linux-kernel - Re: [RFC 2/2] Make x86 calibrate

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Message-ID: <20110331104601.GA8577@elte.hu>
Date:	Thu, 31 Mar 2011 12:46:01 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	Avi Kivity <avi@...hat.com>
Cc:	Robin Holt <holt@....com>, Thomas Gleixner <tglx@...utronix.de>,
	Yinghai Lu <yinghai@...nel.org>,
	Andi Kleen <andi@...stfloor.org>, linux-kernel@...r.kernel.org,
	Ingo Molnar <mingo@...hat.com>,
	Alan Cox <alan@...rguk.ukuu.org.uk>
Subject: Re: [RFC 2/2] Make x86 calibrate_delay run in parallel.


* Avi Kivity <avi@...hat.com> wrote:

> On 03/31/2011 11:57 AM, Ingo Molnar wrote:
> >>
> >>  I am not trying to be argumentative.  I never got an understanding of
> >>  what was going wrong with that earlier patch and am hoping for some
> >>  understanding now.
> >
> > Well, if calibrate_delay() calls run in parallel then different 
> > hyperthreads will impact each other.
> 
> It's different but not more wrong.  If delay() later runs on a thread whose 
> sibling is busy, it will in fact give more accurate results.

No, it's actively wrong: because it makes the delay loop *run faster* when 
other siblings

I.e. this shortens udelay(X)s potentially, which is far more dangerous than the 
current conservative approach of potentially *lengthening* them.

> > Really, there's no good reason why every CPU should be calibrated on a 
> > system running identical CPUs, right? Mixed-frequency systems are rather 
> > elusive on x86.
> 
> Good point.  And udelay() users are probably not sensitive to accuracy anyway 
> (which changes with load and thermal conditions).

True with one important distinction: they are only sensitive to one fact, that 
the delay should not be *shorter* than specified. By shortening udelay() we 
essentially overclock the hardware's tolerances - not good.

Thanks,

	Ingo
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