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Date:   Wed, 5 Dec 2018 17:05:37 -0800
From:   "Paul E. McKenney" <paulmck@linux.ibm.com>
To:     Andrea Parri <andrea.parri@amarulasolutions.com>
Cc:     Josh Triplett <josh@joshtriplett.org>, tglx@linutronix.de,
        mingo@kernel.org, hpa@zytor.com, linux-kernel@vger.kernel.org,
        linux-tip-commits@vger.kernel.org
Subject: Re: [tip:core/rcu] rcutorture: Make initrd/init execute in userspace
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On Thu, Dec 06, 2018 at 01:51:47AM +0100, Andrea Parri wrote:
> > commit 4f8f751961b536f77c8f82394963e8e2d26efd84
> > Author: Paul E. McKenney <paulmck@linux.ibm.com>
> > Date:   Tue Dec 4 14:59:12 2018 -0800
> > 
> >     torture: Explain and simplify odd "for" loop in mkinitrd.sh
> >     
> >     Why a Bourne-shell "for" loop?  And why 192 instances of "a"?  This commit
> >     adds a shell comment to present the answer to these mysteries.  It also
> >     uses a series of factor-of-four Bourne-shell assignments to make it
> >     easy to see how many instances there are, replacing the earlier wall of
> >     'a' characters.
> >     
> >     Reported-by: Josh Triplett <josh@joshtriplett.org>
> >     Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
> > 
> > diff --git a/tools/testing/selftests/rcutorture/bin/mkinitrd.sh b/tools/testing/selftests/rcutorture/bin/mkinitrd.sh
> > index da298394daa2..ff69190604ea 100755
> > --- a/tools/testing/selftests/rcutorture/bin/mkinitrd.sh
> > +++ b/tools/testing/selftests/rcutorture/bin/mkinitrd.sh
> > @@ -40,17 +40,24 @@ mkdir $T
> >  cat > $T/init << '__EOF___'
> >  #!/bin/sh
> >  # Run in userspace a few milliseconds every second.  This helps to
> > -# exercise the NO_HZ_FULL portions of RCU.
> > +# exercise the NO_HZ_FULL portions of RCU.  The 192 instances of "a" was
> > +# empirically shown to give a nice multi-millisecond burst of user-mode
> > +# execution on a 2GHz CPU, as desired.  Modern CPUs will vary from a
> > +# couple of milliseconds up to perhaps 100 milliseconds, which is an
> > +# acceptable range.
> > +#
> > +# Why not calibrate an exact delay?  Because within this initrd, we
> > +# are restricted to Bourne-shell builtins, which as far as I know do not
> > +# provide any means of obtaining a fine-grained timestamp.
> > +
> > +a4="a a a a"
> > +a16="$a4 $a4 $a4 $a4"
> > +a64="$a8 $a8 $a8 $a8"
> 
> Mmh, are you sure you don't want s/a8/a16/ here? ;-)

Indeed I do!  How about the following?

							Thanx, Paul

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

commit 94cae122408cdc55470360868a1a4b8f160e576d
Author: Paul E. McKenney <paulmck@linux.ibm.com>
Date:   Tue Dec 4 14:59:12 2018 -0800

    torture: Explain and simplify odd "for" loop in mkinitrd.sh
    
    Why a Bourne-shell "for" loop?  And why 192 instances of "a"?  This commit
    adds a shell comment to present the answer to these mysteries.  It also
    uses a series of factor-of-four Bourne-shell assignments to make it
    easy to see how many instances there are, replacing the earlier wall of
    'a' characters.
    
    Reported-by: Josh Triplett <josh@joshtriplett.org>
    Signed-off-by: Paul E. McKenney <paulmck@linux.ibm.com>
    Reviewed-by: Josh Triplett <josh@joshtriplett.org>
    [ paulmck: Fix wrong-variable bugs noted by Andrea Parri. ]

diff --git a/tools/testing/selftests/rcutorture/bin/mkinitrd.sh b/tools/testing/selftests/rcutorture/bin/mkinitrd.sh
index da298394daa2..e79eb35c41e2 100755
--- a/tools/testing/selftests/rcutorture/bin/mkinitrd.sh
+++ b/tools/testing/selftests/rcutorture/bin/mkinitrd.sh
@@ -40,17 +40,24 @@ mkdir $T
 cat > $T/init << '__EOF___'
 #!/bin/sh
 # Run in userspace a few milliseconds every second.  This helps to
-# exercise the NO_HZ_FULL portions of RCU.
+# exercise the NO_HZ_FULL portions of RCU.  The 192 instances of "a" was
+# empirically shown to give a nice multi-millisecond burst of user-mode
+# execution on a 2GHz CPU, as desired.  Modern CPUs will vary from a
+# couple of milliseconds up to perhaps 100 milliseconds, which is an
+# acceptable range.
+#
+# Why not calibrate an exact delay?  Because within this initrd, we
+# are restricted to Bourne-shell builtins, which as far as I know do not
+# provide any means of obtaining a fine-grained timestamp.
+
+a4="a a a a"
+a16="$a4 $a4 $a4 $a4"
+a64="$a16 $a16 $a16 $a16"
+a192="$a64 $a64 $a64"
 while :
 do
 	q=
-	for i in \
-		a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a \
-		a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a \
-		a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a \
-		a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a \
-		a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a \
-		a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a a
+	for i in $a192
 	do
 		q="$q $i"
 	done