linux-kernel - Re: Would you help to tell why async printk solution was not taken to upstream kernel ?

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Date:   Mon, 5 Mar 2018 11:14:16 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     "Qixuan.Wu" <qixuan.wu@...ux.alibaba.com>,
        linux-kernel-owner <linux-kernel-owner@...r.kernel.org>,
        Petr Mladek <pmladek@...e.com>, Jan Kara <jack@...e.cz>,
        linux-kernel <linux-kernel@...r.kernel.org>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        "chenggang.qin" <chenggang.qin@...ux.alibaba.com>,
        caijingxian <caijingxian@...ux.alibaba.com>,
        "yuanliang.wyl" <yuanliang.wyl@...baba-inc.com>
Subject: Re: Would you help to tell why async printk solution was not taken
 to upstream kernel ?

On (03/04/18 10:43), Steven Rostedt wrote:
> On Sun, 04 Mar 2018 23:08:23 +0800
> "Qixuan.Wu" <qixuan.wu@...ux.alibaba.com> wrote:
> 
> > Suppose there is one scenario that the system has 100 CPU(0~99). While CPU 0 is 
> > calling slow console, CPU 1~99 are calling printk at the same time. And suppose 
> > CPU 1 will be waiter, as per the patch, 2~99 will return directly. After CPU 0 finish 
> > it's log to console, it will return when it finds CPU 1 are waiting. Then CPU 1 need 
> > flush all logs of CPU(1~99) to the console, which may cause  softlockup or rcu 
> > stall. Above scenario is very unusual and it's very unlikely to happen. 
> 
> Yes, people keep bringing up this scenario.

Yeah.

> It would require a single burst of printks to all CPUs.

That's one possibility. The other one is - console_sem locked by a
preemptible context which gets scheduled out.

> And then no more printks after that. The last one will end up printing
> the entire buffer out the slow console. The thing is, this is a bounded
> time, and no printk will print more than one full buffer worth.

It can print more than "one full buffer worth". In theory and on practice.

> If this is a worry, then set the timeouts for the lockup detection to
> be longer than the time it takes to print one full buffer with the
> slowest console.

I see your point.
But I still think that it makes sense to change that "print it all" approach.
With more clear/explicit watchdog-dependent limits - we do direct printk for
1/2 (or 2/3) of a current watchdog threshold value and offload if there is
more stuff in the logbuf. Implicit "logbuf size * console throughput" is
harder to understand. Disabling watchdog because of printk is a bit too much
of a compromise, probably.

IOW, is logbuf worth of messages so critically important after all that we
are ready to jeopardize the system stability?

	-ss