Hello Eugeniu, On 2020-01-21, Eugeniu Rosca <erosca@xxxxxxxxxxxxxx> 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@xxxxxxxxxxxxx/ > [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/
