linux-kernel - Re: [PATCHv2 4/6] sched_clock: Add support for >32 bit sched

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Date:	Sun, 09 Jun 2013 23:12:08 -0500
From:	Rob Herring <robherring2@...il.com>
To:	Russell King - ARM Linux <linux@....linux.org.uk>
CC:	Stephen Boyd <sboyd@...eaurora.org>, linux-arm-msm@...r.kernel.org,
	Will Deacon <will.deacon@....com>,
	linux-kernel@...r.kernel.org, arm@...nel.org,
	John Stultz <john.stultz@...aro.org>,
	Catalin Marinas <catalin.marinas@....com>,
	Thomas Gleixner <tglx@...utronix.de>,
	linux-arm-kernel@...ts.infradead.org
Subject: Re: [PATCHv2 4/6] sched_clock: Add support for >32 bit sched_clock

On 06/04/2013 05:21 AM, Russell King - ARM Linux wrote:
> On Mon, Jun 03, 2013 at 06:51:59PM -0700, Stephen Boyd wrote:
>> On 06/03/13 15:12, Russell King - ARM Linux wrote:
>>> If you have a 56-bit clock which ticks at a period of 1ns, then
>>> cd.rate = 1, and your sched_clock() values will be truncated to 56-bits.
>>> The scheduler always _requires_ 64-bits from sched_clock.  That's why we
>>> have the complicated code to extend the 32-bits-or-less to a _full_
>>> 64-bit value.
>>>
>>> Let me make this clearer: sched_clock() return values _must_ without
>>> exception monotonically increment from zero to 2^64-1 and then wrap
>>> back to zero.  No other behaviour is acceptable for sched_clock().
>>
>> Ok so you're saying if we have less than 64 bits of useable information
>> we _must_ do something to find where the wraparound will occur and
>> adjust for it so that epoch_ns is always incrementing until 2^64-1. Fair
>> enough. I was trying to avoid more work because on arm architected timer
>> platforms it takes many years for that to happen.
>>
>> I'll see what I can do.
> 
> Well, 56 bits at 1ns intervals is 833 days (2^56 / (1000000000*60*60*24)).
> We used to say that 497 days was enough several years ago, and that got
> fixed.  We used to say 640K was enough memory for anything, and that
> got fixed.

The ARM ARM states a minimum resolution of 40 years AND at least 56-bits
of resolution. So a 1Gz counter would have to have more that 56 bits.

Rob

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