linux-kernel - Re: [PATCH v5 0/2] printk: Console owner and waiter logic cleanup

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Date:   Fri, 12 Jan 2018 12:12:40 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Petr Mladek <pmladek@...e.com>, Tejun Heo <tj@...nel.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        akpm@...ux-foundation.org, linux-mm@...ck.org,
        Cong Wang <xiyou.wangcong@...il.com>,
        Dave Hansen <dave.hansen@...el.com>,
        Johannes Weiner <hannes@...xchg.org>,
        Mel Gorman <mgorman@...e.de>, Michal Hocko <mhocko@...nel.org>,
        Vlastimil Babka <vbabka@...e.cz>,
        Peter Zijlstra <peterz@...radead.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Jan Kara <jack@...e.cz>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Tetsuo Handa <penguin-kernel@...ove.SAKURA.ne.jp>,
        rostedt@...e.goodmis.org, Byungchul Park <byungchul.park@....com>,
        Pavel Machek <pavel@....cz>, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 0/2] printk: Console owner and waiter logic cleanup

On (01/11/18 20:30), Steven Rostedt wrote:
[..]
> Today, printk() can print for a time of A * B, where, as you state
> above:
> 
>    A is the amount of data to print in the worst case
>    B the time call_console_drivers() needs to print a single
> 	  char to all registered and enabled consoles
> 
> In the worse case, the current approach is A is infinite. That is,
> printk() never stops, as long as there is a printk happening on another
> CPU before B can finish. A will keep growing. The call to printk() will
> never return. The more CPUs you have, the more likely this will occur.
> All it takes is a few CPUs doing periodic printks. If there is a slow
> console, where the periodic printk on other CPUs occur quicker than the
> first can finish, the first one will be stuck forever. Doesn't take
> much to have this happen.

console_sem owner can stuck in console_unlock() not because of printk-s
happening right now on other CPUs, but because those printk-s could have
happened while console_sem owner was preempted. when it comes back it has
a ton of pending messages.

I said it before - "we stuck in console_unlock() because others CPUs
printk right now a lot" is not always true. we have preemption. and
the "last console_sem owner prints it all" is not good in this case.

> With my patch, A is fixed to the size of the buffer. A single printk()
> can never print more than that. If another CPU comes in and does a
> printk, then it will take over the task of printing, and release the
> first printk.

yes. and "another CPU" that comes to take over has to print all the
pending messages. from whatever context it's currently in. and bringing
A * B below C can be quite tricky, if possible at all (!). most likely
people will just add more touch_nmi_watchdog().

again, I don't disagree on "let's bound printk". yes, we totally
should! but the bound must be realistic if we want to fix the damn
thing (either with printk_kthread, or hand off, or anything else).

	-ss