linux-kernel - Re: [PATCH v2] clocksource/drivers/arc_timer: Utilize generic sched

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Date:   Mon, 5 Nov 2018 15:48:17 +0100
From:   Daniel Lezcano <daniel.lezcano@...aro.org>
To:     Vineet Gupta <vineet.gupta1@...opsys.com>,
        Alexey Brodkin <alexey.brodkin@...opsys.com>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Cc:     "linux-snps-arc@...ts.infradead.org" 
        <linux-snps-arc@...ts.infradead.org>,
        Thomas Gleixner <tglx@...utronix.de>
Subject: Re: [PATCH v2] clocksource/drivers/arc_timer: Utilize generic
 sched_clock

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@...opsys.com>
>> Cc: Daniel Lezcano <daniel.lezcano@...aro.org>
>> Cc: Vineet Gupta <vgupta@...opsys.com>
>> Cc: Thomas Gleixner <tglx@...utronix.de>
>> ---
> 
> Acked-by: Vineet Gupta <vgupta@...opsys.com>
> 
> @Daniel is this going via timer tree or you want me to pick it up.

Applied, thanks

  -- Daniel

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