linux-kernel - Re: [PATCH] clocksource: Store reg field within struct clocksource

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Message-ID: <alpine.DEB.2.11.1511191130480.3898@nanos>
Date:	Thu, 19 Nov 2015 11:33:47 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
cc:	Marc Gonzalez <marc_gonzalez@...madesigns.com>,
	Daniel Lezcano <daniel.lezcano@...aro.org>,
	LKML <linux-kernel@...r.kernel.org>,
	Mans Rullgard <mans@...sr.com>,
	Viresh Kumar <viresh.kumar@...aro.org>,
	Nicolas Pitre <nico@...aro.org>,
	Tony Lindgren <tony@...mide.com>,
	Sebastian Frias <sebastian_frias@...madesigns.com>
Subject: Re: [PATCH] clocksource: Store reg field within struct clocksource

Russell,

On Wed, 18 Nov 2015, Russell King - ARM Linux wrote:

> On Wed, Nov 18, 2015 at 02:43:34PM +0100, Marc Gonzalez wrote:
> > Since 'struct clocksource' is ____cacheline_aligned, gcc must insert
> > a lot of padding between reg and clksrc in 'struct clocksource_mmio'
> > (for example, L1_CACHE_BYTES = 64 on ARMv7).
> > 
> > Storing reg within 'struct clocksource' removes unnecessary padding,
> > and reg can then be grouped with other hot data. A nice side-effect
> > of this patch is making container_of() unnecessary, which makes the
> > code a bit simpler.
> > 
> > On 32-bit platforms, reg fits in the padding between read and mask,
> > meaning no downside from storing it there.
> 
> Just swap the order of 'reg' and 'clksrc'.

That might reduce the memory footprint, but it does not bring the
iomem pointer closer to the other hotpath clocksource data. So we
still need to touch at minimum two cache lines for reading the time.

With Marcs change we have all hotpath data in a single cacheline.

Thanks,

	tglx


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