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

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Message-ID: <4e36f308-a435-f9c1-2d4f-362e797c764e@prevas.dk>
Date:   Wed, 30 Sep 2020 10:06:24 +0200
From:   Rasmus Villemoes <rasmus.villemoes@...vas.dk>
To:     Petr Mladek <pmladek@...e.com>
Cc:     John Ogness <john.ogness@...utronix.de>,
        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 25/09/2020 10.28, Petr Mladek wrote:
> On Thu 2020-09-24 14:32:49, Rasmus Villemoes wrote:
>> 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.
> 
> Well, printk() is a slow path. It has never been too optimized for
> speed. The main purpose is to report problems and eventually some
> interesting information.

True. But remember that printk is called from _everywhere_, with all
sorts of locks held and/or preemption disabled or whatnot, and every
cycle spent in printk makes those windows wider. Doubling the cost of
every single printk by unconditionally doing vsnprintf() twice is a bad
idea.

Rasmus