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

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Date:   Thu, 24 Sep 2020 14:32:49 +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 24/09/2020 11.54, Rasmus Villemoes wrote:
> 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.

Or, since 128 bytes of stack may be too much, combine the "pre-allocate
a few buffers" with "but fall back to vsprintf(NULL) if we can't get
one". That should allow choosing the X in X*1024 smaller than what
worst-case requires (which will never actually be used, assuming the
machine is doing something useful rather than just printk'ing all day
long) and still works even when a tmp buffer can't be obtained.

Rasmus