linux-kernel - Re: NO_HZ: timer interrupt stuck [Re: Linux 2.6.21-rc1]

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Message-Id: <1172150712.3531.265.camel@laptopd505.fenrus.org>
Date:	Thu, 22 Feb 2007 14:25:12 +0100
From:	Arjan van de Ven <arjan@...radead.org>
To:	Jan Engelhardt <jengelh@...ux01.gwdg.de>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	Luca Tettamanti <kronos.it@...il.com>,
	linux-kernel@...r.kernel.org
Subject: Re: NO_HZ: timer interrupt stuck [Re: Linux 2.6.21-rc1]


> What's the benefit of doing so - and why has not it been done before?
> I mean, I run a regular 2.6.18.6,
> /sys/devices/system/clocksource/clocksource0/current_clocksource (is this
> related?) shows "acpi_pm", but the IRQ0 counter increases at HZ. Maybe I
> am confusing things, but why the need for PIT when clocksource is acpi_pm
> anyway?

you're mixing up 2 concepts:
1) clocksource
2) eventsource

1) is for "what time is it now", and acpi_pm is useful for that, as are
several other things such as rdtsc
2) is for "I need THIS to happen X miliseconds from now". acpi_pm is not
useful for that, nor is rdtsc.

some can be used for both (PIT), but on a concept level the uses are
independent. The advantage of local apic over PIT is that local apic is
cheap to do "one shot" future events with, while the PIT will tick
periodic at a fixed frequency. With tickless idle.. that's not what you
want.


-- 
if you want to mail me at work (you don't), use arjan (at) linux.intel.com
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