Re: [PATCH] printk/console: Do not suppress information about dropped messages

Sergey Senozhatsky
2019-02-27T02:56:30Z

On (02/26/19 17:26), John Ogness wrote:
[..]
> > if (console_seq < log_first_seq) {
> > - len = sprintf(text,
> > - "** %llu printk messages dropped **\n",
> > - log_first_seq - console_seq);
> > + console_dropped_cnt += log_first_seq - console_seq;
> >
> > /* messages are gone, move to first one */
> > console_seq = log_first_seq;
> > console_idx = log_first_idx;
> > - } else {
> > - len = 0;
> > }
> > skip:
> > if (console_seq == log_next_seq)
> > @@ -2435,6 +2441,13 @@ void console_unlock(void)
> > exclusive_console = NULL;
> > }
> >
> > + if (unlikely(console_dropped_cnt)) {
> > + len = sprintf(text,
> > + "** %llu printk messages dropped **\n",
> > + console_dropped_cnt);
> > + console_dropped_cnt = 0;
> > + }
> > +
>
> My only objection to this is that the "messages dropped" only comes if a
> non-supressed message comes. So information about dropped information
> may never get printed unless some task prints something non-supressed.
>
> Imagine a situation where I am expecting a message to come, but don't
> see it because it was dropped. But if no more non-supressed messages
> come, I see neither the expected message nor the dropped message.

I think this is exactly the problem (and thus the patch) we discussed some
3 years ago. I had a number of rather strangely looking serial logs, which
clearly had lost messages but no "%llu printk messages dropped" markers. So
I added `static bool lost_messages' to console_unlock(), set it when printing
loop would discover lost messages, then print "%llu printk messages dropped"
attached to whatever msg was next in the logbuf, regardless of msg->level.
IOW, if lost_messages was set then suppress_message_printing(msg->level)
was not even invoked. Yes, that would sometimes print several "debugging
noise" messages, but the main part was that I would have "%llu printk
messages dropped" markers in the logs; which was much more important to
me.

P.S. I'm very sorry, I'm overloaded with work at the moment; will start
looking at pending patches in a day or two, or three, or four...

-ss