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SubjectRe: Suspect broken frequency transitions on SDM845
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On 04/02/2020 12:53, Valentin Schneider wrote:
> We've been getting some sporadic failures on the big CPUs of a Pixel3
> running mainline [1], here is an example of a correct run (CPU4):
>
> | frequency (kHz) | sysbench events |
> |-----------------+-----------------|
> | 825600 | 236 |
> | 1286400 | 369 |
> | 1689600 | 483 |
> | 2092800 | 600 |
> | 2476800 | 711 |
>
> and here is a failed one (still CPU4):
>
> | frequency (kHz) | sysbench events |
> |-----------------+-----------------|
> | 825600 | 234 |
> | 1286400 | 369 |
> | 1689600 | 449 |
> | 2092800 | 600 |
> | 2476800 | 355 |
>
>
> We've encountered something like this in the past with the exact same
> test on h960 [2] but it is much harder to reproduce reliably this time
> around.
>
> I haven't found much time to dig into this; I did get a run of ~100
> iterations with about ~15 failures, but nothing cpufreq related showed up in
> dmesg. I briefly suspected fast-switch, but it's only used by schedutil, so
> in this test I would expect the frequency transition to be complete before we
> even try to start executing sysbench.
>

I've been adding some more debug stuff in that test case following some of
Lukasz' recommendations, and I still don't find anything that would
explain what I'm seeing.

The raw output of the test is:

CPU0:
300000: 61
576000: 114
825600: 172
1056000: 221
1324800: 278
1612800: 339
CPU4:
825600: 236
1286400: 368
1689600: 479
2092800: 420 <---}
2476800: 339 <---} Both of these are not monotonically increasing...


/sys/kernel/debug/clk/clk_summary doesn't seem to include CPU clocks, or
doesn't get updated because I see no diff from one frequency to another
(even between lowest & highest tested frequency)


/sys/devices/system/cpu/cpu*/cpufreq/stats/time_in_state does get updated,
and seems to hint that I am getting the frequency I'm asking for:

[2020-02-12 14:48:21,706] 2476800 39544
[2020-02-12 14:48:23,929] 2476800 39745

There's about ~10% (200ms) missing here, but that shouldn't lead to about
half the expected performance (I get ~710 "score" out of that 2.477GHz freq
on non-failing runs).


I also made sure to read back
/sys/devices/system/cpu/cpu*/cpufreq/scaling_cur_freq
and I do see the value I've asked for.


Finally, I also probed the thermal state via
/sys/class/thermal/cooling_device*/cur_state
and they are *always* 0 (i.e., no throttling) right after finishing the
execution of the benchmark, which should be close to the "hottest" point.


So AFAICT there is nothing on the cpufreq side that hints at a slow or
unsuccessful frequency transition. Can FW mess about frequencies without
notifying the kernel?

> If anyone has the time and will to look into this, that would be much
> appreciated.
>
> [1]: https://git.linaro.org/people/amit.pundir/linux.git/log/?h=blueline-mainline-tracking
> [2]: https://lore.kernel.org/lkml/d3ede0ab-b635-344c-faba-a9b1531b7f05@arm.com/
>
> Cheers,
> Valentin
>

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