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

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

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