On 2020-01-22, Eugeniu Rosca <erosca@xxxxxxxxxxxxxx> wrote: > 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. > > I cannot fully explain this empirical observation, but it directs my > mind to the following workaround, for which I have a PoC: > - employ vprintk_safe() any time CPU0 is the owner/caller of printk > - tie CPU0-private printk internal IRQ workers to another CPU > > The above makes sure nothing is printed to the serial console on > behalf of CPU0. I don't even hope this to be accepted by community, > but can you please share your opinion the idea itself is sane? It is a problem-specific hack. You will need to be certain that CPU1-7 will never have problems with console printing storms. Be aware that vprintk_safe() is not particularly reliable in many crash scenarios. If seeing oops output on the console is important, this can be a risky hack. Also, be aware that it has its own config option for the safe buffer size: PRINTK_SAFE_LOG_BUF_SHIFT >> 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. > > I didn't dive into the internals of this series, but decoupling the > execution context of the serial driver from the execution context of > the printk callers sounds very good to me (this is what i try to > achieve via vanilla vprintk_safe). I wonder if it's easier to remove > CPU0 from equation with this series applied. Yes, it would be quite easy. The console printers run as dedicated kthreads. It is only a matter of setting the CPU affinity for the related kthread. >> 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. > > If there is any roadmap publicly available, I would appreciate a > reference. I am only aware of the quilt "series" file [0] that is roughly documenting the status of the effort. John Ogness [0] https://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git/tree/patches/series?h=linux-5.4.y-rt-patches
