linux-kernel - Re: [lustre-devel] [PATCH 11/17] staging: lustre: libcfs: discard cfs_time

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Date:   Wed, 4 Apr 2018 08:10:44 +0000
From:   "Dilger, Andreas" <andreas.dilger@...el.com>
To:     NeilBrown <neilb@...e.com>
CC:     James Simmons <jsimmons@...radead.org>,
        "Drokin, Oleg" <oleg.drokin@...el.com>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "Lustre Development List" <lustre-devel@...ts.lustre.org>
Subject: Re: [lustre-devel] [PATCH 11/17] staging: lustre: libcfs: discard
 cfs_time_shift().

On Apr 2, 2018, at 16:26, NeilBrown <neilb@...e.com> wrote:
> On Mon, Apr 02 2018, Dilger, Andreas wrote:
>> On Mar 30, 2018, at 13:02, James Simmons <jsimmons@...radead.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@...radead.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@...ts.lustre.org
> http://lists.lustre.org/listinfo.cgi/lustre-devel-lustre.org

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