linux-kernel - Re: [RFC patch 0/4] TSC calibration improvements

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Message-ID: <alpine.LFD.1.10.0809040840330.3381@nehalem.linux-foundation.org>
Date:	Thu, 4 Sep 2008 08:45:59 -0700 (PDT)
From:	Linus Torvalds <torvalds@...ux-foundation.org>
To:	Ingo Molnar <mingo@...e.hu>
cc:	Thomas Gleixner <tglx@...utronix.de>,
	LKML <linux-kernel@...r.kernel.org>,
	Alok Kataria <akataria@...are.com>,
	Arjan van de Veen <arjan@...radead.org>,
	"H. Peter Anvin" <hpa@...or.com>
Subject: Re: [RFC patch 0/4] TSC calibration improvements

On Thu, 4 Sep 2008, Ingo Molnar wrote:
> 
> i've added them to tip/x86/tsc and merged it into tip/master - if 
> there's test success we can merge it into x86/urgent as well and push it 
> into v2.6.27. Any objections to that merge route?

I don't think it's quite that urgent, and wonder what the downside is of 
just changing the timeout to 10ms. On 32-bit x86, it was 30ms (I think) 
before the merge, so it sounds like 50ms was a bit excessive even before 
the whole "loop five times"/

So _short_ term, I'd really prefer (a) looping just three times and (b) 
looping with a smaller timeout.

Long-term, I actually think even 10ms is actually a total waste. I'll post 
my trial "quick calibration" code that is more likely to fail under 
virtualization or SMM (or, indeed, perhaps even on things like TMTA CPU's 
that can have longer latencies due to translation), but that is really 
fast and knows very intimately when it succeeds. I just need to do 
slightly more testing.

				Linus
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