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Message-ID: <875xpzvt3v.fsf@somnus>
Date: Fri, 11 Oct 2024 12:20:20 +0200
From: Anna-Maria Behnsen <anna-maria@...utronix.de>
To: Frederic Weisbecker <frederic@...nel.org>
Cc: Thomas Gleixner <tglx@...utronix.de>, Jonathan Corbet <corbet@....net>,
linux-kernel@...r.kernel.org, Len Brown <len.brown@...el.com>, "Rafael J.
Wysocki" <rafael@...nel.org>, Arnd Bergmann <arnd@...db.de>,
linux-arch@...r.kernel.org
Subject: Re: [PATCH v2 05/15] timers: Update function descriptions of
sleep/delay related functions
Frederic Weisbecker <frederic@...nel.org> writes:
> Le Thu, Oct 10, 2024 at 10:45:03AM +0200, Anna-Maria Behnsen a écrit :
>> Frederic Weisbecker <frederic@...nel.org> writes:
>> >> @@ -281,7 +281,34 @@ EXPORT_SYMBOL_GPL(schedule_hrtimeout);
>> >>
>> >> /**
>> >> * msleep - sleep safely even with waitqueue interruptions
>> >> - * @msecs: Time in milliseconds to sleep for
>> >> + * @msecs: Requested sleep duration in milliseconds
>> >> + *
>> >> + * msleep() uses jiffy based timeouts for the sleep duration. The accuracy of
>> >> + * the resulting sleep duration depends on:
>> >> + *
>> >> + * * HZ configuration
>> >> + * * sleep duration (as granularity of a bucket which collects timers increases
>> >> + * with the timer wheel levels)
>> >> + *
>> >> + * When the timer is queued into the second level of the timer wheel the maximum
>> >> + * additional delay will be 12.5%. For explanation please check the detailed
>> >> + * description about the basics of the timer wheel. In case this is accurate
>> >> + * enough check which sleep length is selected to make sure required accuracy is
>> >> + * given. Please use therefore the following simple steps:
>> >> + *
>> >> + * #. Decide which slack is fine for the requested sleep duration - but do not
>> >> + * use values shorter than 1/8
>> >
>> > I'm confused, what means 1/x for a slack value? 1/8 means 125 msecs? I'm not
>> > even I understand what you mean by slack. Is it the bucket_expiry - expiry?
>>
>> I was confused as well and had to read it twice... I would propose to
>> rephrase the whole function description:
>>
>>
>> /**
>> * msleep - sleep safely even with waitqueue interruptions
>> * @msecs: Requested sleep duration in milliseconds
>> *
>> * msleep() uses jiffy based timeouts for the sleep duration. Because of the
>> * design of the timer wheel, the maximum additional percentage delay (slack) is
>> * 12.5%. This is only valid for timers which will end up in the second or a
>> * higher level of the timer wheel. For explanation of those 12.5% please check
>> * the detailed description about the basics of the timer wheel.
>
> I've never realized this constant worst percentage of slack. Would be nice to mention
> that somewhere in kernel/time/timer.c
Yes, we can explicitly add it (I will put it on the TODO list). It's
possible to calculate it on your own with the overview of levels and
granularity,...
> However this doesn't need a second to apply. It only takes crossing levels above
> 0. Or am I missing something?
s/the second/level 1/
more clear? Then it's the same number as used in the timer wheel
documentation.
>> *
>> * The slack of timers which will end up in the first level depends on:
>> *
Same here: s/the first level/level 0/
>> * * sleep duration (msecs)
>> * * HZ configuration
>> *
>> * To make sure the sleep duration with the slack is accurate enough, a slack
>> * value is required (because of the design of the timer wheel it is not
>
> But where is it required?
The callsite has to decide which accuracy/slack is required for their
use case (this was also part of the discussion which leads to this
queue).
>> * possible to define a value smaller than 12.5%). The following check makes
>> * clear, whether the sleep duration with the defined slack and with the HZ
>> * configuration will meet the constraints:
>> *
>> * ``msecs >= (MSECS_PER_TICK / slack)``
>> *
>> * Examples:
>> *
>> * * ``HZ=1000`` with ``slack=25%``: ``MSECS_PER_TICK / slack = 1 / (1/4) = 4``:
>> * all sleep durations greater or equal 4ms will meet the constraints.
>> * * ``HZ=1000`` with ``slack=12.5%``: ``MSECS_PER_TICK / slack = 1 / (1/8) = 8``:
>> * all sleep durations greater or equal 8ms will meet the constraints.
>> * * ``HZ=250`` with ``slack=25%``: ``MSECS_PER_TICK / slack = 4 / (1/4) = 16``:
>> * all sleep durations greater or equal 16ms will meet the constraints.
>> * * ``HZ=250`` with ``slack=12.5%``: ``MSECS_PER_TICK / slack = 4 / (1/8) = 32``:
>> * all sleep durations greater or equal 32ms will meet the constraints.
>
> But who defines those slacks and where? I'm even more confused now...
I think I know where the confusion comes from. I rephrase it once more
and turned around the calculation:
/**
* msleep - sleep safely even with waitqueue interruptions
* @msecs: Requested sleep duration in milliseconds
*
* msleep() uses jiffy based timeouts for the sleep duration. Because of the
* design of the timer wheel, the maximum additional percentage delay (slack) is
* 12.5%. This is only valid for timers which will end up in level 1 or a
* higher level of the timer wheel. For explanation of those 12.5% please check
* the detailed description about the basics of the timer wheel.
*
* The slack of timers which will end up in level 0 depends on sleep
* duration (msecs) and HZ configuration and can be calculated in the
* following way (with the timer wheel design restriction that the slack
* is not less than 12.5%):
*
* ``slack = MSECS_PER_TICK / msecs``
*
* When the allowed slack of the callsite is known, the calculation
* could be turned around to find the minimal allowed sleep duration to meet
* the constraints. For example:
*
* * ``HZ=1000`` with ``slack=25%``: ``MSECS_PER_TICK / slack = 1 / (1/4) = 4``:
* all sleep durations greater or equal 4ms will meet the constraints.
* * ``HZ=1000`` with ``slack=12.5%``: ``MSECS_PER_TICK / slack = 1 / (1/8) = 8``:
* all sleep durations greater or equal 8ms will meet the constraints.
* * ``HZ=250`` with ``slack=25%``: ``MSECS_PER_TICK / slack = 4 / (1/4) = 16``:
* all sleep durations greater or equal 16ms will meet the constraints.
* * ``HZ=250`` with ``slack=12.5%``: ``MSECS_PER_TICK / slack = 4 / (1/8) = 32``:
* all sleep durations greater or equal 32ms will meet the constraints.
*
* See also the signal aware variant msleep_interruptible().
*/
Hopefully this attempt clarifies the confusion?
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