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

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Message-ID: <20181109061204.GC599@jagdpanzerIV>
Date:   Fri, 9 Nov 2018 15:12:04 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
To:     Tetsuo Handa <penguin-kernel@...ove.sakura.ne.jp>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Petr Mladek <pmladek@...e.com>,
        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 20:37), Tetsuo Handa wrote:
> On 2018/11/08 13:45, Sergey Senozhatsky wrote:
> > So, can we just do the following? /* a sketch */
> > 
> > lockdep.c
> > 	printk_safe_enter_irqsave(flags);
> > 	lockdep_report();
> > 	printk_safe_exit_irqrestore(flags);
> 
> If buffer size were large enough to hold messages from out_of_memory(),
> I would like to use it for out_of_memory() because delaying SIGKILL
> due to waiting for printk() to complete is not good. Surely we can't
> hold all messages because amount from dump_tasks() is unpredictable.
> Maybe we can hold all messages from dump_header() except dump_tasks().
> 
> But isn't it essentially same with
> http://lkml.kernel.org/r/1493560477-3016-1-git-send-email-penguin-kernel@I-love.SAKURA.ne.jp
> which Linus does not want?

Dunno. I guess we still haven't heard from Linus because he did quite a good
job setting up his 'email filters' ;)

Converting the existing users to buffered printk is not so simple.
Apparently there are different paths; some can afford buffered printk, some
cannot. Some of 'cont' users tend to get advantage of transparent 'cont'
context: start 'cont' output in function A: A()->pr_cont(), continue it in
B: A()->B()->pr_cont(), and then in C: A()->B()->C()->pr_cont(), and
finally flush in A: A()->pr_cont(\n). And then some paths have the
early_printk requirement. We can break the 'transparent cont' by passing
buffer pointers around [it can get a bit hairy; looking at lockdep patch],
but early_printk requirement is a different beast.

So in my email I was not advertising printk_safe as a "buffered printk for
everyone", I was just talking about lockdep. It's a bit doubtful that Peter
will ACK lockdep transition to buffered printk.

	-ss