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From: Dave Chinner <david@fromorbit.com>
To: Waiman Long <longman@redhat.com>
Cc: Alexander Viro <viro@zeniv.linux.org.uk>,
	Jonathan Corbet <corbet@lwn.net>,
	linux-kernel@vger.kernel.org, linux-fsdevel@vger.kernel.org,
	linux-mm@kvack.org, linux-doc@vger.kernel.org,
	"Luis R. Rodriguez" <mcgrof@kernel.org>,
	Kees Cook <keescook@chromium.org>,
	Linus Torvalds <torvalds@linux-foundation.org>,
	Jan Kara <jack@suse.cz>,
	"Paul E. McKenney" <paulmck@linux.vnet.ibm.com>,
	Andrew Morton <akpm@linux-foundation.org>,
	Ingo Molnar <mingo@kernel.org>,
	Miklos Szeredi <mszeredi@redhat.com>,
	Matthew Wilcox <willy@infradead.org>,
	Larry Woodman <lwoodman@redhat.com>,
	James Bottomley <James.Bottomley@HansenPartnership.com>,
	"Wangkai (Kevin C)" <wangkai86@huawei.com>,
	Michal Hocko <mhocko@kernel.org>
Subject: Re: [PATCH v3 4/4] fs/dcache: Eliminate branches in nr_dentry_negative accounting
Date: Wed, 12 Sep 2018 08:13:15 +1000	[thread overview]
Message-ID: <20180911221315.GH5631@dastard> (raw)
In-Reply-To: <1536693506-11949-5-git-send-email-longman@redhat.com>

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.
-- 
Dave Chinner
david@fromorbit.com

  reply	other threads:[~2018-09-11 22:13 UTC|newest]

Thread overview: 16+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2018-09-11 19:18 [PATCH v3 0/4] fs/dcache: Track # of negative dentries Waiman Long
2018-09-11 19:18 ` [PATCH v3 1/4] fs/dcache: Fix incorrect nr_dentry_unused accounting in shrink_dcache_sb() Waiman Long
2018-09-11 22:02   ` Dave Chinner
2018-09-12 15:41     ` Waiman Long
2018-09-11 19:18 ` [PATCH v3 2/4] fs: Don't need to put list_lru into its own cacheline Waiman Long
2018-09-11 22:02   ` Dave Chinner
2018-09-11 19:18 ` [PATCH v3 3/4] fs/dcache: Track & report number of negative dentries Waiman Long
2018-09-11 22:08   ` Dave Chinner
2018-09-12 15:40     ` Waiman Long
2018-09-11 19:18 ` [PATCH v3 4/4] fs/dcache: Eliminate branches in nr_dentry_negative accounting Waiman Long
2018-09-11 22:13   ` Dave Chinner [this message]
2018-09-12 15:44     ` Waiman Long
2018-09-12  2:36   ` Matthew Wilcox
2018-09-12 15:49     ` Waiman Long
2018-09-12 15:55       ` Matthew Wilcox
2018-09-12 16:11         ` Waiman Long

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