Re: [PATCH] MIPS: Optimize spinlocks.

[Ralf Baechle <ralf@linux-mips.org>]

On Wed, Feb 24, 2010 at 08:55:12AM -0800, David Daney wrote:

> It is possible that by choosing a better nudge_writes()
> implementation for R10K, that the 3% degradation could be erased.
> Perhaps:
> 
> #define nudge_writes() do { } while (0)

raw_spin_unlock must provide a barrier so this wouldn't be a valid
implementation for nudge_writes().  Implementing it as barrier() this
is a pure compiler barrier is the most liberal valid implementation.

> Basically you want something that is fast, but that also forces the
> write to be globally visible as soon as possible.  Some processors
> have a prefetch instruction that does this.  On other processors a
> NOP is optimal as they don't combine writes in the write back
> buffer.
> 
> There is a wbflush() function that could potentially be used, but
> its implementation is too heavy on Octeon.

For IP27 which is a strongly ordered system nudge_writes() is implemented
as barrier().

Another experiment I did was alignment.  A branch on an R10000 has a
significant execution time penalty if it's delay slot is overlapping a
128 byte S-cache boundary.  Suitable alignment however didn't not seem
to make any difference at all on R10000.

  Ralf