From: Heiko Carstens <heiko.carstens@de.ibm.com>
To: Linus Torvalds <torvalds@linux-foundation.org>,
Tony Luck <tony.luck@intel.com>,
linux-kernel@vger.kernel.org
Subject: Re: [PATCH] lockref: remove cpu_relax() again
Date: Thu, 5 Sep 2013 16:13:05 +0200 [thread overview]
Message-ID: <20130905141305.GB24274@osiris> (raw)
In-Reply-To: <20130905131814.GA24274@osiris>
On Thu, Sep 05, 2013 at 03:18:14PM +0200, Heiko Carstens wrote:
> d472d9d9 "lockref: Relax in cmpxchg loop" added a cpu_relax() call to the
> CMPXCHG_LOOP() macro. However to me it seems to be wrong since it is very
> likely that the next round will succeed (or the loop will be left).
> Even worse: cpu_relax() is very expensive on s390, since it means yield
> "my virtual cpu to the hypervisor". So we are talking of several 1000 cycles.
>
> In fact some measurements show the bad impact of the cpu_relax() call on
> s390 using Linus' test case that "stats()" like mad:
>
> Without converting s390 to lockref:
> Total loops: 81236173
>
> After converting s390 to lockref:
> Total loops: 31896802
>
> After converting s390 to lockref but with removed cpu_relax() call:
> Total loops: 86242190
All of those should have been "converting s390 to ARCH_USE_CMPXCHG_LOCKREF"
instead of "to lockref" of course .. ;)
next prev parent reply other threads:[~2013-09-05 14:13 UTC|newest]
Thread overview: 13+ messages / expand[flat|nested] mbox.gz Atom feed top
2013-09-05 13:18 [PATCH] lockref: remove cpu_relax() again Heiko Carstens
2013-09-05 14:13 ` Heiko Carstens [this message]
2013-09-05 14:48 ` Luck, Tony
2013-09-05 15:31 ` Linus Torvalds
2013-09-05 17:35 ` Luck, Tony
2013-09-05 17:53 ` Linus Torvalds
2013-09-05 18:57 ` Luck, Tony
2013-09-05 19:21 ` Linus Torvalds
2013-09-05 19:45 ` Luck, Tony
2013-09-05 19:50 ` Linus Torvalds
2013-09-05 19:56 ` Luck, Tony
2013-09-06 18:36 ` Tony Luck
2013-09-05 17:54 ` Waiman Long
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