linux-kernel - Re: [PATCH v3 4/4] fs/dcache: Eliminate branches in nr_dentry

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Date:   Wed, 12 Sep 2018 11:44:24 -0400
From:   Waiman Long <longman@...hat.com>
To:     Dave Chinner <david@...morbit.com>
Cc:     Alexander Viro <viro@...iv.linux.org.uk>,
        Jonathan Corbet <corbet@....net>, linux-kernel@...r.kernel.org,
        linux-fsdevel@...r.kernel.org, linux-mm@...ck.org,
        linux-doc@...r.kernel.org, "Luis R. Rodriguez" <mcgrof@...nel.org>,
        Kees Cook <keescook@...omium.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>,
        "Paul E. McKenney" <paulmck@...ux.vnet.ibm.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Ingo Molnar <mingo@...nel.org>,
        Miklos Szeredi <mszeredi@...hat.com>,
        Matthew Wilcox <willy@...radead.org>,
        Larry Woodman <lwoodman@...hat.com>,
        James Bottomley <James.Bottomley@...senPartnership.com>,
        "Wangkai (Kevin C)" <wangkai86@...wei.com>,
        Michal Hocko <mhocko@...nel.org>
Subject: Re: [PATCH v3 4/4] fs/dcache: Eliminate branches in
 nr_dentry_negative accounting

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