linux-kernel - Re: A few questions and issues with dynticks, NOHZ and powertop

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Message-ID: <20100405205213.GA4576@core.coreip.homeip.net>
Date:	Mon, 5 Apr 2010 13:52:13 -0700
From:	Dmitry Torokhov <dmitry.torokhov@...il.com>
To:	Dominik Brodowski <linux@...inikbrodowski.net>,
	Arjan van de Ven <arjan@...ux.intel.com>,
	Adam Belay <abelay@...ell.com>,
	Len Brown <len.brown@...el.com>, linux-kernel@...r.kernel.org,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...e.hu>,
	Peter Zijlstra <peterz@...radead.org>,
	Alan Stern <stern@...land.harvard.edu>
Subject: Re: A few questions and issues with dynticks, NOHZ and powertop

On Mon, Apr 05, 2010 at 10:41:43PM +0200, Dominik Brodowski wrote:
> On Sun, Apr 04, 2010 at 08:42:43PM -0700, Arjan van de Ven wrote:
> > On 4/4/2010 3:47, Dominik Brodowski wrote:
> > >Hey,
> > >
> > >On Sat, Apr 03, 2010 at 04:53:26PM -0700, Dmitry Torokhov wrote:
> > >>On Sun, Apr 04, 2010 at 12:33:28AM +0200, Dominik Brodowski wrote:
> > >>>
> > >>>4) SynPS/2 touchpad:
> > >>>Why does moving the touchpad lead to sooo many IRQs? I can't look as fast
> > >>>as the mouse pointer seems to get new data:
> > >>>   62,5% (473,1)<interrupt>  : PS/2 keyboard/mouse/touchpad
> > >>>
> > >>
> > >>80 pps @ 6 bytes/packet = 480 interrupts/sec.
> > >>
> > >>You can try using psmouse.rate=40 to limit it to 40 pps which should
> > >>bring it to the rate of standard PS/2 mouse at the expense of
> > >>sensitivity...
> > >
> > >as a sidenote: if we know -- like here -- that the next IRQ will be issued
> > >soon, in approximately 1.75 ms (well, at least on my system), might it make
> > >sense to make tick_nohz_get_sleep_length() smarter to know about this?
> > 
> > yes and no.
> > 
> > if you are very sure (95%+ or so) then absolutely it needs to know about this
> > so that the C state selection code can make a better decision.
> > Right now it tries to look at history to guess this delay.
> > 
> > Unfortunately we do not currently have such a concept in the code to make this
> > work... but it'd be really nice to have.
> 
> Dmitry, are we "very sure" in this touchpad case?
> 

Psmouse driver tries to not rely on any timing data really... But yes,
we do expect the next interrupt to arrive "shortly" and I guess the
driver could do some data gathering to collect average time between
interrupts. The question is - is it worth it?

-- 
Dmitry
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