Date: Thu, 19 Nov 2015 10:33:54 +0000
From: Russell King - ARM Linux <linux@arm.linux.org.uk>
To: Marc Gonzalez <marc_gonzalez@sigmadesigns.com>
Cc: Thomas Gleixner <tglx@linutronix.de>,
        Daniel Lezcano <daniel.lezcano@linaro.org>,
        LKML <linux-kernel@vger.kernel.org>, Mans Rullgard <mans@mansr.com>,
        Viresh Kumar <viresh.kumar@linaro.org>,
        Nicolas Pitre <nico@linaro.org>, Tony Lindgren <tony@atomide.com>,
        Sebastian Frias <sebastian_frias@sigmadesigns.com>
Subject: Re: [PATCH] clocksource: Store reg field within struct clocksource
Message-ID: <20151119103354.GU8644@n2100.arm.linux.org.uk>
References: <564C8086.80804@sigmadesigns.com>
 <20151118172114.GS8644@n2100.arm.linux.org.uk>
 <564D9605.4010207@sigmadesigns.com>
MIME-Version: 1.0
Content-Type: text/plain; charset=us-ascii
Content-Disposition: inline
In-Reply-To: <564D9605.4010207@sigmadesigns.com>
User-Agent: Mutt/1.5.23 (2014-03-12)
Sender: linux-kernel-owner@vger.kernel.org
Content-Length: 1820
Lines: 43

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.

-- 
FTTC broadband for 0.8mile line: currently at 9.6Mbps down 400kbps up
according to speedtest.net.
--
To unsubscribe from this list: send the line "unsubscribe linux-kernel" in
the body of a message to majordomo@vger.kernel.org
More majordomo info at  http://vger.kernel.org/majordomo-info.html
Please read the FAQ at  http://www.tux.org/lkml/