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

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Date:   Tue, 11 Sep 2018 19:36:10 -0700
From:   Matthew Wilcox <willy@...radead.org>
To:     Waiman Long <longman@...hat.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>,
        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 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.

You're substituting your judgement here for the compiler's.  I don't
see a reason why the compiler couldn't choose to use a cmov in order
to do this:

	if (dentry->d_flags & DCACHE_LRU_LIST)
		this_cpu_inc(nr_dentry_negative);

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.