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SubjectRe: [PATCH v2] clocksource/drivers/arc_timer: Utilize generic sched_clock
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Hi Daniel,

On Sun, 2018-11-18 at 03:17 +0100, Daniel Lezcano wrote:
> On 05/11/2018 15:39, Daniel Lezcano wrote:
> > On 24/10/2018 00:33, Vineet Gupta wrote:
> > > On 10/17/2018 04:30 AM, 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 running
> > > > @ 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>
> > > > ---
> > >
> > > Acked-by: Vineet Gupta <vgupta@synopsys.com>
> > >
> > > @Daniel is this going via timer tree or you want me to pick it up.
> >
> > I will take care of it.
>
> Please resend without the arch Kconfig change

I'm wondering if there's a problem with arc/arc/Kconfig change going
through your tree? This way it will be really atomic change and it will be
much easier to back-port (and that's what we'd really like to happen).

If Vineet is OK with that IMHO it's safe to keep it in the one and only commit.

Otherwise should I just split this patch in 2 and still submit them as series or
have 2 completely not-related patches one for you and one for Vineet?

In that case do I understand correctly that we may enable GENERIC_SCHED_CLOCK
for ARC even before proposed change for arc_timer.c gets merged - i.e. with no
special GENERIC_SCHED_CLOCK driver we'll safely fall-back to jiffie-based
sched clock which we anyways use now when GENERIC_SCHED_CLOCK is disabled, right?

-Alexey
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