linux-kernel - Re: [PATCH V3] perf: x86: Improve accuracy of perf/sched clock

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Message-ID: <20150901085703.GJ16853@twins.programming.kicks-ass.net>
Date:	Tue, 1 Sep 2015 10:57:03 +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 V3] perf: x86: Improve accuracy of perf/sched clock

On Tue, Sep 01, 2015 at 11:36:38AM +0300, Adrian Hunter wrote:
> On 21/08/15 12:05, 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).  That is achieved by using the
> > 'clocks_calc_mult_shift()' function.
> > 
> > Signed-off-by: Adrian Hunter <adrian.hunter@...el.com>
> 
> Ping?

I've queued it. Thanks
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