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

On Sun, 2011-01-02 at 20:21 -0500, Nicolas Pitre wrote:

On Mon, 3 Jan 2011, Russell King - ARM Linux wrote:

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On Wed, Dec 29, 2010 at 08:31:58PM -0500, Nicolas Pitre wrote:

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On Tue, 21 Dec 2010, Russell King - ARM Linux wrote:

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On Mon, Dec 20, 2010 at 08:21:41PM -0500, Nicolas Pitre wrote:

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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.

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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?

Sure.  My current understanding is that the core might be trying to play 
it safe as the mult and shift factors used to be provided by each clock 
source drivers, and they were often the result of some guessing from the 
authors of those drivers.  Now that those factors are programmatically 
determined via the clocksource_register_khz interface (is this the case 
for them all?) then it should be possible to tighten that code a bit and 
make it optimal.

Its not yet the case for all clocksources, I've had some trouble getting
arch maintainers to pick up the conversion patches I've sent out so far
(Russell jumped in and did the arm conversion for me, which lightens my
load dramatically, so thanks again for that!). 

And yes, the consolidation was done as I was seeing a lot of mult/shift
copy-pasta in clocksources (which is my fault, as the advice on how to
calculate it was poor), and as idle time lengths are growing I saw a
potential hazard of breaking platforms as idle lengths grow, or users
being frustrated that one platform might seemingly use more power then
another due to the mult/shift pair needlessly limiting it.

So the consolidation is mainly just to get the expectations unified and
make it so changes don't have to be done on a clocksource by clocksource
basis.

thanks
-john