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SubjectRe: [PATCH v2] clocksource/drivers/arc_timer: Utilize generic sched_clock
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On 19/11/2018 10:31, Alexey Brodkin wrote:
> 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?

The ARC's Kconfig part does not apply on tip/timers/core.

As you described, sending a separate arc timer change is fine IMO.





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