linux-kernel - Re: [PATCH printk 3/5] printk: use buffer pool for sprint buffers

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Date:   Thu, 24 Sep 2020 18:06:53 +0900
From:   Sergey Senozhatsky <sergey.senozhatsky@...il.com>
To:     Petr Mladek <pmladek@...e.com>
Cc:     Sergey Senozhatsky <sergey.senozhatsky@...il.com>,
        John Ogness <john.ogness@...utronix.de>,
        Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH printk 3/5] printk: use buffer pool for sprint buffers

On (20/09/24 10:54), Petr Mladek wrote:
> > Just a question:
> > 
> > Can dynamic_textbuf be a PER_CPU array of five textbuf[1024] buffers
> > (for normal printk, nmi, hard irq, soft irq and one extra buffer for
> > recursive printk calls)?
> 
> That would be my preferred fallback when the approach with
> vsprintf(NULL, ) is not acceptable for some reasons.

OK.

> But I still think that calling vsprintf(NULL, ) is the most trivial
> and good enough solution.

It's probably good enough.

> IMHO, the solution with per-CPU buffers is not that trivial, for
> example:
> 
>   What if recursive printk() is interrupted by NMI and it causes
>   yet another recursion?
> 
>   Is one level of recursion enough?

We might try the current approach - when we, for example, have
recursion in printk_safe() we just end up writing data to the
same per-CPU buffer. We need to limit the depth of recursion
one way or another. With per-CPU counter we will just bail out
of "deeply recursive printk" without attempting to store its
messages; with the buffers approach we will write the data to
a static buffer and see how badly it will be overlapped at the
end. Just a thought.

	-ss