linux-kernel - Re: [PATCH v2] x86: Reduce clock calibration time during slave cpu startup

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Message-ID: <20110805131638.GA27779@sgi.com>
Date:	Fri, 5 Aug 2011 08:16:38 -0500
From:	Jack Steiner <steiner@....com>
To:	Ingo Molnar <mingo@...e.hu>
Cc:	tglx@...utronix.de, davej@...hat.com, yinghan@...gle.com,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] x86: Reduce clock calibration time during slave cpu
	startup

On Fri, Aug 05, 2011 at 12:46:35PM +0200, Ingo Molnar wrote:
> 
> * Jack Steiner <steiner@....com> wrote:
> 
> > +/*
> > + * Check if another cpu is in the same socket and has already been calibrated.
> > + * If found, use the previous value. This assumes all cores in the same physical
> > + * socket have the same core frequency.
> > +
> > +unsigned long __cpuinit calibrate_delay_is_known(void)
> > +{
> > +	int i, cpu = smp_processor_id();
> > +
> > +	if (!tsc_disabled && !cpu_has(&cpu_data(cpu), X86_FEATURE_CONSTANT_TSC))
> > +		return 0;
> > +
> > +	for_each_online_cpu(i)
> > +		if (cpu_data(i).phys_proc_id == cpu_data(cpu).phys_proc_id)
> > +			return cpu_data(i).loops_per_jiffy;
> 
> Hm, why do we have to make such an assumption? Cannot we query the 
> core frequency?

>> See V2 of the patch. The above assumption was removed & replaced by a check for
>> X86_FEATURE_CONSTANT_TSC & using the TSC for __delay(). If all cores see a
>> constant TSC frequency, then core frequency should not matter.
>> 
>> Does this make sense....


Ahhh..  I see what you mean. I failed to update the comment in the code.
Aside from the bogus comment (I'll send a V3 with a fixed comment), does the patch
look ok.


--- jack
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