On (03/12/19 13:38), Petr Mladek wrote: > > Hmm. OK. So one of the things with printk is that it's fully sequential. > > We call console drivers one by one. Slow consoles can affect what appears > > on the fast consoles; fast console have no impact on slow ones. > > > > call_console_drivers() > > for_each_console(c) > > c->write(c, text, text_len); > > > > So a list of (slow_serial serial netcon) console drivers is a camel train; > > fast netcon is not fast anymore, and slow consoles sometimes are the reason > > we have dropped messages. And if we drop messages we drop them for all > > consoles, including fast netcon. Turning that sequential pipline into a > > bunch of per-console kthreads/irq and letting fast consoles to be fast is > > not a completely bad thing. Let's think more about this, I'd like to read > > more opinions. > > Per-console kthread sounds interesting but there is the problem with > reliability. I mean that kthread need not get scheduled. Correct, it has to get scheduled. From that point of view IRQ offloading looks better - either to irq_work (like John suggested) or to serial drivers' irq handler (poll uart xmit + logbuf). kthread offloading is not super reliable. That's why I played tricks with CPU affinity - scheduler sometimes schedule printk_kthread on the same CPU which spins in console_unlock() loop printing the messages, so printk_kthread offloading never happens. It was first discovered by Jan Kara (back in the days of async-printk patch set). I think at some point Jan's async-printk patch set had two printk kthreads. We also had some concerns regarding offloading on UP systems. > Some of these problems might get solved by the per-console loglevel > patchset. Yes, some. > Sigh, any feature might be useful in some situation. But we always > have to consider the cost and the gain. I wonder how common is > to actively use two consoles at the same time and what would > be the motivation. Facebook fleet for example. The motivation is - to have a fancy fast console that does things which simple serial consoles cannot do and a slow serial console, which is sometimes more reliable, as last resort. Fancy stuff usually means dependencies - net, mm, etc. So when fancy console stop working, slow serial console still does. -ss
