Re: poor cyclictest results with 5.0 series rt for arm64

[Paul Thomas <pthomas8589@gmail.com>]

On Tue, Aug 13, 2019 at 5:53 AM Sebastian Andrzej Siewior
<bigeasy@linutronix.de> wrote:
>
> On 2019-07-17 18:28:51 [-0400], Paul Thomas wrote:
> > On Wed, Jul 17, 2019 at 6:21 AM Thomas Gleixner <tglx@linutronix.de> wrote:
> > >
> > > On Wed, 17 Jul 2019, Thomas Gleixner wrote:
> > > > On Wed, 17 Jul 2019, Thomas Gleixner wrote:
> > > >
> > > > The one below should fix it for real.
> > >
> > > That still has an issue which needs deeper investigation. Just keep the
> > > offending patch reverted for now.
> >
> > OK, sounds good.
>
> could you please try the following hunk?
>
> diff --git a/arch/arm64/kernel/entry.S b/arch/arm64/kernel/entry.S
> index 6808cd9526c67..63c6960262861 100644
> --- a/arch/arm64/kernel/entry.S
> +++ b/arch/arm64/kernel/entry.S
> @@ -648,7 +648,8 @@ alternative_if ARM64_HAS_IRQ_PRIO_MASKING
>         orr     x24, x24, x0
>  alternative_else_nop_endif
>
> -       cbnz    x24, 2f                                 // preempt count != 0
> +       cbz     x24, 1f                                 // (need_resched + count) == 0
> +       cbnz    w24, 2f                                 // count != 0
>
>         ldr     w24, [tsk, #TSK_TI_PREEMPT_LAZY]        // get preempt lazy count
>         cbnz    w24, 2f                                 // preempt lazy count != 0
>
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.

thanks,
Paul