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

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Message-ID: <20151119103354.GU8644@n2100.arm.linux.org.uk>
Date:	Thu, 19 Nov 2015 10:33:54 +0000
From:	Russell King - ARM Linux <linux@....linux.org.uk>
To:	Marc Gonzalez <marc_gonzalez@...madesigns.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	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

On Thu, Nov 19, 2015 at 10:27:33AM +0100, Marc Gonzalez wrote:
> On 18/11/2015 18:21, 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'.
> 
> You already suggested that the last time (April 1st).
> What problem is this supposed to solve?
> Swapping the fields does not change the amount of padding required,
> and does not place reg close to the hot data.
> 
> On a 32-bit platform, with L1_CACHE_BYTES = 64
> 
> sizeof(struct unaligned_clocksource) = 80
> sizeof(struct clocksource) = 128
> sizeof(struct clocksource_mmio)  = 192, reg at +0,   clksrc at +64
> sizeof(struct clocksource_mmio2) = 192, reg at +128, clksrc at +0
> 
> Same amount of padding.

Maybe the ____cacheline_aligned is inappropriate then, because it means
any wrapping of struct clocksource has exactly the same problem.

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