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Subject[PATCH tip/core/rcu 02/10] torture: Explain and simplify odd "for" loop in mkinitrd.sh
Date
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. ]
---
.../selftests/rcutorture/bin/mkinitrd.sh | 23 ++++++++++++-------
1 file changed, 15 insertions(+), 8 deletions(-)

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
--
2.17.1
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