Re: [PATCH printk v2 09/12] printk: add functions to prefer direct printing

[Helge Deller]

Hi John,

On 4/7/22 16:35, John Ogness wrote:
> On 2022-04-07, Helge Deller <deller@xxxxxx> wrote:
>>>>  - emergency reboot/shutdown
>>>>  - cpu and rcu stalls
>>>>  - hard and soft lockups
>>>>  - hung tasks
>>>>  - warn
>>>>  - sysrq
>>
>> I agree that those needs direct printing.
>> Did you maybe checked how much (e.g. in percentage) of a normal log is
>> then still left to be printed threadened?  Just in case it's not much,
>> is there still a benefit to introduce threadened printing?
>
> A "normal" log can still contain many many messages. There have been
> reports [0] where the system died because _printk_ killed it. This was
> not any of the scenarios that we want direct printing for. It was just a
> lot of messages that some driver wanted to output.
>
> Until now the response was always to reduce printk usage or avoid it
> altogether. printk has a bad reputation because it is so unpredictable
> and expensive. So we are fixing printk. With threaded printers, we make
> a dramatic change. printk becomes 100% lockless and consistently quite
> fast. You can use it to debug or log messages within any code in the
> kernel and not need to worry about horrible latencies being introduced.
>
> For drivers that start dumping tons of messages, there is no concern
> that the system will die because of the messages.

Thank you for this good explanation!

In my case - while I debug low-level kernel code - I then just need to use
pr_warn() or pr_emerg() and get it printed non-threadened. That's sufficient for me.

>> Or another example, e.g. when running on a slow console (the best
>> testcase I know of is a 9600 baud serial port), where some printks are
>> threadened. Then a warn printk() has to be shown, doesn't it mean all
>> threadened printks have to be flushed (waited for) until the warning
>> can be shown. Will there then still be a measureable time benefit?
>
> Now you are talking about something else, unrelated to the threaded
> printing. A warn is considered a serious non-normal event. In this case,
> printk temporarily falls back to the direct, slow, unpredictable,
> printk-caller-context behavior we know today.

Good.

> If we see that direct printing is being used during normal operation
> (i.e. developers are performing warns just for fun), then we should not
> mark warns for direct printing. We will need to get some real-world
> experience here, but the goal is that systems do not use direct printing
> unless a real serious emergency is going on. Otherwise, the system risks
> dying because of the messages rather than the problem being reported.

I agree, it's good if this gets fixed at some point.

Thanks!
Helge

> John
>
> [0] https://lore.kernel.org/all/20190306171943.12345598@xxxxxxxxxxxxxxxx