Re: [PATCH] MIPS: Don't branch to eret in TLB refill.

[David Daney <ddaney@caviumnetworks.com>]

Maciej W. Rozycki wrote:
> On Tue, 12 May 2009, David Daney wrote:
> 
>>>> +			/*
>>>> +			 * Find the split point.
>>>> +			 */
>>>> +			if (uasm_insn_has_bdelay(relocs, split - 1))
>>>> +				split--;
>>>> +		}
>>> The code itself makes sense. Does this case actually happen much, or was
>>> this just an itch?
>>>
>> For my CPU it was happening 100% of the time when I add the soon to be
>> submitted hugeTLBfs support patch.  Although I have not measured it, this code
>> is so hot that keeping the normal case fitting on a single cache line should
>> be a big win.
> 
>  Rather than this hack,

I don't really know what to say about that comment.

* We are synthesizing optimized TLB refill handlers, even small 
improvements yield big gains in system performance.

* The optimization you suggest below, although a good one, is somewhat 
different and would make a good follow on patch.

* I am trying to make forward progress and not have The perfect be the 
enemy of the good.

> I'd suggest microoptimising the code by shuffling 
> it such that unless the handler fits in 128 bytes entirely (I'm not sure 
> if that ever happens for XTLB refill) the part built by 
> build_get_pgd_vmalloc64() is placed in the TLB handler slot, saving an 
> unnecessary unconditional branch there.  This way the problem of an 
> unconditional branch to ERET will solve automagically as a side-effect.  
> Unless the vmalloc part does not fit in 128 bytes, that is, in which case 
> it would have to overflow back to the XTLB slot.  It should be pretty 
> straightforward to code. ;)
> 
>   Maciej
>