linux-kernel - Re: [PATCH 3/3] lockdep: Use line-buffered printk() for lockdep messages.

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Message-ID: <20181108124413.GB30440@jagdpanzerIV>
Date:   Thu, 8 Nov 2018 21:44:13 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        Dmitriy Vyukov <dvyukov@...gle.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Alexander Potapenko <glider@...gle.com>,
        Fengguang Wu <fengguang.wu@...el.com>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        LKML <linux-kernel@...r.kernel.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Andrew Morton <akpm@...ux-foundation.org>, linux-mm@...ck.org,
        Ingo Molnar <mingo@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>,
        Will Deacon <will.deacon@....com>
Subject: Re: [PATCH 3/3] lockdep: Use line-buffered printk() for lockdep
 messages.

On (11/08/18 12:53), Petr Mladek wrote:
> > lockdep.c
> > 	printk_safe_enter_irqsave(flags);
> > 	lockdep_report();
> > 	printk_safe_exit_irqrestore(flags);
> 
> All this looks nice. Let's look it also from the other side.
> The following comes to my mind:
> 
> a) lockdep is not the only place when continuous lines get mixed.
>    This patch mentions also RCU stalls. The other patch mentions
>    OOM. I am sure that there will be more.
> 
> b) It is not obvious where printk_safe() would be necessary.
>    While buffered printk is clearly connected with continuous
>    lines.
> 
> c) I am not sure that disabling preemption would always be
>    acceptable.
> 
> d) We might need to increase the size of the per-CPU buffers if
>    they are used more widely.
> 
> e) People would need to learn a new (printk_safe) API when it is
>    use outside printk sources.
> 
> f) Losing the entire log is more painful than loosing one line
>    when the buffer never gets flushed.
> 
> Sigh, no solution is perfect. If only we could agree that one
> way was better than the other.

I agree with what you are saying. All of the above (in my email)
was for lockdep only, that's why I did mention lockdep several times.
Like I said, a random and wild idea.
I'm not proposing printk_safe as a "better" buffered printk for
everyone. The buffered_printk patch is pretty big, and comes with a
price tag.

If lockdep and OOM people will ACK buffered printk transition in
its current form, then we can go ahead.

It's debatable if we need a fixed size list of buffers; or we can
do kmalloc()+cont fallback. But if we will have ACKs, then we can
move forward.

	-ss