Hi,
On Thu, Apr 20, 2017 at 11:54 AM, Eric Anholt <e...@anholt.net> wrote:
> Stefan Wahren <stefan.wah...@i2se.com> writes:
>
>> Hi,
>>
>>> Doug Anderson <diand...@chromium.org> hat am 18. April 2017 um 22:41
>>> geschrieben:
>>>
>>>
>>> It's hard to know for sure that all of this time is really in
>>> urb_enqueue(). Possible we could have task switched out and been
>>> blocked elsewhere. Using ftrace to get more fine-grained timings
>>> would be useful. ktime_get(), ktime_sub(), and ktime_to_us() are your
>>> friends here if you want to use trace_printk.
>>
>> i'm a newbie to ftrace, so i hope this would be helpful.
>>
>> # connect PL2303 to the onboard hub
>> # echo 0 > options/sleep-time
>> # echo 0 > function_profile_enabled
>> # echo 1 > function_profile_enabled
>> # ./usb_test
>> # Waiting for at least 20 seconds and then disconnect PL2303
>> # echo 0 > function_profile_enabled
>> # cat trace_stat/function0
>>
>> Function Hit Time Avg
>> s^2
>> -------- --- ---- ---
>> ---
>> bcm2835_handle_irq 361347 219567633 us 607.636 us
>> 1485199 us
>> __handle_domain_irq 1082482 212639551 us 196.437 us
>> 3642030 us
>> generic_handle_irq 1082482 100592051 us 92.927 us
>> 50511334 us
>> irq_exit 1082482 98197771 us 90.715 us
>> 29649040 us
>> handle_level_irq 1082482 95812379 us 88.511 us
>> 51910093 us
>
> If I'm reading this output right, we're spending half of our interrupt
> processing time in irq_exit(), so even if dwc2's interrupt was free (the
> generic_handle_irq() chain), we'd be eating about half the CPU getting
> back out of the interrupt handler, right?
>
> I don't really know anything about DWC2 or USB, but is there any way we
> could mitigate the interrupt frequency with this hardware? If nothing
> else, could we loop reading gintsts until it reads back 0?
Take ftrace with a little bit of a grain of salt, especially on older
/ slower ARMs (without the arch timer). Whenever ftrace takes a log
it grabs a timestamp. This can be an expensive (ish) operation. Even
on newer CPUs it's still not free if you call it as much as ftrace,
but on older CPUs it's extra expensive.
I spent a chunk of time working on optimizations for that on exynos
since it showed up in profiles as an expensive operation (Chrome asks
for the time a lot during its internal profiling). Some of that type
of data is in commit 3252a646aa2c ("clocksource: exynos_mct: Only use
32-bits where possible").
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