On Mon, Jan 30, 2012 at 05:45:19PM +0000, stephane eranian wrote:
> There you go, no attachment, not sure the omap list
> supports this.
Cheers Stephane.
> There is something quite interesting to observe.
>
> While I run perf record -e cycles -F 100 noploop 10, I watch
> /proc/interrupts. The number of interrupts is way lower than
> expected. Therefore the number of samples is way too low:
>
> $ perf record -e cycles -F 100 noploop 10
> $ perf report -D | tail -20
> cycles stats:
> TOTAL events: 535
> MMAP events: 11
> COMM events: 2
> EXIT events: 2
> SAMPLE events: 520
>
> The delta in /proc/interrupts on CPU1 is 520 interrupts.
Yes, that is about half of what you'd expect. Running on my A9 platform
(vexpress) I get:
$ perf record -e cycles -F 100 noploop 10
$ perf report -D | tail -20
cycles stats:
TOTAL events: 1007
MMAP events: 18
COMM events: 2
EXIT events: 2
SAMPLE events: 985
> So looks like the frequency adjustment which is hooked off of the
> timer tick is either not called at each timer tick, the timer ticks are
> not at regular interval, or the math is wrong.
My hunch is that that the interval is probably varying, but I don't know much
about OMAP4 and its clocks.
> If I go with the fixed period mode:
> $ perf stat -e cycles noploop 10
> noploop for 10 seconds
> Performance counter stats for 'noploop 10':
> 10079156960 cycles # 0.000 GHz
> 10.004547117 seconds time elapsed
>
> That means, if I want 100 samples/sec: = 10079156960/(10*100)=10079157
> $ perf record -e cycles -c 10079157 noploop 10
> $ perf report -D | tail -20
> cycles stats:
> TOTAL events: 1003
> MMAP events: 11
> COMM events: 2
> EXIT events: 2
> THROTTLE events: 1
> UNTHROTTLE events: 1
> SAMPLE events: 986
>
> Now, we're getting the right answer!
Just to confirm, for me:
$ perf stat -e cycles ./noploop 10
noploop for 10 seconds
Performance counter stats for './noploop 10':
4001163930 cycles # 0.000 GHz
10.006534024 seconds time elapsed
$ perf record -e cycles -c 4001163 ./noploop 10
$ perf report -D | tail -20
Aggregated stats:
TOTAL events: 1020
MMAP events: 18
COMM events: 2
EXIT events: 2
SAMPLE events: 998
cycles stats:
TOTAL events: 1020
MMAP events: 18
COMM events: 2
EXIT events: 2
SAMPLE events: 998
which is close enough :)
> We need to elucidate what's going on in perf_event_task_tick().
> I have tried with my throttling fix and it did not help. We are
> not subject to throttling with such a low rate.
Ok. I would start by looking at the clock ticks if I were you, since this
seems to be alright on my board.
Will
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