Re: [RFC PATCH v1 00/25] printk: new implementation

[John Ogness]

On 2019-03-14, Petr Mladek <pmladek@xxxxxxxx> wrote:
>>> I suggest the following way forward (separate patchsets):
>>>
>>>     1. Replace log buffer (least controversial thing)
>> 
>> Yes. I will post a series that only implements the ringbuffer using
>> your simplified API. That will be enough to remove printk_safe and
>> actually does most of the work of updating devkmsg, kmsg_dump, and
>> syslog.
>
> Great. I just wonder if it is going to be fully lockless or
> still using the prb_lock. I could understand the a fully lockless
> solution will be much more complicated. But I think that it would
> make a lot of things easier in the long run. Especially it might
> help to avoid the big-kernel-lock-like approach.

I will re-visit my lockless implementation and see if I can minimize
complexity. We have since identified a couple other negative affects of
the prb_lock for PREEMPT_RT relating to CPU isolated setups. So the
lockless version is starting to look more attractive from a PREEMPT_RT
perspective as well.

>>>     2. Reliable offload to kthread (would be useful anyway)
>> 
>> Yes. I would like to implement per-console kthreads for this
>> series. I think the advantages are obvious. For PREEMPT_RT the
>> offloading will need to be always active. (PREEMPT_RT cannot call the
>> console->write() from atomic contexts.) But I think this would be
>> acceptable at first. It would certainly be better than what
>> PREEMPT_RT is doing now.
>
> I would personally start with one kthread. I am afraid that
> the discussion about it will be complicated enough. We could
> always make it more complicated later.
>
> I understand the immediate offloading might be necessary for
> PREEMPT_RT. But a more conservative approach is needed for
> non-rt kernels.
>
> Well, if there won't be a big difference in the complexity
> between one and more threads then we could mix it. But
> I personally see this a two steps that are better be done
> separately.

I will create the series in a way that a complete solution with 1
kthread exists and all following patches in the series add per-console
kthreads. Then we have a clean cut if we decide we only want the first
part of the series.

>>>     3. Atomic consoles (a lot of tricky code, might not be
>>> 		worth the effort)
>> 
>> I think this will be necessary. PREEMPT_RT cannot support reliable
>> emergency console messages without it. And for kernel developers this
>> is also very helpful. People like PeterZ are using their own patches
>> because the mainline kernel is not providing this functionality.
>> 
>> The decision about _when_ to use it is still in the air. But I guess
>> we'll worry about that when we get that far. There's enough to do
>> until then.
>
> I am sure that there are situations where the direct output
> to atomic consoles would help with debugging. PeteZ and Steven
> are using their own patches for a reason.
>
> Let's see how the code is complicated and how many consoles
> might get supported a reasonable way.

Agreed.

I will post each of the above series as RFCv2 because I expect we still
need some discussion. Especially if I post the fully lockless version of
the ringbuffer.

I have taken a *lot* of notes about things that need changing during
this thread. I think a lot of great feedback has come out of this
RFC. Thank you to everyone that responded (publicly and privately)! I'll
need several weeks before the RFCv2 for the ringbuffer is ready.

John Ogness