Re: [PATCH] arch,locking: Ciao arch_mutex_cpu_relax()

[Vineet Gupta]

Hi Peter,

On Monday 23 June 2014 12:24 PM, Peter Zijlstra wrote:
> On Fri, Jun 20, 2014 at 11:21:13AM -0700, Davidlohr Bueso wrote:
>> diff --git a/arch/arc/include/asm/processor.h b/arch/arc/include/asm/processor.h
>> index d99f9b3..8e1bf6b 100644
>> --- a/arch/arc/include/asm/processor.h
>> +++ b/arch/arc/include/asm/processor.h
>> @@ -62,6 +62,8 @@ unsigned long thread_saved_pc(struct task_struct *t);
>>  #define cpu_relax()	do { } while (0)
>>  #endif
>>  
>> +#define arch_cpu_relax() cpu_relax()
>> +
>>  #define copy_segments(tsk, mm)      do { } while (0)
>>  #define release_segments(mm)        do { } while (0)
> 
> I'm not at all sure that cpu_relax() definition ARC has is valid. We
> rely on cpu_relax() being at least a barrier() all over the place, and
> it doesn't need to be SMP only. You can have a UP wait loop waiting for
> an interrupt for example.
> 
> Vineet?

Over the years we've not had any trouble with !SMP cpu_relax() being a no-op (and
barrier version was only required when we hit a hard hang in our our initial SMP
code). UP busy wait looping would be frowned upon in general.

However what we have now is just a code optimization quirk for !SMP since a
compiler barrier will cause gcc to dump out and reload scratch regs - specially
for our deep reg file.

Here's what I get with current UP kernel switching to compiler barrier

./scripts/bloat-o-meter vmlinux-pre-cpu-relax  vmlinux | head
add/remove: 1/0 grow/shrink: 75/5 up/down: 1218/-32 (1186)
function                                     old     new   delta
path_init                                    708     826    +118
sys_semtimedop                              2540    2640    +100
...
__slab_alloc.isra.constprop                  564     560      -4
deactivate_slab                              886     878      -8

So it doesn't look too bad, although I've not run any performance tests. We can
switch UP to barrier if you feel it is needed semantically.

-Vineet


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