linux-kernel - Re: [PATCH v3 03/33] vsprintf: Convert to printbuf

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Date:   Wed, 15 Jun 2022 11:09:35 +0200
From:   Rasmus Villemoes <linux@...musvillemoes.dk>
To:     Kent Overstreet <kent.overstreet@...il.com>,
        linux-kernel@...r.kernel.org
Cc:     pmladek@...e.com, rostedt@...dmis.org
Subject: Re: [PATCH v3 03/33] vsprintf: Convert to printbuf

On 04/06/2022 21.30, Kent Overstreet wrote:
> This converts vsnprintf() to printbufs: instead of passing around raw
> char * pointers for current buf position and end of buf, we have a real
> type!
> 
> This makes the calling convention for our existing pretty printers a lot
> saner and less error prone, plus printbufs add some new helpers that
> make the code smaller and more readable, with a lot less crazy pointer
> arithmetic.
> 
> There are a lot more refactorings to be done: this patch tries to stick
> to just converting the calling conventions, as that needs to be done all
> at once in order to avoid introducing a ton of wrappers that will just
> be deleted.
> 
> Thankfully we have good unit tests for printf, and they have been run
> and are all passing with this patch.

So, as the primary author of those tests, a somewhat active contributor
to vsprintf.c and being listed as R: for both files, why wasn't I cc'ed
on this?

Anyway, my main concern with this is that performance goes down the
drain and the generated code will be awful. Have you done any
measurements and/or looked at disassembly? Thanks to
-fno-strict-aliasing (or perhaps just because we're writing through a
char* pointer which IIRC may alias anything), I think the compiler will
be forced to reload prt->pos and prt->size over and over and over. I may
be wrong, of course, that happens often. Perhaps __restrict could help, IDK.

> ---
>  include/linux/kernel.h |    4 +

Please don't expand that dumping ground. Please, if printbufs will
become a thing (whether or not vsprintf internally will be refactored to
use them), add a new linux/printf.h where these things can go, and the
declarations of vsprintf() and close friends can eventually be moved.

Rasmus