[PATCH 0/5] Kernel mode NEON for XOR and RAID6

[Dave Martin]

On Fri, Jun 21, 2013 at 10:58:21AM -0400, Christopher Covington wrote:
> Hi Ard,
> 
> On 06/21/2013 06:08 AM, Ard Biesheuvel wrote:
> > On 21 June 2013 11:33, Will Deacon <will.deacon at arm.com> wrote:
> >> On Sat, Jun 08, 2013 at 04:09:56AM +0100, Nicolas Pitre wrote:
> >>> On Fri, 7 Jun 2013, Will Deacon wrote:
> >>>> What's the earliest toolchain we claim to support nowadays? If that can't
> >>>> deal with the intrinsics then we either need to bump the requirement, or
> >>>> write this using hand-coded asm. In the case of the latter, I don't think
> >>>> the maintenance overhead of having two implementations is worth it.
> >>>
> >>> We have many different minimum toolchain version requirements attached
> >>> to different features being enabled already, ftrace being one of them if
> >>> I remember correctly.  For these Neon optimizations the minimum gcc
> >>> version is v4.6.
> >>>
> >>> Given that this is going to be interesting mostly to server systems, and
> >>> given that ARM server deployments are rather new, I don't see the point
> >>> of compiling a new server environment using an older gcc version.
> >>
> >> I've mulled over this, had some discussions with our toolchain guys and
> >> have concluded the following:
> >>
> >>   - The intrinsics are actually ok. I was sceptical at first, but I've been
> >>     assured that they should do a reasonable job (echoing your performance
> >>     figures).
> >>
> >>   - The current approach is targetting servers and isn't (yet) suitable for
> >>     mobile.
> >>
> >> So, given that the patches do the right thing wrt GCC version, the only
> >> remaining point is that we need to keep an eye out for people trying to
> >> re-use this stuff for mobile (likely crypto, as I mentioned earlier). When
> >> that happens, we should consider revisiting the benchmark/power figures.
> >>
> > 
> > OK, so a number of points have been raised in this discussion, let me
> > address them one by one:
> > 
> > Should we allow NEON to be used in the kernel?
> > 
> > The consensus is not to allow floating point. However, NEON is
> > different, as the performance gains are considerable and there is no
> > dependency on support code, which makes it not as hairy as
> > conventional (pre-v3) VFP. Also, managing the vfpstates is easily
> > doable if NEON is only used outside interrupt context and with
> > preemption disabled.
> > 
> > 
> > Does my series implement it correctly?
> > 
> > I have addressed Russell's first round of comments. Happy to take
> > another round if necessary.
> > 
> > 
> > Should we allow NEON intrinsics in the kernel?
> > Should we allow GCC-generated NEON in the kernel?
> > 
> > Only if the implementation is clear on which minimum version of GCC it
> > requires. We could use my examples to set a precedent on what is a
> > suitable way to use NEON intrinsics or the vectorizer in kernel code
> > (which includes coding it such that it can be reused for arm64 with no
> > modifications)
> > 
> > 
> > Is kernel mode NEON suitable for mobile?
> > 
> > To me, it is unclear why kernel and userland are so different in this
> > respect. However, kernel mode NEON is separately configurable from
> > Kconfig so it can be disabled at will.
> > 
> > 
> > Is there a point to doing a boot time benchmark to select the optimal
> > implementation of an algorithm?
> > 
> > Perhaps not but unrelated to kernel mode NEON.

Significant benchmarks on the boot path would be unacceptable, unless they
are *fast* (and by fast, I mean fast on all platforms, not just fast on
the fast platforms).  If one second gets added onto the boot path for each
optimised algorithm, that sounds like a fail.  If all the benchmarks
combined take one second in total, that's no quite as bad.

Maybe benchmarks could be time-bounded (i.e., see how much data we can
chug though in X milliseconds) instead of size-bounded.  This would avoid
unreasonable slowdown on slow platforms, while avoiding trivially small
benchmark payloads on faster platforms which may typically have a more
complex architecture, bigger caches etc. which would cause them to take
longer to reach saturated performance when running a particular algorithm.

Cheers
---Dave