Re: poor cyclictest results with 5.0 series rt for arm64

[Paul Thomas <pthomas8589@gmail.com>]

On Wed, Aug 14, 2019 at 7:26 AM Sebastian Andrzej Siewior
<bigeasy@linutronix.de> wrote:
>
> On 2019-08-13 14:02:39 [-0400], Paul Thomas wrote:
> > I still get large latencies, like this:
> > root@xu5:/opt/rt-tests# ./cyclictest -h ./cyclictest -S -m -n -p 99 -i
> > 200 -h 400 -D 900
> > # /dev/cpu_dma_latency set to 0us
> > policy: fifo: loadavg: 0.10 0.10 0.03 1/134 1901
> >
> > T: 0 ( 1886) P:99 I:200 C:2724618 Min:      5 Act:    6 Avg:    6 Max:      45
> > T: 1 ( 1887) P:99 I:200 C:2724525 Min:      5 Act:    7 Avg:    6 Max:      29
> > T: 2 ( 1888) P:99 I:200 C:2724246 Min:      5 Act:    6 Avg:    6 Max:    6338
> > T: 3 ( 1889) P:99 I:200 C:2724339 Min:      5 Act:    8 Avg:    6 Max:     141
> >
> > This is with the Lazy preempt commit in there.
>
> I re-read the thread and figured out that you talk about 5.0 while I was
> testing on 5.2. On my ARM64 box I end up with
>
> |T: 0 ( 9577) P:99 I:250 C:19979015 Min:      3 Act:    4 Avg:    3 Max:      19
> |T: 1 ( 9578) P:99 I:250 C:19978958 Min:      3 Act:    3 Avg:    3 Max:      24
> |T: 2 ( 9579) P:99 I:250 C:19978901 Min:      3 Act:    4 Avg:    4 Max:      22
> |T: 3 ( 9580) P:99 I:250 C:19978843 Min:      3 Act:    3 Avg:    3 Max:      19
>
> with the patch I sent you. Without it, had spikes between 130us and
> 160us. Since you said it is not working, I looked more and came up with
> this:
>
> diff --git a/arch/arm64/include/asm/preempt.h b/arch/arm64/include/asm/preempt.h
> index 3bfad251203b5..ca1c6fe8dd347 100644
> --- a/arch/arm64/include/asm/preempt.h
> +++ b/arch/arm64/include/asm/preempt.h
> @@ -73,6 +73,8 @@ static inline bool __preempt_count_dec_and_test(void)
>         if (!pc || !READ_ONCE(ti->preempt_count))
>                 return true;
>  #ifdef CONFIG_PREEMPT_LAZY
> +       if ((pc & ~PREEMPT_NEED_RESCHED))
> +               return false;
>         if (current_thread_info()->preempt_lazy_count)
>                 return false;
>         return test_thread_flag(TIF_NEED_RESCHED_LAZY);
>
> but this shouldn't make any difference for you (but it is a bug fix).
> Staring more into it, I don't see anything wrong.
>
> v5.0 is not supported any more. Do you think, that you could re-test on
> 5.2-RT with those two patches I sent you?

Hi Sebastian,

Yes, for the test yesterday it was using 5.2.0-rt1, I should have
explained that in the first response, since I posted this we have
moved to 5.2.

I added the second patch as well and it was still happening. I took
another trace data set in case this is helpful.

I added an attachment to the bugzilla bug:
https://bugzilla.kernel.org/show_bug.cgi?id=204189

The attachment is here:
https://bugzilla.kernel.org/attachment.cgi?id=284391

We don't actually need Lazy Preempt, so now that we know what the
issues is we can just turn it off in the config? However I suspect
everyone wants to get to the bottom of what's going on at this point?

thanks,
Paul