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

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Date:   Thu, 24 Sep 2020 11:54:03 +0200
From:   Rasmus Villemoes <rasmus.villemoes@...vas.dk>
To:     Petr Mladek <pmladek@...e.com>,
        John Ogness <john.ogness@...utronix.de>
Cc:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>,
        Sergey Senozhatsky <sergey.senozhatsky@...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 23/09/2020 17.11, Petr Mladek wrote:
> On Tue 2020-09-22 17:44:14, John Ogness wrote:
>> vprintk_store() is using a single static buffer as a temporary
>> sprint buffer for the message text. This will not work once
>> @logbuf_lock is removed. Replace the single static buffer with a
>> pool of buffers.
> 
> The buffer is used because we do not know the length of the
> formatted message to reserve the right space in the ring buffer
> in advance.
> 
> There was the idea to call vsprintf(NULL, fmt, args) to count
> the length in advance.

sprintf is dog slow. If you do this, perhaps say "we can afford to use
128 bytes of stack" and do vsprintf(stackbuf, 128, fmt, args) to do the
counting, and in the vast majority of cases where the text fits we don't
need to do vsprintf() again.

Rasmus