Re: [PATCH v2] iio: adc: ad7944: Add support for "3-wire mode"

[Arnaldo Carvalho de Melo <acme@kernel.org>]

On Mon, Mar 18, 2024 at 02:29:23PM +0000, Jonathan Cameron wrote:
> On Mon, 18 Mar 2024 15:09:32 +0200
> Andy Shevchenko <andy.shevchenko@gmail.com> wrote:
> 
> > On Mon, Mar 18, 2024 at 2:41 PM Jonathan Cameron
> > <Jonathan.Cameron@huawei.com> wrote:
> > > > >  struct ad7944_adc {
> > > > >     struct spi_device *spi;
> > > > > +   enum ad7944_spi_mode spi_mode;
> > > > >     /* Chip-specific timing specifications. */
> > > > >     const struct ad7944_timing_spec *timing_spec;
> > > > >     /* GPIO connected to CNV pin. */
> > > > > @@ -58,6 +75,9 @@ struct ad7944_adc {
> > > > >      } sample __aligned(IIO_DMA_MINALIGN);
> > > > >  };  
> > > >
> > > > Have you run `pahole` to see if there is a better place for a new member?  
> > >
> > > I know this matters for structures where we see lots of them, but do we actually
> > > care for one offs?  Whilst it doesn't matter here I'd focus much more
> > > on readability and like parameter grouping for cases like this than wasting
> > > a few bytes.  
> > 
> > This is _also_ true, but think more about cache line contamination.
> > Even not-so-important bytes may decrease the performance. In some
> > cases it's tolerable, in some it is not (high-speed ADC). In general I
> > assume that the developer has to understand many aspects of the
> > software and cache line contamination may be last but definitely not
> > least.
> > 
> 
> Not totally sure what you are covering with contamination as many aspects
> around caches and that's not really a standard term for any of them (as
> far as I know).
> 
> It's part of a multi cacheline allocation anyway (because it's tacked on the
> end of the iio device struct, so fairly unlikely to share with other allocations
> and definitely not on ARM because of the trailing __aligned(IIO_DMA_MINALIGN)
> elements.
> 
> If it matters more locally, then pahole is more likely to push you to pack

You mean 'pahole --reorganize', right? Yeah, I need to take into account
explicit __attribute__((__aligned__)) at the start of cachelines as a
hint that the fields in a cacheline can't be moved outside of that
cacheline or plain leave that cacheline members alone, as-is.

I also need to get perf's data-type profiling as an input for 'pahole
--reorganize', with that we may take into account the existing
__aligned__ markings and combine it with what we get from data-type
profiling.

- Arnaldo

> things together in a fashion that makes false sharing and similar perf issues
> more likely if you are grouping things for packing purposes rather than
> logical groups.
> 
> If you just mean cache pressure then fair enough if we squeeze everything into
> one cacheline and that doesn't cause false sharing.
> 'Maybe' this will fit on x86. On Arm64 it's not going to
> make any difference, just moving the padding around a bit within the line.
> 
> So I'd argue premature optimization for a small, one off, structure.
> 
> Jonathan
> 
> 
>