Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67;
From:   "Dilger, Andreas" <andreas.dilger@intel.com>
To:     NeilBrown <neilb@suse.com>
CC:     James Simmons <jsimmons@infradead.org>,
        "Drokin, Oleg" <oleg.drokin@intel.com>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Linux Kernel Mailing List <linux-kernel@vger.kernel.org>,
        "Lustre Development List" <lustre-devel@lists.lustre.org>
Subject: Re: [lustre-devel] [PATCH 11/17] staging: lustre: libcfs: discard
 cfs_time_shift().
Thread-Topic: [lustre-devel] [PATCH 11/17] staging: lustre: libcfs: discard
 cfs_time_shift().
Thread-Index: AQHTyFmnPaab2m4I+kyw4VsVO58/aKPuV+IAgAAv/ICAAjWlAA==
Date:   Wed, 4 Apr 2018 08:10:44 +0000
Message-ID: <CB90FEDF-3F67-46A7-8EC3-ABC788E7978C@intel.com>
References: <152229732768.27689.4800156943561963977.stgit@noble>
 <152229760863.27689.4681198496926109808.stgit@noble>
 <alpine.LFD.2.21.1803302001440.9681@casper.infradead.org>
 <0B6A147C-7E98-4B00-AF32-AF8843484CF9@intel.com>
 <87muyl1bzs.fsf@notabene.neil.brown.name>
In-Reply-To: <87muyl1bzs.fsf@notabene.neil.brown.name>
Accept-Language: en-US
Content-Language: en-US
Content-Type: text/plain; charset="us-ascii"
Content-ID: <AC47B2A255C0A040BDE0C278AED4F2BB@intel.com>
Content-Transfer-Encoding: 8BIT
MIME-Version: 1.0
Sender: linux-kernel-owner@vger.kernel.org
Precedence: bulk

On Apr 2, 2018, at 16:26, NeilBrown <neilb@suse.com> wrote:
> On Mon, Apr 02 2018, Dilger, Andreas wrote:
>> On Mar 30, 2018, at 13:02, James Simmons <jsimmons@infradead.org> wrote:
>>>> This function simply multiplies by HZ and adds jiffies.
>>>> This is simple enough to be opencoded, and doing so
>>>> makes the code easier to read.
>>>> 
>>>> Same for cfs_time_shift_64()
>>> 
>>> Reviewed-by: James Simmons <jsimmons@infradead.org>
>> 
>> Hmm, I thought we were trying to get rid of direct HZ usage in modules,
>> because of tickless systems, and move to e.g. msecs_to_jiffies() or similar?
> 
> Are we?  I hadn't heard but I could easily have missed it.
> Documentation/scheduler/completion.txt does say
> 
>    Timeouts are preferably calculated with
>    msecs_to_jiffies() or usecs_to_jiffies().
> 
> but is isn't clear what they are preferred to.  Do you remember where
> you heard? or have a reference?

I thought the goal was to avoid hard-coding the HZ value so that kernels
could have variable clock rates in the future.

Cheers, Andreas

> $ git grep ' \* *HZ'  |wc
>   2244   15679  170016
> $ git grep msecs_to_jiffies | wc
>   3301   13151  276725
> 
> so msecs_to_jiffies is slightly more popular than "* HZ" (even if you add
> in "HZ *").  But that could just be a preference for using milliseconds
> over using seconds.
> 
> $ git grep msecs_to_jiffies   | grep -c '[0-9]000'
> 587
> 
> so there are only 587 places that msecs_to_jiffies is clearly used in
> place of multiplying by HZ.
> 
> If we were to pursue this, I would want to add secs_to_jiffies() to
> include/linux/jiffies.h and use that.
> 
> Thanks,
> NeilBrown
> _______________________________________________
> lustre-devel mailing list
> lustre-devel@lists.lustre.org
> http://lists.lustre.org/listinfo.cgi/lustre-devel-lustre.org

Cheers, Andreas
--
Andreas Dilger
Lustre Principal Architect
Intel Corporation