[Ksummit-discuss] [TECH TOPIC] asynchronous printk

[Geert Uytterhoeven]

On Tue, Jul 19, 2016 at 9:02 AM, Hannes Reinecke <hare at suse.com> wrote:
>> If the printk would show up on the console, handle it inline immediately
>> before returning, so that the user sees it on the console immediately in
>> case the very next line hangs the system.  That also helps with the
>> debugging approach of copy/pasting many instances of pr_alert("%s:%d:
>> here\n", __func__, __LINE__) and looking for the last one that shows up.
>>
>> If the printk would *not* show up on the console, and would only show up
>> asynchronously in dmesg or a log somewhere, then go ahead and throw it
>> to the asynchronous printk_kthread context to handle and return, because
>> if the next line crashes, userspace wouldn't get the opportunity to read
>> and log it anyway.
>>
>> Combined with a mechanism like "if the kernel panics, try as hard as
>> possible to dump out all the pending printks before dying", that seems
>> like a reasonable default behavior that shouldn't result in surprises.
>> If the kernel is alive enough that userspace can still log things (such
>> as if the display hangs but the kernel and userspace are still running),
>> then the kernel should also still be alive enough to process the pending
>> printks.
>>
> But it still leaves us with a possible priority inversion.
> How should we deal with situations where the async thread is running and
> someone is issuing a synchronous printk?
> Should we skip the asynchonous ones and print the synchronous one first,
> risking out-of-order messages but a higher probability that the urgent
> message is actually printed ?

There are already out-of-order messages for SMP and continuation,
some of these we want to fix, but always for the same loglevel.
If two messages have a different log level, I see no reason to (try to) keep
ordering. If keeping them together is required, the user should use pr_cont.

Gr{oetje,eeting}s,

                        Geert

--
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert at 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