linux-kernel - Re: [PATCH/RFC] msleep() with hrtimers

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Message-ID: <21003.1184596954@lwn.net>
Date:	Mon, 16 Jul 2007 08:42:34 -0600
From:	corbet@....net (Jonathan Corbet)
To:	Roman Zippel <zippel@...ux-m68k.org>
cc:	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH/RFC] msleep() with hrtimers 

Hey, Roman,

> One possible problem here is that setting up that timer can be 
> considerably more expensive, for a relative timer you have to read the 
> current time, which can be quite expensive (e.g. your machine now uses the 
> PIT timer, because TSC was deemed unstable).

That's a possibility, I admit I haven't benchmarked it.  I will say that
I don't think it will be enough to matter - msleep() is not a hot-path
sort of function.  Once the system is up and running it almost never
gets called at all - at least, on my setup.

> One question here would be, is it really a problem to sleep a little more?

"A little more" is a bit different than "twenty times as long as you
asked for."  That "little bit more" added up to a few seconds when
programming a device which needs a brief delay after tweaking each of
almost 200 registers.

> BTW there is another thing to consider. If you already run with hrtimer/ 
> dyntick, there is not much reason to keep HZ at 100, so you could just 
> increase HZ to get the same effect.

Except that then, with the current implementation, you're paying for the
higher HZ whenever the CPU is busy.  I bet that doesn't take long to
overwhelm any added overhead in the hrtimer msleep().

In the end, I did this because I thought msleep() should do what it
claims to do, because I thought that getting a known-to-expire timeout
off the timer wheel made sense, and to make a tiny baby step in the
direction of reducing the use of jiffies in the core code.

jon
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