linux-kernel - Re: [RFC][PATCHv6 00/12] printk: introduce printing kernel thread

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Date:   Mon, 18 Dec 2017 14:51:12 +0100
From:   Petr Mladek <pmladek@...e.com>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     Steven Rostedt <rostedt@...dmis.org>, Tejun Heo <tj@...nel.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Jan Kara <jack@...e.cz>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Rafael Wysocki <rjw@...ysocki.net>,
        Pavel Machek <pavel@....cz>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        linux-kernel@...r.kernel.org
Subject: Re: [RFC][PATCHv6 00/12] printk: introduce printing kernel thread

On Mon 2017-12-18 19:36:24, Sergey Senozhatsky wrote:
> it takes call_console_drivers() 0.01+ of a second to print some of
> the messages [I think we can ignore raw_spin_lock(&console_owner_lock)
> and fully blame call_console_drivers()]. so vprintk_emit() seems to be
> gazillion times faster and i_do_printks can add tons of messages while
> some other process prints just one.
> 
> to be more precise, I see from the traces that i_do_printks can add 1100
> messages to the logbuf while call_console_drivers() prints just one.

This sounds interesting.

A solution would be to add some "simple" throttling. We could add
a per-CPU or per-process counter that would count the number
of lines added while other CPU is processing one line.

The counter would be incremented only when the CPU is actively
printing.

One question is how to clear the counter. One possibility
would be to limit it to one scheduling period or so.

Best Regards,
Petr