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

[Rasmus Villemoes <rasmus.villemoes@prevas.dk>]

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