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

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Date:   Wed, 12 Sep 2018 12:11:21 -0400
From:   Waiman Long <longman@...hat.com>
To:     Matthew Wilcox <willy@...radead.org>
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>,
        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 09/12/2018 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.

Cheers,
Longman