linux-kernel - Re: [PATCH V2] perf: x86: Improve accuracy of perf/sched clock

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Message-ID: <20150817095617.GA20948@worktop>
Date:	Mon, 17 Aug 2015 11:56:17 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Adrian Hunter <adrian.hunter@...el.com>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Arnaldo Carvalho de Melo <acme@...nel.org>,
	Andy Lutomirski <luto@...capital.net>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-kernel@...r.kernel.org,
	Stephane Eranian <eranian@...gle.com>,
	Andi Kleen <ak@...ux.intel.com>
Subject: Re: [PATCH V2] perf: x86: Improve accuracy of perf/sched clock

On Mon, Aug 17, 2015 at 10:34:03AM +0300, Adrian Hunter wrote:
> On 29/07/15 00:14, Adrian Hunter wrote:
> > When TSC is stable perf/sched clock is based on it.
> > However the conversion from cycles to nanoseconds
> > is not as accurate as it could be.  Because
> > CYC2NS_SCALE_FACTOR is 10, the accuracy is +/- 1/2048
> > 
> > The change is to calculate the maximum shift that
> > results in a multiplier that is still a 32-bit number.
> > For example all frequencies over 1 GHz will have
> > a shift of 32, making the accuracy of the conversion
> > +/- 1/(2^33)
> > 
> > Signed-off-by: Adrian Hunter <adrian.hunter@...el.com>
> 
> Is this OK?

Yes I think so, sorry I got backlogged with preparation for Seattle.

Thomas, if you see this, could you merge with my ACK on, otherwise I'll
queue it the normal route once I'm back.
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