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Subject: Re: [PATCH printk 3/5] printk: use buffer pool for sprint buffers
To:     Petr Mladek <pmladek@suse.com>
Cc:     John Ogness <john.ogness@linutronix.de>,
        Sergey Senozhatsky <sergey.senozhatsky.work@gmail.com>,
        Sergey Senozhatsky <sergey.senozhatsky@gmail.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Linus Torvalds <torvalds@linux-foundation.org>,
        Greg Kroah-Hartman <gregkh@linuxfoundation.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        linux-kernel@vger.kernel.org
References: <20200922153816.5883-1-john.ogness@linutronix.de>
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 <20200923151129.GC6442@alley>
 <aef3626f-56a0-9040-fc0e-224ada032d02@prevas.dk>
 <2c95c16b-03e7-eadd-d3af-bedc6b0b471e@prevas.dk>
 <20200925082822.GL29288@alley>
From:   Rasmus Villemoes <rasmus.villemoes@prevas.dk>
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Date:   Wed, 30 Sep 2020 10:06:24 +0200
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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