Re: [PATCH] clocksource: Store reg field within struct clocksource

Russell King - ARM Linux
2015-11-19T10:34:09Z

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.