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

[Kent Overstreet]

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.