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From: Stephane Gasparini <stephane.gasparini@linux.intel.com>
To: Peter Zijlstra <peterz@infradead.org>
Cc: Len Brown <lenb@kernel.org>,
	x86@kernel.org, linux-pm@vger.kernel.org,
	linux-kernel@vger.kernel.org, Len Brown <len.brown@intel.com>
Subject: Re: [PATCH] x86: Calculate MHz using APERF/MPERF for cpuinfo and scaling_cur_freq
Date: Fri, 1 Apr 2016 11:30:48 +0200	[thread overview]
Message-ID: <7239BF36-2C27-4325-A5B8-01BC4CA58B4C@linux.intel.com> (raw)
In-Reply-To: <20160401082923.GH11035@twins.programming.kicks-ass.net>

my comment was about your comment that MSR have wrapped however many times



> On Apr 1, 2016, at 10:03 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> 
> That is; if userspace doesn't request a freq reading we can go without
> reading this for a very long time.
> 
>> +
>> +	rdmsrl(MSR_IA32_APERF, aperf);
>> +	rdmsrl(MSR_IA32_MPERF, mperf);
>> +
>> +	aperf_delta = aperf - s->aperf;
>> +	mperf_delta = mperf - s->mperf;
> 
> That means these delta's can be arbitrarily large, in fact the MSRs can
> have wrapped however many times.

The MSRs will not wrap that often.

—
Steph




> On Apr 1, 2016, at 10:29 AM, Peter Zijlstra <peterz@infradead.org> wrote:
> 
> On Fri, Apr 01, 2016 at 10:23:23AM +0200, Peter Zijlstra wrote:
>> 
>> Trim your emails
>> 
>> On Fri, Apr 01, 2016 at 10:16:42AM +0200, Stephane Gasparini wrote:
>> 
>>>> That means these delta's can be arbitrarily large, in fact the MSRs can
>>>> have wrapped however many times.
>>> 
>>> 64 bits is 18 446 744 073 709 551 615
>>> 
>>> so even assuming a 10 GHz frequency if my math are good this is more than
>>> 58 years before the MSR wrap around, assuming the device ran always at max
>>> freq.
>> 
>> fair enough.. but going with 10Ghz, cpu_khz would be 10e6 ~ 33 bits,
> 
> I can't do maths this morning; 23 bits
> 
>> which effectively reduces the wrap/overflow time to just 31 bits, which
>> per that frequency is just ~1/4th of a second.
> 
> 41 giving lots more, but a reasonable time to wrap/overflow.
> --
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  reply	other threads:[~2016-04-01  9:30 UTC|newest]

Thread overview: 12+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
     [not found] <6e0c25e64e0fb65a42dfc63ad5f660302e07cd87.1459485198.git.len.brown@intel.com>
2016-04-01  4:37 ` [PATCH] x86: Calculate MHz using APERF/MPERF for cpuinfo and scaling_cur_freq Len Brown
2016-04-01  7:56   ` [PATCH] x86: Calculate MHz using APERF/MPERF for cpuinfo and scaling_cur_freqy Thomas Gleixner
2016-04-01  8:03   ` [PATCH] x86: Calculate MHz using APERF/MPERF for cpuinfo and scaling_cur_freq Peter Zijlstra
2016-04-01  8:16     ` Stephane Gasparini
2016-04-01  8:23       ` Peter Zijlstra
2016-04-01  8:29         ` Peter Zijlstra
2016-04-01  9:30           ` Stephane Gasparini [this message]
2016-04-01  9:38             ` Peter Zijlstra
2016-04-01  9:50             ` Borislav Petkov
2016-04-02  5:22               ` Len Brown
2016-04-01  8:16   ` Peter Zijlstra
2016-04-24 16:38   ` Pavel Machek

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