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

[Waiman Long]

On 09/11/2018 06:13 PM, Dave Chinner wrote:
> On Tue, Sep 11, 2018 at 03:18:26PM -0400, Waiman Long wrote:
>> Because the accounting of nr_dentry_negative depends on whether a dentry
>> is a negative one or not, branch instructions are introduced to handle
>> the accounting conditionally. That may potentially slow down the task
>> by a noticeable amount if that introduces sizeable amount of additional
>> branch mispredictions.
>>
>> To avoid that, the accounting code is now modified to use conditional
>> move instructions instead, if supported by the architecture.
> I think this is a case of over-optimisation. It makes the code
> harder to read for extremely marginal benefit, and if we ever need
> to add any more code for negative dentries in these paths the first
> thing we'll have to do is revert this change.
>
> Unless you have numbers demonstrating that it's a clear performance
> improvement, then NACK for this patch.
>
> Cheers,
>
> Dave.

Yes, this is an optimization.

Unfortunately I don't have any performance number as I had not seen any
significant performance difference outside of the noise range with these
set of changes. I am not fine with not taking this patch.

Cheers,
Longman