Re: [PATCH V2] cpuidle: Change ktime_get() with local_clock()

[Rafael J. Wysocki]

On Friday, April 22, 2016 08:42:40 AM Peter Zijlstra wrote:
> On Thu, Apr 21, 2016 at 09:41:14PM +0200, Rafael J. Wysocki wrote:
> > On Thu, Apr 21, 2016 at 10:56 AM, Daniel Lezcano
> > <daniel.lezcano@xxxxxxxxxx> wrote:
> > > The ktime_get() can have a non negligeable overhead, use local_clock()
> > > instead.
> > >
> > > In order to test the difference between ktime_get() and local_clock(),
> > > a quick hack has been added to trigger, via debugfs, 10000 times a
> > > call to ktime_get() and local_clock() and measure the elapsed time.
> > >
> > > Then the average value, the min and max is computed for each call.
> > >
> > > From userspace, the test above was called 100 times every 2 seconds.
> > >
> > > So, ktime_get() and local_clock() have been called 1000000 times in
> > > total.
> > >
> > > The results are:
> > >
> > > ktime_get():
> > > ============
> > >  * average: 101 ns (stddev: 27.4)
> > >  * maximum: 38313 ns
> > >  * minimum: 65 ns
> > >
> > > local_clock():
> > > ==============
> > >  * average: 60 ns (stddev: 9.8)
> > >  * maximum: 13487 ns
> > >  * minimum: 46 ns
> > >
> > > The local_clock() is faster and more stable.
> > >
> > > Even if it is a drop in the ocean, changing the ktime_get() by the
> > > local_clock() allows to save 80ns at idle time (entry + exit). And
> > > in some circumstances, especially when there are several CPUs racing
> > > for the clock access, we save tens of microseconds.
> > >
> > > The idle duration resulting from a diff is converted from nanosec to
> > > microsec. This could be done with integer division (div 1000) - which is
> > > an expensive operation or by 10 bits shifting (div 1024) - which is fast
> > > but unprecise.
> > >
> > > The following table gives some results at the limits.
> > >
> > >  ------------------------------------------
> > > |   nsec   |   div(1000)   |   div(1024)   |
> > >  ------------------------------------------
> > > |   1e3    |        1 usec |      976 nsec |
> > >  ------------------------------------------
> > > |   1e6    |     1000 usec |      976 usec |
> > >  ------------------------------------------
> > > |   1e9    |  1000000 usec |   976562 usec |
> > >  ------------------------------------------
> > >
> > > There is a linear deviation of 2.34%. This loss of precision is acceptable
> > > in the context of the resulting diff which is used for statistics. These
> > > ones are processed to guess estimate an approximation of the duration of the
> > > next idle period which ends up into an idle state selection. The selection
> > > criteria takes into account the next duration based on large intervals,
> > > represented by the idle state's target residency.
> > >
> > > The 2^10 division is enough because the approximation regarding the 1e3
> > > division is lost in all the approximations done for the next idle duration
> > > computation.
> > >
> > > Signed-off-by: Daniel Lezcano <daniel.lezcano@xxxxxxxxxx>
> > 
> > Looks good to me.
> > 
> > Peter, are you happy with the changelog now?
> 
> Yep, works for me:
> 
> Acked-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>

OK, applied.  Thanks!