linux-kernel - Re: Ques: [kernel/time/*] Is there any disadvantage in using sleep

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Message-ID: <alpine.DEB.2.11.1512142042500.4336@nanos>
Date:	Mon, 14 Dec 2015 20:53:40 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Aniroop Mathur <aniroop.mathur@...il.com>
cc:	Clemens Ladisch <clemens@...isch.de>,
	John Stultz <john.stultz@...aro.org>, a.mathur@...sung.com,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: Ques: [kernel/time/*] Is there any disadvantage in using
 sleep_range for more than 20ms delay ?

On Tue, 15 Dec 2015, Aniroop Mathur wrote:
> On Sun, Dec 13, 2015 at 3:17 PM, Clemens Ladisch <clemens@...isch.de> wrote:
> > Aniroop Mathur wrote:
> >>> 2. If we use msleep(40), is it possible that process could wake up after
> >>> 60 ms or 70 ms or 100 ms or more ?
> >
> > Yes, if lots of other processes compete for the CPU.
> 
> You mean to say "yes" for process competing for changing state
> from "waiting" to "ready" or "ready" to "running" ?

There is no state ready. We change from [un]interruptible to running,
but that's just the wakeup by the timer callback. When the task gets
on the CPU is a different question.
 
> Regarding above, could you please help to confirm below things?
> 1. Using msleep(40) with HZ=1000 (1ms), process can still be in
> "waiting" state and will not move to "ready" state even after 42,45 or 50 ms.
> Right ?

kernel/time/timer.c:apply_slack()

> 2. Using usleep_range(40000, 41000), it is guaranteed that process will
> change its state from waiting to ready state before or at 41 ms.
> Right ?

1) It is supposed to be woken up by the timer in that range, but there
   is no guarantee. Depends also on your kernel configuration and
   hardware capabilities.

2) The state changes from [un]interruptible to running. And again
   that does not tell you when it gets on the CPU.

> Regarding usleep_range disadvatage:
> 1. Usleep_range has a disadvantage that it does not allow the system to
> do optimal timer batching for power savings. Except that, Is there any
> other disadvantage?

Higher CPU usage.

> 2. Is the impact of optimal timer batching in systems like android or ubuntu
> with moderate cpu speed significant or it can be negligible also?
> To understand the impact better, it would be really appreciating if you could
> kindly explain in detail with some case and data input.

  http://bfy.tw/3HaV
  http://bfy.tw/3Han
  ....

Thanks,

	tglx
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