Re: [PATCH v3] clocksource/drivers/arc_timer: Utilize generic sched_clock

[Alexey Brodkin <alexey.brodkin@synopsys.com>]

On Mon, 2018-11-19 at 23:09 +0100, Daniel Lezcano wrote:
> On 19/11/2018 22:53, Alexey Brodkin wrote:
> > Hi Daniel,
> > 
> > On Mon, 2018-11-19 at 22:50 +0100, Daniel Lezcano wrote:
> > > On 19/11/2018 12:29, Alexey Brodkin wrote:
> > > > It turned out we used to use default implementation of sched_clock()
> > > > from kernel/sched/clock.c which was as precise as 1/HZ, i.e.
> > > > by default we had 10 msec granularity of time measurement.
> > > > 
> > > > Now given ARC built-in timers are clocked with the same frequency as
> > > > CPU cores we may get much higher precision of time tracking.
> > > > 
> > > > Thus we switch to generic sched_clock which really reads ARC hardware
> > > > counters.
> > > > 
> > > > This is especially helpful for measuring short events.
> > > > That's what we used to have:
> > > > ------------------------------>8------------------------
> > > > $ perf stat /bin/sh -c /root/lmbench-master/bin/arc/hello > /dev/null
> > > > 
> > > >  Performance counter stats for '/bin/sh -c /root/lmbench-master/bin/arc/hello':
> > > > 
> > > >          10.000000      task-clock (msec)         #    2.832 CPUs utilized
> > > >                  1      context-switches          #    0.100 K/sec
> > > >                  1      cpu-migrations            #    0.100 K/sec
> > > >                 63      page-faults               #    0.006 M/sec
> > > >            3049480      cycles                    #    0.305 GHz
> > > >            1091259      instructions              #    0.36  insn per cycle
> > > >             256828      branches                  #   25.683 M/sec
> > > >              27026      branch-misses             #   10.52% of all branches
> > > > 
> > > >        0.003530687 seconds time elapsed
> > > > 
> > > >        0.000000000 seconds user
> > > >        0.010000000 seconds sys
> > > > ------------------------------>8------------------------
> > > > 
> > > > And now we'll see:
> > > > ------------------------------>8------------------------
> > > > $ perf stat /bin/sh -c /root/lmbench-master/bin/arc/hello > /dev/null
> > > > 
> > > >  Performance counter stats for '/bin/sh -c /root/lmbench-master/bin/arc/hello':
> > > > 
> > > >           3.004322      task-clock (msec)         #    0.865 CPUs utilized
> > > >                  1      context-switches          #    0.333 K/sec
> > > >                  1      cpu-migrations            #    0.333 K/sec
> > > >                 63      page-faults               #    0.021 M/sec
> > > >            2986734      cycles                    #    0.994 GHz
> > > >            1087466      instructions              #    0.36  insn per cycle
> > > >             255209      branches                  #   84.947 M/sec
> > > >              26002      branch-misses             #   10.19% of all branches
> > > > 
> > > >        0.003474829 seconds time elapsed
> > > > 
> > > >        0.003519000 seconds user
> > > >        0.000000000 seconds sys
> > > > ------------------------------>8------------------------
> > > > 
> > > > Note how much more meaningful is the second output - time spent for
> > > > execution pretty much matches number of cycles spent (we're runnign
> > > > @ 1GHz here).
> > > > 
> > > > Signed-off-by: Alexey Brodkin <abrodkin@synopsys.com>
> > > > Cc: Daniel Lezcano <daniel.lezcano@linaro.org>
> > > > Cc: Vineet Gupta <vgupta@synopsys.com>
> > > > Cc: Thomas Gleixner <tglx@linutronix.de>
> > > > ---
> > > 
> > > Applied, thanks.
> > 
> > Maybe a bit too late but I completely forgot to add stable tag into Cc list.
> > Any chance to add it still?
> 
> Yes, no problem, I've added it.

Cool, thanks!

-Alexey