linux-kernel - Re: [PATCH] doc: add note on usleep

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Message-ID: <20161227215626.GA5336@amd>
Date:   Tue, 27 Dec 2016 22:56:26 +0100
From:   Pavel Machek <pavel@....cz>
To:     Nicholas Mc Guire <hofrat@...dl.org>
Cc:     Thomas Gleixner <tglx@...utronix.de>,
        Jonathan Corbet <corbet@....net>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org
Subject: Re: [PATCH] doc: add note on usleep_range range

On Tue 2016-12-13 04:58:43, Nicholas Mc Guire wrote:
> useleep_range() with a delta of 0 makes no sense and only prevents the
> timer subsystem from optimizing interrupts. As any user of usleep_range()
> is in non-atomic context the timer jitter is in the range of 10s of 
> microseconds anyway.
> 
> This adds a note making it clear that a range of 0 is a bad idea.
> 
> Signed-off-by: Nicholas Mc Guire <hofrat@...dl.org>
> ---
> 
> as of 4.9.0 there are about 20 cases of usleep_ranges() that have 
> min==max and none of them really look like they are necessary, so 
> it does seem like a relatively common misunderstanding worth
> noting in the documentation.
> 
> Patch is against 4.9.0 (localversion-next is 20161212)
> 
>  Documentation/timers/timers-howto.txt | 7 +++++++
>  1 file changed, 7 insertions(+)
> 
> diff --git a/Documentation/timers/timers-howto.txt b/Documentation/timers/timers-howto.txt
> index 038f8c7..b5cdf82 100644
> --- a/Documentation/timers/timers-howto.txt
> +++ b/Documentation/timers/timers-howto.txt
> @@ -93,6 +93,13 @@ NON-ATOMIC CONTEXT:
>  			tolerances here are very situation specific, thus it
>  			is left to the caller to determine a reasonable range.
>  
> +			A range of 0, that is usleep_range(100,100) or the 
> +			like, do not make sense as this code is in a 
> +			non-atomic section and a system can not be expected 
> +			to have jitter 0. For any non-RT code any delta

Would it be possible to fix english here?

"to have zero jitter" at least. I believe it is "does not".

I don't see how atomic vs. non-atomic context makes difference. There
are sources of jitter that affect atomic context...

> +			less than 50 microseconds probably is only preventing
> +			timer subsystem optimization but providing no benefit.

And I don't trust you here. _If_ it prevents timer optimalization,
_then_ it provides benefit, at least in the average case.

									Pavel
-- 
(english) http://www.livejournal.com/~pavelmachek
(cesky, pictures) http://atrey.karlin.mff.cuni.cz/~pavel/picture/horses/blog.html

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