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

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Date:   Thu, 28 May 2020 11:08:55 -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 07:25:01PM +0200, Geert Uytterhoeven wrote:
> Hi Paul,
> 
> On Thu, May 28, 2020 at 6:28 PM Paul E. McKenney <paulmck@...nel.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@...nel.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@...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?
> > >
> > > 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.

Fair enough, and thank you for the explanation!  Though in this case,
these divisions are nowhere near anything even vaguely resembling a
fastpath.  They instead happen at the end of the run, while doing output.

So I am restricting to 64BIT for the time being.  Yeah, I know, lazy of
me.  ;-)

							Thanx, Paul