linux-kernel - Re: [BUG] Threaded printk breaks early debugging

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Message-ID: <Yqb9xOBiY/262lhk@google.com>
Date:   Mon, 13 Jun 2022 18:05:08 +0900
From:   Sergey Senozhatsky <senozhatsky@...omium.org>
To:     John Ogness <john.ogness@...utronix.de>
Cc:     Sergey Senozhatsky <senozhatsky@...omium.org>,
        Petr Mladek <pmladek@...e.com>,
        Peter Geis <pgwipeout@...il.com>,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
        "open list:ARM/Rockchip SoC..." <linux-rockchip@...ts.infradead.org>
Subject: Re: [BUG] Threaded printk breaks early debugging

On (22/06/13 10:36), John Ogness wrote:
> >> IMHO, no. Especially in that situation, we do not want printk causing
> >> that atomic section to become even longer. If the machine has entered
> >> normal operation, we want printk out of the way.
> >
> > At the same time printk throttles itself in such cases: new messages are
> > not added at much higher pace that they are printed at. So we lower the
> > chances of missing messages.
> 
> That is true if there is only 1 printk caller.

Well, which is the case when num_online_cpus() == 1?

> For SMP systems with printing handovers, it might not help at all.
> I firmly believe that sprinkling randomness into printk (i.e. system)
> latencies is not the answer. We need to keep printk lockless and out
> of the system's way unless there is a real emergency happening.

Yeah sure.

> This particular thread is not about missed messages due to printk not
> "throttling the system", but rather the kernel buffers not getting
> flushed in an emergency. This, of course, needs to be properly handled.

True, but Peter mentioned

  "I noticed with threading enabled during large bursts the console
   drops an excessive amount of messages. It's especially apparent
   during the handover from earlycon to the normal console."