linux-kernel - Re: [PATCH] x86: tsc based udelay should have rdtsc

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Message-ID: <20090626072504.GJ14078@elte.hu>
Date:	Fri, 26 Jun 2009 09:25:04 +0200
From:	Ingo Molnar <mingo@...e.hu>
To:	"Pallipadi, Venkatesh" <venkatesh.pallipadi@...el.com>
Cc:	linux-kernel@...r.kernel.org, tglx@...utronix.de, hpa@...or.com
Subject: Re: [PATCH] x86: tsc based udelay should have rdtsc_barrier


* Pallipadi, Venkatesh <venkatesh.pallipadi@...el.com> wrote:

> 
> delay_tsc needs rdtsc_barrier to provide proper delay.
> 
> Output from a test driver using hpet to cross check delay
> provided by udelay().
> 
> Before:
> [   86.794363] Expected delay 5us actual 4679ns
> [   87.154362] Expected delay 5us actual 698ns
> [   87.514162] Expected delay 5us actual 4539ns
> [   88.653716] Expected delay 5us actual 4539ns
> [   94.664106] Expected delay 10us actual 9638ns
> [   95.049351] Expected delay 10us actual 10126ns
> [   95.416110] Expected delay 10us actual 9568ns
> [   95.799216] Expected delay 10us actual 9638ns
> [  103.624104] Expected delay 10us actual 9707ns
> [  104.020619] Expected delay 10us actual 768ns
> [  104.419951] Expected delay 10us actual 9707ns
> 
> After:
> [   50.983320] Expected delay 5us actual 5587ns
> [   51.261807] Expected delay 5us actual 5587ns
> [   51.565715] Expected delay 5us actual 5657ns
> [   51.861171] Expected delay 5us actual 5587ns
> [   52.164704] Expected delay 5us actual 5726ns
> [   52.487457] Expected delay 5us actual 5657ns
> [   52.789338] Expected delay 5us actual 5726ns
> [   57.119680] Expected delay 10us actual 10755ns
> [   57.893997] Expected delay 10us actual 10615ns
> [   58.261287] Expected delay 10us actual 10755ns
> [   58.620505] Expected delay 10us actual 10825ns
> [   58.941035] Expected delay 10us actual 10755ns
> [   59.320903] Expected delay 10us actual 10615ns
> [   61.306311] Expected delay 10us actual 10755ns
> [   61.520542] Expected delay 10us actual 10615ns

Nice fix!

	Ingo
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