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

[Daniel Lezcano <daniel.lezcano@linaro.org>]

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.



[1]
https://git.kernel.org/pub/scm/linux/kernel/git/tip/tip.git/tree/Documentation/process/stable-kernel-rules.rst?h=timers/core


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