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

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Date:   Tue, 25 Jun 2019 10:44:19 +0200
From:   John Ogness <john.ogness@...utronix.de>
To:     Sergey Senozhatsky <sergey.senozhatsky.work@...il.com>
Cc:     linux-kernel@...r.kernel.org,
        Peter Zijlstra <peterz@...radead.org>,
        Petr Mladek <pmladek@...e.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Andrea Parri <andrea.parri@...rulasolutions.com>,
        Thomas Gleixner <tglx@...utronix.de>,
        Sergey Senozhatsky <sergey.senozhatsky@...il.com>
Subject: Re: [RFC PATCH v2 1/2] printk-rb: add a new printk ringbuffer implementation

On 2019-06-25, Sergey Senozhatsky <sergey.senozhatsky.work@...il.com> wrote:
>>>> +	struct prb_reserved_entry e;
>>>> +	char *s;
>>>> +
>>>> +	s = prb_reserve(&e, &rb, 32);
>>>> +	if (s) {
>>>> +		sprintf(s, "Hello, world!");
>>>> +		prb_commit(&e);
>>>> +	}
>>>
>>> A nit: snprintf().
>>>
>>> sprintf() is tricky, it may write "slightly more than was
>>> anticipated" bytes - all those string_nocheck(" disabled"),
>>> error_string("pK-error"), etc.
>> 
>> Agreed. Documentation should show good examples.
> 
> 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.

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

John Ogness