linux-kernel - Re: [RFC][PATCH] kernel.h: Add generic roundup

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Message-ID: <CAHk-=wg5HqJ2Kfgpub+tCWQ2_FiFwEW9H1Rm+an-BLGaGvDDXw@mail.gmail.com>
Date:   Thu, 23 May 2019 08:10:44 -0700
From:   Linus Torvalds <torvalds@...ux-foundation.org>
To:     Steven Rostedt <rostedt@...dmis.org>
Cc:     LKML <linux-kernel@...r.kernel.org>,
        Ben Skeggs <bskeggs@...hat.com>,
        David Airlie <airlied@...ux.ie>,
        Daniel Vetter <daniel@...ll.ch>,
        Leon Romanovsky <leon@...nel.org>,
        Doug Ledford <dledford@...hat.com>,
        Jason Gunthorpe <jgg@...pe.ca>,
        "Darrick J. Wong" <darrick.wong@...cle.com>,
        linux-xfs@...r.kernel.org,
        dri-devel <dri-devel@...ts.freedesktop.org>,
        nouveau@...ts.freedesktop.org,
        linux-rdma <linux-rdma@...r.kernel.org>,
        Andrew Morton <akpm@...ux-foundation.org>
Subject: Re: [RFC][PATCH] kernel.h: Add generic roundup_64() macro

On Thu, May 23, 2019 at 7:00 AM Steven Rostedt <rostedt@...dmis.org> wrote:
>
> +# define roundup_64(x, y) (                            \
> +{                                                      \
> +       typeof(y) __y = y;                              \
> +       typeof(x) __x = (x) + (__y - 1);                \
> +       do_div(__x, __y);                               \
> +       __x * __y;                                      \
> +}                                                      \

The thing about this is that it absolutely sucks for power-of-two arguments.

The regular roundup() that uses division has the compiler at least
optimize them to shifts - at least for constant cases. But do_div() is
meant for "we already know it's not a power of two", and the compiler
doesn't have any understanding of the internals.

And it looks to me like the use case you want this for is very much
probably a power of two. In which case division is all kinds of just
stupid.

And we already have a power-of-two round up function that works on
u64. It's called "round_up()".

I wish we had a better visual warning about the differences between
"round_up()" (limited to powers-of-two, but efficient, and works with
any size) and "roundup()" (generic, potentially horribly slow, and
doesn't work for 64-bit on 32-bit).

Side note: "round_up()" has the problem that it uses "x" twice.

End result: somebody should look at this, but I really don't like the
"force division" case that is likely horribly slow and nasty.

                  Linus