Re: [PATCH] MIPS: Optimize spinlocks.

[David Daney <ddaney@caviumnetworks.com>]

On 02/25/2010 06:15 AM, Ralf Baechle wrote:
> 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().

That barrier is separate (and present).  The sole purpose of 
nudge_writes() is to make speed up the global visibility of the 
releasing write, it does not have anything to do with locking semantics.

>  Implementing it as barrier() this
> is a pure compiler barrier is the most liberal valid implementation.

No, the most liberal would be a true NOP: 'do { } while (0)'.

>
>> 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
>