linux-kernel - Re: [PATCH] refperf: work around 64-bit division

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Date:   Sun, 31 May 2020 13:03:17 -0700
From:   "Paul E. McKenney" <paulmck@...nel.org>
To:     Arnd Bergmann <arnd@...db.de>
Cc:     Nathan Chancellor <natechancellor@...il.com>,
        Josh Triplett <josh@...htriplett.org>,
        "Joel Fernandes (Google)" <joel@...lfernandes.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Mathieu Desnoyers <mathieu.desnoyers@...icios.com>,
        Lai Jiangshan <jiangshanlai@...il.com>,
        Stephen Rothwell <sfr@...b.auug.org.au>, rcu@...r.kernel.org,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
Subject: Re: [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@...il.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@...db.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@...el.com>
    Reported-by: Valdis Klētnieks <valdis.kletnieks@...edu>
    Acked-by: Randy Dunlap <rdunlap@...radead.org> # build-tested
    Signed-off-by: Arnd Bergmann <arnd@...db.de>
    Signed-off-by: Paul E. McKenney <paulmck@...nel.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);
 	}