linux-kernel - Re: [RFC PATCH] x86:Use cpu_khz for loops_per

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Date:	Tue, 3 Jun 2008 21:01:54 -0700
From:	"Alok kataria" <alokkataria1@...il.com>
To:	"Arjan van de Ven" <arjan@...radead.org>
Cc:	akataria@...are.com, "Thomas Gleixner" <tglx@...utronix.de>,
	"Ingo Molnar" <mingo@...e.hu>, LKML <linux-kernel@...r.kernel.org>,
	"Dan Hecht" <dhecht@...are.com>, "Tim Mann" <mann@...are.com>,
	"Zachary Amsden" <zach@...are.com>,
	"Sahil Rihan" <srihan@...are.com>
Subject: Re: [RFC PATCH] x86:Use cpu_khz for loops_per_jiffy calculation

On Tue, Jun 3, 2008 at 6:20 PM, Arjan van de Ven <arjan@...radead.org> wrote:
> On Tue, 03 Jun 2008 17:41:09 -0700
> Alok Kataria <akataria@...are.com> wrote:
>
>> On X86 platform we can use the value of cpu_khz computed during tsc
>> calibration to calculate the loops_per_jiffy value. Its very
>> important to keep the error in lpj values to minimum as any error in
>> that may result in kernel panic in check_timer.
>> In virtualization environment on a highly overloaded host, the guest
>> delay calibration may sometimes result in errors beyond the ~50% that
>> timer_irq_works can handle, resulting in the guest panicking.
> \
>
> can you guarantee that the rate tsc ticks at is the same as the current
> CPU frequency? Answer: You can't....
>

I think at the boot time atleast we can assume that, no ?
If you are referring to the cpu frequency changing at run time (aka
dynamic frequency scaling),
in that case the time_cpufreq_notifier should take care of updating
the loops_per_jiffy
value for that corresponding cpu.

> sadly we do need to calibrate this...
>
> In addition, clearly you can have different cpus in a system run at a
> different rate (both in terms of cpu_khz and, independently, in terms
> of tsc rate)
>

Again yes at run time frequency's may change but they shouldn't at boottime.
AFAIK, i don't think there are X86 MP systems with asymmetric cpus
i.e. systems with different
base frequencies. If thats not true then there sure is a problem.

Thanks,
Alok

>
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