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SubjectRe: [RFC PATCH v1 00/25] printk: new implementation
Hi Eugeniu,

On Wed, Jan 22, 2020 at 3:34 AM Eugeniu Rosca <erosca@de.adit-jv.com> wrote:
> On Wed, Jan 22, 2020 at 12:56:48AM +0100, John Ogness wrote:
> > On 2020-01-21, Eugeniu Rosca <erosca@de.adit-jv.com> wrote:
> > > This [1] is a fairly old thread, but I only recently stumbled upon it,
> > > while co-investigating below audio distortions [2] on R-Car3 ARM64
> > > boards, which can be reproduced by stressing [3] the serial console.
> > >
> > > The investigation started a few months ago, when users reported audio
> > > drops during the first seconds of system startup. Only after a few
> > > weeks it became clear (thanks to some people in Cc) that the
> > > distortions were contributed by the above-average serial console load
> > > during the early boot. Once understood, we were able to come up with a
> > > synthetic test [2-3].
> > >
> > > I thought it would be interesting to share below reproduction matrix,
> > > in order to contrast vanilla to linux-rt-devel [4], as well as to
> > > compare various preemption models.
> > >
> > > | Ser.console Ser.console
> > > | stressed at rest or disabled
> > > --------------------------------------------
> > > v5.5-rc6 (PREEMPT=y) | distorted clean
> > > v5.4.5-rt3 (PREEMPT=y) | distorted clean
> > > v5.4.5-rt3 (PREEMPT_RT=y) | clean clean
> > >
> > > My feeling is that the results probably do not surprise linux-rt
> > > people.
> > >
> > > My first question is, should there be any improvement in the case of
> > > v5.4.5-rt3 (PREEMPT=y), which I do not sense? I would expect so, based
> > > on the cover letter of this series (pointing out the advantages of the
> > > redesigned printk mechanism).
> >
> > The problem you are reporting is not the problem that the printk rework
> > is trying to solve.
>
> In general, agreed. But there are some quirks and peculiarities in how
> the issue (increased audio latency) manifests itself on R-Car3, which
> might create some room for a (relatively simple) loophole solution in
> the printk mechanism.
>
> With that said, I need to diverge a bit from the platform-agnostic
> scope of this series.
>
> So, what's specific to R-Car3, based on my testing, is that the issue
> can only be reproduced if the printk storm originates on CPU0 (it does
> not matter if from interrupt or task context, both have been tested). If
> the printk storm is initiated on any other CPU (there are 7 secondary
> ones on R-Car H3), there is no regression in the audio quality/latency.

The secure stuff is running on CPU0, isn't it?
Is that a coincidence?

Gr{oetje,eeting}s,

Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
-- Linus Torvalds

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