linux-fsdevel.vger.kernel.org archive mirror
 help / color / mirror / Atom feed
From: Waiman Long <longman@redhat.com>
To: Matthew Wilcox <willy@infradead.org>
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>,
	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 12:11:21 -0400	[thread overview]
Message-ID: <6247b212-923f-f8a1-3f97-c346b606a7b6@redhat.com> (raw)
In-Reply-To: <20180912155557.GA18304@bombadil.infradead.org>

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

      reply	other threads:[~2018-09-12 16:11 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
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 [this message]

Reply instructions:

You may reply publicly to this message via plain-text email
using any one of the following methods:

* Save the following mbox file, import it into your mail client,
  and reply-to-all from there: mbox

  Avoid top-posting and favor interleaved quoting:
  https://en.wikipedia.org/wiki/Posting_style#Interleaved_style

* Reply using the --to, --cc, and --in-reply-to
  switches of git-send-email(1):

  git send-email \
    --in-reply-to=6247b212-923f-f8a1-3f97-c346b606a7b6@redhat.com \
    --to=longman@redhat.com \
    --cc=James.Bottomley@HansenPartnership.com \
    --cc=akpm@linux-foundation.org \
    --cc=corbet@lwn.net \
    --cc=jack@suse.cz \
    --cc=keescook@chromium.org \
    --cc=linux-doc@vger.kernel.org \
    --cc=linux-fsdevel@vger.kernel.org \
    --cc=linux-kernel@vger.kernel.org \
    --cc=linux-mm@kvack.org \
    --cc=lwoodman@redhat.com \
    --cc=mcgrof@kernel.org \
    --cc=mhocko@kernel.org \
    --cc=mingo@kernel.org \
    --cc=mszeredi@redhat.com \
    --cc=paulmck@linux.vnet.ibm.com \
    --cc=torvalds@linux-foundation.org \
    --cc=viro@zeniv.linux.org.uk \
    --cc=wangkai86@huawei.com \
    --cc=willy@infradead.org \
    /path/to/YOUR_REPLY

  https://kernel.org/pub/software/scm/git/docs/git-send-email.html

* If your mail client supports setting the In-Reply-To header
  via mailto: links, try the mailto: link
Be sure your reply has a Subject: header at the top and a blank line before the message body. 
This is a public inbox, see mirroring instructions
for how to clone and mirror all data and code used for this inbox;
as well as URLs for NNTP newsgroup(s).