How to get better precision out of getrusage on the ARM?

[Patrick Doyle]

Hello Alexandre,
I missed that patch.  I'll have to take a look at that.  Thank you.

Regardless, it seems that I need to modify tcb_clksrc.c to register
itself through CLOCKSOURCE_OF_DECLARE(), set the appropriate kernel
configuration options, and use clock_gettime(CLOCK_THREAD_CPUTIME_ID)
in order to get better than 1 tick resolution out of clock_gettime for
my threads.  Unfortunately, the tcb_clksrc.c modifications appear to
be incompatible with the design intent of atmel_tclib.c.  I am waiting
for the opportunity to talk to Atmel about this.  (I need to ping my
local FAE about this again).  I also need to turn up the heat on this
back burner'ed task.  (I got some numbers, none of us are happy with
them, and I need to see what I can do to get better numbers, when I
can make the time to do so.)

--wpd


On Mon, Jan 18, 2016 at 7:16 PM, Alexandre Belloni
<alexandre.belloni at free-electrons.com> wrote:
> Hi Patrick,
>
> I was wondering whether you had the chance to test this patch from the
> RT tree:
> http://git.kernel.org/cgit/linux/kernel/git/rt/linux-rt-devel.git/commit/?h=v4.4-rt2&id=57142bdff523a67657d0b2603eaa91df58b88bd8
>
> when trying your sched_clock experiment?
>
> On 30/12/2015 at 10:00:46 -0500, Patrick Doyle wrote :
>> Continuing on...
>> I now have a CLOCKSOURCE_OF_DECLARED()'ed 10 MHz clock source running
>> on my ARM processor (Atmel SAMA5D2 Xplained board).  It registers
>> itself through sched_clock_register() to provide a high resolution
>> sched clock.  Once I turned on "Full dynticks CPU time accounting"
>> (CONFIG_VIRT_CPU_ACCOUNTING_GEN), I was able to get better than jiffy
>> resolution from my calls to getrusage(RUSAGE_THREAD,..).  But things
>> still aren't quite right.  I am using getrusage() to provide some
>> runtime profile information to an existing application (that was
>> ported to run on Linux instead of a custom RTOS).  I have code that
>> looks like:
>>
>> tick()
>> // commented out code that used to do something
>> tock()
>>
>> where tick() & tock() are my profile "start" and "stop" points that
>> call getrusage() to record and and accumulate time spent between calls
>> to tick() & tock().  Most of the time, I get a delta of 0 between the
>> two calls, which I expect.  But occasionally, I get a delta ranging
>> between 800us and 1000us, which I don't understand at all.  It seems
>> like my thread is being "charged" for time spent doing something else.
>> Perhaps an interrupt occurred and its time got charged to my thread;
>> perhaps a higher priority thread ran for 1ms, I don't know (yet).
>>
>> Does anybody have any suggestions as to where I might look, or as to
>> what kernel CONFIG options might make the most sense for an
>> application such as this?
>>
>> --wpd
>>
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>
> --
> Alexandre Belloni, Free Electrons
> Embedded Linux, Kernel and Android engineering
> http://free-electrons.com