linux-kernel - Re: Interrupt latency on some 945GM platforms

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Date:	Tue, 14 Sep 2010 11:55:00 +0200
From:	Andi Kleen <andi@...stfloor.org>
To:	Vasily Khoruzhick <anarsoul@...il.com>
Cc:	Venkatesh Pallipadi <venki@...gle.com>,
	intel-gfx@...ts.freedesktop.org,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org
Subject: Re: Interrupt latency on some 945GM platforms

Vasily Khoruzhick <anarsoul@...il.com> writes:

>> Whats the clockevent in this case ("Tick Device" section of
>> /proc/timer_list). clocksource= option only changes the clocksource used
>> to maintain
>> timeofday. But, timer interrupt (clockevent) source will not change.
>> Wondering how just the clocksource change is making the diff here..
>> 
>> Also, if clocksource tsc has a higher rating than HPET. The reason
>> HPET is getting used as clocksource in the first place seems to be due
>> to TSC is not a dependable clocksource on this platform (may be it
>> stops in C3). So, I am not sure forcing it to tsc will be a good
>> thing. May be clocksource=acpi_pm is a better thing to try.
>> 
>> Thanks,
>> Venki
>
> I investigated it a bit and found out that single nohz=off option helps. Just 
> changing clocksource doesn't help, but it works smoothly with any clocksource 
> with nohz=off. So, it seems that something wrong with intel driver while 
> system is in tickless mode. 

It could be simply that nohz=off prevents the system from
going into lower idle states and it is related to that.

You could test this theory by running with processor.max_cstate=1

-Andi

-- 
ak@...ux.intel.com -- Speaking for myself only.
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