From 0d8a9ea9764a0e34e17e3b80a2be3855de239d6e Mon Sep 17 00:00:00 2001 From: "Paul E. McKenney" Date: Tue, 4 Dec 2018 14:59:12 -0800 Subject: [PATCH] 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 Signed-off-by: Paul E. McKenney Reviewed-by: Josh Triplett [ paulmck: Fix wrong-variable bugs noted by Andrea Parri. ] --- tools/testing/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 da29839..e79eb35 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 -- 2.7.4