[PATCH] ARM: sched_clock: improve mult/shift accuracy with high frequency clocks

[Russell King - ARM Linux]

On Wed, Dec 29, 2010 at 08:31:58PM -0500, Nicolas Pitre wrote:
> On Tue, 21 Dec 2010, Russell King - ARM Linux wrote:
> 
> > On Mon, Dec 20, 2010 at 08:21:41PM -0500, Nicolas Pitre wrote:
> > > 
> > > The minsec argument to clocks_calc_mult_shift() [ which appears to be
> > > misnamed by the way ] is used to clamp the magnitude of the mult factor
> > > so that the given range won't overflow a 64 bit result.  The mult factor
> > > is itself already limited to a maximum of 32 bits.  Given that the largest
> > > clock tick range we may have is also 32 bits, there is no way our usage of
> > > any computed mult factor would ever overflow 64 bits.
> > > 
> > > The choice of 60 seconds here for minsec is rather arbitrary.  If the
> > > mult factor wasn't already limited to 32 bits, this value of 60 would
> > > create overflows for clocks which take more than 60 seconds to wrap.
> > 
> > 60 seconds was arbitary, chosen from the selection of clock rates which
> > I had available at the time (the highest of which was 24MHz).
> 
> Fair enough.  It is just not universally optimal given the different 
> clock ranges this code now covers.
> 
> > > And for clocks which take less than 60 seconds to wrap then we do lose
> > > precision as the mult factor is made smaller (fewer bits) to be adjusted
> > > to a range which is larger than what we actually use.  This currently
> > > affects clocks faster than 71MHz.
> > > 
> > > We could use minsec = 0 instead of 60 and a good 32-bit mult factor would
> > > be computed in all cases.  But let's be formal and provide the actual
> > > range to clocks_calc_mult_shift(). None of the cyc_to_fixed_sched_clock()
> > > users are affected as they all are using clocks < 71MHz.
> > 
> > Maybe __clocksource_updatefreq_scale() needs a similar fix too, instead
> > of assuming 5 seconds?  It also has access to the mask and rate.
> 
> There is a comment to that effect right above the 
> clocks_calc_mult_shift() indicating that this is a known issue in that 
> case already.  
> 
> And looking at that code, it appears that there is a balance to be made 
> between cs->max_idle_ns and cs->mult, the former currently being 
> determined by the later.  But if cs->mult is maximized to 32 bits, that 
> leaves only 31 bits for cs->max_idle_ns which corresponds to approx 2 
> seconds only.
> 
> So this is not clear to me what would be the optimal mult/shift values 
> in the __clocksource_updatefreq_scale() context, while this is rather 
> obvious in the init_sched_clock() context.  Hence this patch.

As clocksources are about precision (I believe it has been stated so in
the past) it seems that fudging the shift and multiplier to get an
extended period out of the clock is not the correct approach.

Maybe Thomas or John can shed some light on this?