linux-kernel - Re: [PATCH 3/3] clocksource: exynos_mct: Optimize register reads with ldmia

lists.openwall.net		lists / announce owl-users owl-dev john-users john-dev passwdqc-users yescrypt popa3d-users / oss-security kernel-hardening musl sabotage tlsify passwords / crypt-dev xvendor / Bugtraq Full-Disclosure linux-kernel linux-netdev linux-ext4 linux-hardening linux-cve-announce PHC
Open Source and information security mailing list archives

Hash Suite: Windows password security audit tool. GUI, reports in PDF.

[<prev] [next>] [<thread-prev] [thread-next>] [day] [month] [year] [list]

Message-ID: <alpine.DEB.2.10.1406042004140.3319@nanos>
Date:	Wed, 4 Jun 2014 20:05:47 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Doug Anderson <dianders@...omium.org>
cc:	Kukjin Kim <kgene.kim@...sung.com>,
	Tomasz Figa <t.figa@...sung.com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	Vincent Guittot <vincent.guittot@...aro.org>,
	Chirantan Ekbote <chirantan@...omium.org>,
	David Riley <davidriley@...omium.org>, olof@...om.net,
	linux-samsung-soc@...r.kernel.org, linux-kernel@...r.kernel.org,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCH 3/3] clocksource: exynos_mct: Optimize register reads
 with ldmia

On Wed, 4 Jun 2014, Doug Anderson wrote:

> As we saw in (clocksource: exynos_mct: cache mct upper count), the
> time spent reading the MCT shows up fairly high in real-world
> profiles.  That means that it's worth some optimization.
> 
> We get a roughly 10% speedup in userspace gettimeofday() by using an
> ldmia to read the two halfs of the MCT.  That seems like a worthwhile
> thing to do.
> 
> Before: 1173084 us for 1000000 gettimeofday in userspace
> After:  1045674 us for 1000000 gettimeofday in userspace
> 
> NOTE: we could actually do better than this if we really wanted to.
> Technically we could register the clocksource as a 32-bit timer and
> only use the "lower" half.  Doing so brings us down to 1014429 us for
> 1000000 gettimeofday in userspace (and doesn't even require assembly
> code).  That would be an alternative to this change.

I was about to ask exactly that question: What's the advantage of the
64 bit dance there? AFAICT nothing.
 
Thanks,

	tglx
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@...r.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/