linux-kernel - Re: [rcu:dev.2020.05.26a 56/72] refperf.c:undefined reference to `_

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Date:   Thu, 28 May 2020 06:51:41 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Geert Uytterhoeven <geert@...ux-m68k.org>
Cc:     kbuild test robot <lkp@...el.com>, kbuild-all@...ts.01.org,
        Linux Kernel Mailing List <linux-kernel@...r.kernel.org>
Subject: Re: [rcu:dev.2020.05.26a 56/72] refperf.c:undefined reference to
 `__umoddi3'

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@...el.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@...el.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?

							Thanx, Paul

> |         }
> |
> |         // Print the average of all experiments
> | @@ -386,7 +386,7 @@ static int main_func(void *arg)
> |         strcat(buf, "Threads\tTime(ns)\n");
> |
> |         for (exp = 0; exp < nreaders; exp++) {
> | -               sprintf(buf1, "%d\t%llu\n", exp + 1,
> reader_tasks[exp].result_avg);
> | +               sprintf(buf1, "%d\t%llu.%03d\n", exp + 1,
> reader_tasks[exp].result_avg / 1000,
> (int)(reader_tasks[exp].result_avg % 1000));
> 
> do_div() for 64-by-32 division/modulo
> 
> |                 strcat(buf, buf1);
> |         }
> 
> 
> Gr{oetje,eeting}s,
> 
>                         Geert
> 
> -- 
> Geert Uytterhoeven -- There's lots of Linux beyond ia32 -- geert@...ux-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