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

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Date:	Mon, 16 Jul 2007 17:43:22 +0200 (CEST)
From:	Roman Zippel <zippel@...ux-m68k.org>
To:	Jonathan Corbet <corbet@....net>
cc:	linux-kernel@...r.kernel.org, Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>
Subject: Re: [PATCH/RFC] msleep() with hrtimers 

Hi,

On Mon, 16 Jul 2007, Jonathan Corbet wrote:

> > 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.

That's a bit my problem - we have to consider other setups as well.
Is it worth converting all msleep users behind their back or should we 
just a provide a separate function for those who care?
I would really like to keep hrtimer and kernel timer separate and make it 
obvious who is using what, as the usage requirements are somewhat 
different.

> > 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.

Which driver is this? I'd like to look at this, in case there's some other 
hidden problem. 

> > 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().

Actually if that's the case I'd consider this a bug, where is that extra 
cost coming from?

> 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.

I know that Ingo considers everything HZ related evil, but it really is 
not - it keeps Linux scalable. Unless you need the high resolution the 
timer wheel performance is still pretty hard to beat. That 
"known-to-expire" stuff is really the least significant problem to 
consider here, please just forget about it.
I don't want to keep anyone from using hrtimer, if it's just some driver 
go wild, but in generic code we have to consider portability issues. Using 
jiffies as a time base is still unbeatable cheap in the general case, so 
we have to carefully consider whether using a different time source is 
required. There is nothing wrong with using jiffies if it fits the bill 
and in many cases it still does.

bye, Roman
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