Re: [PATCHv2 4/6] sched_clock: Add support for >32 bit sched_clock

From: Russell King - ARM Linux
Date: Mon Jun 10 2013 - 12:08:31 EST

On Mon, Jun 10, 2013 at 09:31:21PM +0530, anish singh wrote:
> On Mon, Jun 10, 2013 at 9:08 PM, Russell King - ARM Linux
> <linux@xxxxxxxxxxxxxxxx> wrote:
> Least I can do is to say "Thanks".
> > On Mon, Jun 10, 2013 at 08:46:36PM +0530, anish singh wrote:
> >> Probably a trivial question.I was wondering why this particular requirement
> >> exists in the first place.I looked into this commit 112f38a4a3 but couldn't
> >> gather the reason.
> >
> > You're looking at a commit introducing an implementation. The requirement
> > isn't driven by the implementation. It's driven by the code and the maths
> > in the core scheduler, and its been a requirement for years.
> >
> > sched_clock() needs to be monotonic, and needs to wrap at 64-bit, because
> > calculations are done by comparing the difference of two 64-bit values
> > returned from this function.
> Yes, and this is the question.If it is 32 bit then also it can overflow but
> it will happen relatively fast.So I guess that is the reason why we use 64 bit
> and this will avoid recalculations for recalibration.

And that's why 112f38a4a3 is there - to ensure that we extend a 32-bit
or smaller counter all the way up to the full 64-bits. This replaces
the previous generation code which only extended it to 63-bits. Problems
were reported!

> > Let's take a trivial example - if you have a 16 bit counter, and you have
> > a value of 0xc000 ns, and next time you read it, it has value 0x0001 ns,
> > then what value do you end up with when you calculate the time passed
> > using 64-bit maths.
> >
> > That's 0x0000000000000001 - 0x000000000000c000. The answer is a very big
> > number which is not the correct 16385. This means that things like process
> > timeslice counting and scheduler fairness is compromised - I'd expect even
> So you mean when counter overflows the scheduler doesn't handle it?

There is no handling of counter overflows at scheduler level because
the specification for sched_clock() is that this function _will_ return
a monotonically increasing 64-bit value from 0 to the maximum 64-bit

The reason for this is that there are popular architectures around
which do this natively, so the powers that be do not want additional
useless code cluttering their architectures.
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