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

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Date:   Wed, 15 Jun 2022 14:44:29 -0400
From:   Kent Overstreet <kent.overstreet@...il.com>
To:     Rasmus Villemoes <linux@...musvillemoes.dk>,
        linux-kernel@...r.kernel.org
Cc:     pmladek@...e.com, rostedt@...dmis.org
Subject: Re: [PATCH v3 03/33] vsprintf: Convert to printbuf

On 6/15/22 05:09, Rasmus Villemoes wrote:
> 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?

Apologies for the oversight.

> 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.

If we care that much about sprintf performance we must have some 
benchmarks somewhere - could you point me at them?


>> ---
>>   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.

kernel.h is indeed a dumping ground that needs to be reorganized, but 
this patch series is about printbufs, not reorganizing header files (and 
my doctor tells me I need to be taking my blood pressure meds more 
regularly before taking something like that on). For now, prt_printf() 
goes with the other printf() functions.