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

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 your chart, v5.4.5-rt3 (PREEMPT_RT=y) is the only configuration that
is _not_ disabling hardware interrupts during UART activity. I would
guess your problem is due to interrupts being disabled for unacceptable
lengths of time. You need a low-latency system, so PREEMPT_RT=y _is_ the
correct (and only) solution if a verbose serial console is a must.

The printk rework focusses on making printk non-interfering by
decoupling console printing from printk() callers. However, the console
printing itself will still do just as much interrupt disabling as
before. That is driver-related, not printk-related.

> And the other question is, how would you, generally speaking, tackle
> the problem, given that backporting the linux-rt patches is *not* an
> option and enabling serial console is a must?

The linux-rt patches (which include this printk rework) *are* being
ported to mainline now. My recommendation is to continue using the
linux-rt patches (with PREEMPT_RT=y) until PREEMPT_RT is available
mainline.

John Ogness

> [1] https://lore.kernel.org/lkml/20190212143003.48446-1-john.ogness@linutronix.de/
> [2] H3ULCB> speaker-test -f24_LE -c2 -t wav -Dplughw:rcarsound -b 4000
> https://vocaroo.com/9NV98mMgdjX
> [3] https://github.com/erosca/linux/tree/stress-serial
> [4] https://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git/

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