stickiness" to the cacheline to the core, and thus adding the extra
> loop can make the ping-pong issue even worse because now there is more
> of it.
Actually, if it's using the ll/sc, then I don't see why arm64 should
even change. It doesn't really even change the pattern: the initial
load of the value is just replaced with a "ll" that gets a non-zero
value, and then we re-try without even doing the "sc" part.
End result: exact same "load once, then do ll/sc to update". Just
using a slightly different instruction pattern.
But maybe "ll" does something different to the cacheline than a regular "ld"?
Alternatively, the machine you used is using LSE, and the "swp" thing
has some horrid behavior when it fails.
So I take it back. I'm actually surprised that arm64 performs worse. I
don't think it should. But numbers walk, bullshit talks, and it
clearly does make for worse numbers on arm64.
Linus
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next prev parent reply other threads:[~2019-02-14 18:17 UTC|newest]
Thread overview: 10+ messages / expand[flat|nested] mbox.gz Atom feed top
2019-02-13 20:32 [PATCH v3 0/2] locking/rwsem: Remove arch specific rwsem files Waiman Long
2019-02-13 20:32 ` [PATCH v3 1/2] " Waiman Long
2019-02-13 20:32 ` [PATCH v3 2/2] locking/rwsem: Optimize down_read_trylock() Waiman Long
2019-02-14 10:33 ` Peter Zijlstra
2019-02-14 14:53 ` Waiman Long
2019-02-14 17:51 ` Linus Torvalds
2019-02-14 18:09 ` Linus Torvalds [this message]
2019-02-15 18:35 ` Will Deacon
2019-02-14 18:02 ` Will Deacon
2019-02-14 18:35 ` Waiman Long
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