On Thu, Dec 06, 2018 at 01:51:47AM +0100, Andrea Parri wrote:
> > commit 4f8f751961b536f77c8f82394963e8e2d26efd84
> > Author: Paul E. McKenney <paulmck@xxxxxxxxxxxxx>
> > 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@xxxxxxxxxxxxxxxx>
> > Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxx>
> >
> > 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@xxxxxxxxxxxxx>
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@xxxxxxxxxxxxxxxx>
Signed-off-by: Paul E. McKenney <paulmck@xxxxxxxxxxxxx>
Reviewed-by: Josh Triplett <josh@xxxxxxxxxxxxxxxx>
[ 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
|