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

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Message-ID: <87r35ctcrp.fsf@intel.com>
Date:   Tue, 13 Dec 2016 11:10:50 +0200
From:   Jani Nikula <jani.nikula@...ux.intel.com>
To:     Nicholas Mc Guire <hofrat@...dl.org>,
        Thomas Gleixner <tglx@...utronix.de>
Cc:     Jonathan Corbet <corbet@....net>, linux-kernel@...r.kernel.org,
        linux-doc@...r.kernel.org, Nicholas Mc Guire <hofrat@...dl.org>,
        Dan Carpenter <dan.carpenter@...cle.com>,
        Julia Lawall <julia.lawall@...6.fr>
Subject: Re: [PATCH] doc: add note on usleep_range range

On Tue, 13 Dec 2016, Nicholas Mc Guire <hofrat@...dl.org> 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.

So I don't really have anything to do with the timer subsystem, I'm just
their "consumer", so take this with a grain of salt.

Documentation is good, but I don't think this will be enough.

I think the only thing that will work is to detect and complain about
things like this automatically. Some ideas:

* WARN_ON(min == max) or WARN_ON_ONCE(min == max) in usleep_range()
  might be drastic, but it would get the job done eventually.

* If you want to avoid the runtime overhead (and complaints about the
  backtraces), you could wrap usleep_range() in a macro that does
  BUILD_BUG_ON(min == max) if the parameters are build time constants
  (they usually are). But you'd have to fix all the problem cases first.

* You could try (to persuade Julia or Dan) to come up with a
  cocci/smatch check for usleep_range() calls where min == max, so we
  could get bug reports for this. This probably works on expressions, so
  this would catch also cases where the parameters aren't built time
  constants.

BR,
Jani.

>
> 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
> +			less than 50 microseconds probably is only preventing
> +			timer subsystem optimization but providing no benefit.
> +
>  	SLEEPING FOR LARGER MSECS ( 10ms+ )
>  		* Use msleep or possibly msleep_interruptible

-- 
Jani Nikula, Intel Open Source Technology Center