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

Russell King - ARM Linux
2015-11-19T10:36:57Z

On Thu, Nov 19, 2015 at 11:33:47AM +0100, Thomas Gleixner wrote:
> 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.

Right, and what it's doing is polluting struct clocksource with lots
of ifdefs which determine how much data is contained in there. Seems
to me to be totally insane.

The basic cause of this problem is the ____cacheline_aligned annotation
which effectively prevents wrapping struct clocksource to provide
implementation specific data.

Maybe your idea is that struct clocksource should be bloated with all
implementation specific data in the long term?

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