linux-kernel - Re: [PATCH 2/2] Subject: printk: Don't trap random context in infinite log

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Message-ID: <20171104042408.GB539@tigerII.localdomain>
Date:   Sat, 4 Nov 2017 13:24:08 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Tejun Heo <tj@...nel.org>
Cc:     Petr Mladek <pmladek@...e.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        linux-kernel@...r.kernel.org, kernel-team@...com,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [PATCH 2/2] Subject: printk: Don't trap random context in
 infinite log_buf flush

On (11/02/17 06:52), Tejun Heo wrote:
> 
> When printk flushing isn't contended, whoever creates messages get to
> flush them, which is great in both fairness and keeping log delivery
> synchronous.  However, when console drivers can't keep up with the
> rate of new messages, which happens a lot, who ends up with the
> flushing duty is determined by timing and everyone else's messages
> become asynchronous.


Hello Tejun,

thanks for the patch set. we are currently looking at another approach:
lkml.kernel.org/r/20171102134515.6eef16de@...dalf.local.home

would you be interested in taking a look?

there are some concerns, like a huge number of printk-s happening while
console_sem is locked. e.g. console_lock()/console_unlock() on one of the
CPUs, or console_lock(); printk(); ... printk(); console_unlock();

>
> the problem of "the last printk()", which will take
> over and do the flush.
> 
> CPU0                                    CPU1  ~  CPU99
> console_lock();
>                                        printk(); ... printk();
> console_unlock();
>                                            IRQ on CPU2
>                                             printk()
>                                              // take over console_sem
>                                              console_unlock()
>

	-ss