Re: [PATCH v3 4/4] fs/dcache: Eliminate branches in nr_dentry_negative accounting

From: Waiman Long
Date: Wed Sep 12 2018 - 12:11:27 EST

On 09/12/2018 11:55 AM, Matthew Wilcox wrote:
> On Wed, Sep 12, 2018 at 11:49:22AM -0400, Waiman Long wrote:
>>> unless our macrology has got too clever for the compilre to see through
>>> it. In which case, the right answer is to simplify the percpu code,
>>> not to force the compiler to optimise the code in the way that makes
>>> sense for your current microarchitecture.
>> I had actually looked at the x86 object file generated to verify that it
>> did use cmove with the patch and use branch without. It is possible that
>> there are other twists to make that happen with the above expression. I
>> will need to run some experiments to figure it out. In the mean time, I
>> am fine with dropping this patch as it is a micro-optimization that
>> doesn't change the behavior at all.
> I don't understand why you included it, to be honest. But it did get
> me looking at the percpu code to see if it was too clever. And that
> led to the resubmission of rth's patch from two years ago that I cc'd
> you on earlier.
> With that patch applied, gcc should be able to choose to use the
> cmov if it feels that would be a better optimisation. It already
> makes one different decision in dcache.o, namely that it uses addq
> $0x1,%gs:0x0(%rip) instead of incq %gs:0x0(%rip). Apparently this
> performs better on some CPUs.
> So I wouldn't spend any more time on this patch.

Thank for looking into that. Well I am not going to look further into
this unless I have no other thing to do which is unlikely.