Hi Paul,

On Thu, May 28, 2020 at 6:28 PM Paul E. McKenney <paulmck@kernel.org> wrote:
> On Thu, May 28, 2020 at 05:31:33PM +0200, Geert Uytterhoeven wrote:
> > On Thu, May 28, 2020 at 3:51 PM Paul E. McKenney <paulmck@kernel.org> wrote:
> > > On Thu, May 28, 2020 at 09:04:38AM +0200, Geert Uytterhoeven wrote:
> > > > On Thu, May 28, 2020 at 5:26 AM kbuild test robot <lkp@intel.com> wrote:
> > > > > tree:   https://git.kernel.org/pub/scm/linux/kernel/git/paulmck/linux-rcu.git dev.2020.05.26a
> > > > > head:   63fdce1252f16032c9e1eb7244bb674ba4f84855
> > > > > commit: bd5b16d6c88da451a46d068a25fafad8e83d14a6 [56/72] refperf: Allow decimal nanoseconds
> > > > > config: m68k-allyesconfig (attached as .config)
> > > > > compiler: m68k-linux-gcc (GCC) 9.3.0
> > > > > reproduce (this is a W=1 build):
> > > > >         wget https://raw.githubusercontent.com/intel/lkp-tests/master/sbin/make.cross -O ~/bin/make.cross
> > > > >         chmod +x ~/bin/make.cross
> > > > >         git checkout bd5b16d6c88da451a46d068a25fafad8e83d14a6
> > > > >         # save the attached .config to linux build tree
> > > > >         COMPILER_INSTALL_PATH=$HOME/0day COMPILER=gcc-9.3.0 make.cross ARCH=m68k
> > > > >
> > > > > If you fix the issue, kindly add following tag as appropriate
> > > > > Reported-by: kbuild test robot <lkp@intel.com>
> > > > >
> > > > > All errors (new ones prefixed by >>, old ones prefixed by <<):
> > > > >
> > > > > m68k-linux-ld: kernel/rcu/refperf.o: in function `main_func':
> > > > > >> refperf.c:(.text+0x762): undefined reference to `__umoddi3'
> > > > > >> m68k-linux-ld: refperf.c:(.text+0x8f2): undefined reference to `__udivdi3'
> > > > > m68k-linux-ld: refperf.c:(.text+0x97c): undefined reference to `__udivdi3'
> > > >
> > > > | --- a/kernel/rcu/refperf.c
> > > > | +++ b/kernel/rcu/refperf.c
> > > > | @@ -375,7 +375,7 @@ static int main_func(void *arg)
> > > > |                 if (torture_must_stop())
> > > > |                         goto end;
> > > > |
> > > > | -               reader_tasks[exp].result_avg =
> > > > process_durations(exp) / ((exp + 1) * loops);
> > > > | +               reader_tasks[exp].result_avg = 1000 *
> > > > process_durations(exp) / ((exp + 1) * loops);
> > > >
> > > > div64_ul() for 64-by-unsigned-long division
> > >
> > > Ah, thank you for the explanation!
> > >
> > > This is just a performance-test module intended for SMP systems, so
> > > I don't see much point in making it work on m68k, which looks to be
> > > UP-only.  But it is clearly useful to prevent the test bots from building
> > > refperf on m68k.  So one approach would be for me to make its Kconfig
> > > option depend on SMP.  Another would be to make it depend on 64BIT.
> > > Still another would be to make it depend on !M68K.
> > >
> > > I could potentially dump out the numbers in picoseconds, then
> > > do the averaging and other division operations in userspace,
> > > but that is strange enough to cause more trouble than it is worth.
> > > (An rcu_read_lock()/rcu_read_unlock() pair takes -how- long???)  Though if
> > > there was some point in running this on m68k, it might be worth it (with
> > > "PICOSECONDS" in all caps or some such), but in this case it is not.
> > > But this would probably require more data to be dumped to allow userspace
> > > to do the operations, increasing the probability of lost printk()s.  :-/
> > >
> > > Left to myself, I would take the easy way out and make this depend
> > > on 64BIT.
> > >
> > > But you must have run into this situation before.  Any thoughts?
> >
> > Oh, this is not just on m68k. I expect the build bots to start complaining
> > about other 32-bit platforms, too, like i386 and arm32 ;-)
> >
> > While restricting this to 64BIT will fix the issue, are you sure people
> > on 32-bit SMP platforms don't want to run this code?
>
> In the unlikely event that they do, we can go from there.
>
> > So I'd go for div64_ul() and do_div().
>
> OK, I will bite...  Plus my feeble web search failed to satisfy my
> idle curiosity on this point.  ;-)
>
> Why can't these 32-bit SMP platforms supply the API that the compiler
> expects, so that normal C-language arithmetic just works?

This is done on purpose, to avoid people accidentally introducing expensive
64-bit divisions on 32-bit platforms.

Gr{oetje,eeting}s,

                        Geert

-- 
Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@linux-m68k.org

In personal conversations with technical people, I call myself a hacker. But
when I'm talking to journalists I just say "programmer" or something like that.
                                -- Linus Torvalds