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SubjectRe: [PATCH] refperf: work around 64-bit division
On Sat, May 30, 2020 at 10:01:36AM +0200, Arnd Bergmann wrote:
> On Sat, May 30, 2020 at 5:52 AM Nathan Chancellor
> <natechancellor@gmail.com> wrote:
> > On Fri, May 29, 2020 at 10:15:51PM +0200, Arnd Bergmann wrote:
> > > strcat(buf, "Threads\tTime(ns)\n");
> > >
> > > for (exp = 0; exp < nruns; exp++) {
> > > + u64 avg;
> > > + u32 rem;
> > > +
> > > if (errexit)
> > > break;
> > > - sprintf(buf1, "%d\t%llu.%03d\n", exp + 1, result_avg[exp] / 1000, (int)(result_avg[exp] % 1000));
> > > +
> > > + avg = div_s64_rem(result_avg[exp], 1000, &rem);
> >
> > Shouldn't this be div_u64_rem? result_avg is u64.
>
> Yes, you are right. Actually that would be an important optimization
> since div_u64_rem() optimizes for constant divisors while div_s64_rem
> uses the slow path.
>
> > > + sprintf(buf1, "%d\t%llu.%03d\n", exp + 1, avg, rem);
> >
> > Would %03u be the better specifier since rem is u32?
>
> Yes, though this makes no difference in practice.
>
> Paul, should I send a fixup for these two, or do you prefer to just
> edit it in place?

And here is the update, thank you all!

Thanx, Paul

------------------------------------------------------------------------

commit 0dd4132157c2cf6bec2a0a6e04163067323abdb1
Author: Arnd Bergmann <arnd@arndb.de>
Date: Fri May 29 14:36:26 2020 -0700

refperf: Work around 64-bit division

A 64-bit division was introduced in refperf, breaking compilation
on all 32-bit architectures:

kernel/rcu/refperf.o: in function `main_func':
refperf.c:(.text+0x57c): undefined reference to `__aeabi_uldivmod'

Fix this by using div_u64 to mark the expensive operation.

[ paulmck: Update primitive and format per Nathan Chancellor. ]
Fixes: bd5b16d6c88d ("refperf: Allow decimal nanoseconds")
Reported-by: kbuild test robot <lkp@intel.com>
Reported-by: Valdis Klētnieks <valdis.kletnieks@vt.edu>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Signed-off-by: Arnd Bergmann <arnd@arndb.de>
Signed-off-by: Paul E. McKenney <paulmck@kernel.org>

diff --git a/kernel/rcu/refperf.c b/kernel/rcu/refperf.c
index 99434e7..3b72925 100644
--- a/kernel/rcu/refperf.c
+++ b/kernel/rcu/refperf.c
@@ -478,7 +478,7 @@ static int main_func(void *arg)
if (torture_must_stop())
goto end;

- result_avg[exp] = 1000 * process_durations(nreaders) / (nreaders * loops);
+ result_avg[exp] = div_u64(1000 * process_durations(nreaders), nreaders * loops);
}

// Print the average of all experiments
@@ -489,9 +489,13 @@ static int main_func(void *arg)
strcat(buf, "Runs\tTime(ns)\n");

for (exp = 0; exp < nruns; exp++) {
+ u64 avg;
+ u32 rem;
+
if (errexit)
break;
- sprintf(buf1, "%d\t%llu.%03d\n", exp + 1, result_avg[exp] / 1000, (int)(result_avg[exp] % 1000));
+ avg = div_u64_rem(result_avg[exp], 1000, &rem);
+ sprintf(buf1, "%d\t%llu.%03u\n", exp + 1, avg, rem);
strcat(buf, buf1);
}

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