Re: [Issue] access cpu_cycles PMU conter in r8a7791

[Chai Wen <chaiw.fnst@cn.fujitsu.com>]

On 10/20/2014 06:14 PM, Geert Uytterhoeven wrote:

> Hi Wen-san,
> 
> On Wed, Oct 15, 2014 at 2:13 PM, Chai Wen <chaiw.fnst@cn.fujitsu.com> wrote:
>> I am planing to use pmu counter cpu_cycles to measure the elapsed time of some small pieces of code.
>> But I found that I can not make the PMU work properly.
>>
>> My cpu is called r8a7791, a 2-core cpu. And the kernel version is 3.10.31.
>> The following is my simple code to test it and its result.
>>
>> I found that no matter what's the scale of this loop:
>>         for (i = 0; i < loops; i++) {
>>                 __asm__ __volatile__("mov r0, r0\n\t");
>>         }
>>         the cycles counts got via:
>>         __asm__ __volatile__("MRC p15, 0, %0, c9, c13, 0\n\t" : "=r"(count));
>>         are not significantly different from each other. I am confused about these values.
>>
>> Any comment or help is appreciated, thanks a lot.
>>
>> ============
>> num of counters: 6
>> before count: 0
>> loops is 100 takes cycles 104665
>> num of counters: 6
>> before count: 0
>> loops is 500 takes cycles 104650
>> num of counters: 6
>> before count: 0
>> loops is 1000 takes cycles 104827
>> num of counters: 6
>> before count: 0
>> loops is 5000 takes cycles 120351
>> num of counters: 6
>> before count: 0
>> loops is 10000 takes cycles 105041
>> num of counters: 6
>> before count: 0
>> loops is 50000 takes cycles 115470
>> num of counters: 6
>> before count: 0
>> loops is 100000 takes cycles 107905
>> num of counters: 6
>> before count: 0
>> loops is 500000 takes cycles 120427
>> num of counters: 6
>> before count: 9
>> loops is 1000000 takes cycles 145033
> 
> I get similar results when compiling your test program as a loadable
> module. When built-in, the results look saner, but still not perfect.
> 
> After wrapping everything in local_irq_save(flags) / local_irq_restore(flags),
> and moving the printk() outside the measurement loop, it looks much better:



Hi Geert Uytterhoeven

Thank you very much for your test and detail explanation in your another reply.

Indeed,  printk is a rather heavy operation. The result became much saner just after I removed
the printk in the loop. And it was rather stable.

It is a mistake of my pool knowledge of printk.
Glad to see that this code could help you ;).

thanks
chai wen

> 
> On r8a7791:
> 
> num of counters: 6
> before count: 0
> loops is 100 takes cycles 26
> num of counters: 6
> before count: 0
> loops is 1000 takes cycles 265
> num of counters: 6
> before count: 0
> loops is 10000 takes cycles 2656
> num of counters: 6
> before count: 0
> loops is 100000 takes cycles 26563
> num of counters: 6
> before count: 0
> loops is 1000000 takes cycles 265625
> num of counters: 6
> before count: 0
> loops is 10000000 takes cycles 2656250
> num of counters: 6
> before count: 0
> loops is 100000000 takes cycles 26562500
> 
> On r8a7740:
> 
> num of counters: 6
> before count: 0
> loops is 100 takes cycles 5
> num of counters: 6
> before count: 0
> loops is 1000 takes cycles 40
> num of counters: 6
> before count: 0
> loops is 10000 takes cycles 391
> num of counters: 6
> before count: 0
> loops is 100000 takes cycles 3907
> num of counters: 6
> before count: 0
> loops is 1000000 takes cycles 39063
> num of counters: 6
> before count: 0
> loops is 10000000 takes cycles 390626
> num of counters: 6
> before count: 0
> loops is 100000000 takes cycles 3906255
> 
> Thanks a lot for your test code, I was just about to do high-resolution
> measurements on r8a7740 anyway ;-)
> 
> Gr{oetje,eeting}s,
> 
>                         Geert
> 
> --
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org
> 
> In personal conversations with technical people, I call myself a hacker. But
> when I'm talking to journalists I just say "programmer" or something like that.
>                                 -- Linus Torvalds
> .
> 



-- 
Regards

Chai Wen