Re: [RFC PATCH v2 1/2] printk-rb: add a new printk ringbuffer implementation

[Sergey Senozhatsky]

On (06/25/19 10:44), John Ogness wrote:
> > In vprintk_emit(), are we going to always reserve 1024-byte
> > records, since we don't know the size in advance, e.g.
> > 
> > 	printk("%pS %s\n", regs->ip, current->name)
> > 		prb_reserve(&e, &rb, ????);
> > 
> > or are we going to run vscnprintf() on a NULL buffer first,
> > then reserve the exactly required number of bytes and afterwards
> > vscnprintf(s) -> prb_commit(&e)?
> 
> (As suggested by Petr) I want to use vscnprintf() on a NULL
> buffer. However, a NULL buffer is not sufficient because things like the
> loglevel are sometimes added via %s (for example, in /dev/kmsg). So
> rather than a NULL buffer, I would use a small buffer on the stack
> (large enough to store loglevel/cont information). This way we can use
> vscnprintf() to get the exact size _and_ printk_get_level() will see
> enough of the formatted string to parse what it needs.

OK. I guess this should work except for the cases when we want to
printk that we are running out of stack :)

More seriously, tho, sometimes messages come with dictionaries of
key/value pairs. I don't think we impose any strict limits on the
number of key/value pair or on the overall size of the dictionary
each record can have (up to a single PAGE, I'd guess. I really need
to check printk code). Finding a sufficiently large buffer size
might be a bit of a task.

> > I'm asking this because, well, if the most common usage
> > pattern (printk->prb_reserve) will always reserve fixed
> > size records (aka data blocks), then you _probably_ (??)
> > can drop the 'variable size records' requirement from prb
> > design and start looking at records (aka data blocks) as
> > fixed sized chunks of bytes, which are always located at
> > fixed offsets.
>
> The average printk message size is well under 128 bytes.

Do you also count in dictionary of properties (key/value pairs) which
records can carry?

For printks from core kernel 128 bytes would be a good estimation,
for dev_printk() and so on - I'm not exactly sure.

cat /dev/kmsg

This one, for instance, is a single logbuf record

6,560,2470340,-;hid-generic 0003:093A:2510.0001: input,hidraw0: USB HID v1.11 Mouse [PixArt USB Optical Mouse] on usb-0000:00:14.0-3/input0
 SUBSYSTEM=hid
 DEVICE=+hid:0003:093A:2510.0001

I suspect that it's larger than 128 bytes.

> It would be quite wasteful to always reserve 1K blocks.

Agreed.

	-ss