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Message-Id: <20181206000809.GX4170@linux.ibm.com>
Date: Wed, 5 Dec 2018 16:08:09 -0800
From: "Paul E. McKenney" <paulmck@...ux.ibm.com>
To: Josh Triplett <josh@...htriplett.org>
Cc: tglx@...utronix.de, mingo@...nel.org, hpa@...or.com,
linux-kernel@...r.kernel.org, linux-tip-commits@...r.kernel.org
Subject: Re: [tip:core/rcu] rcutorture: Make initrd/init execute in userspace
On Wed, Dec 05, 2018 at 02:25:24PM -0800, Josh Triplett wrote:
> On Tue, Dec 04, 2018 at 03:04:23PM -0800, Paul E. McKenney wrote:
> > On Tue, Dec 04, 2018 at 02:24:13PM -0800, Josh Triplett wrote:
> > > On Tue, Dec 04, 2018 at 02:09:42PM -0800, tip-bot for Paul E. McKenney wrote:
> > > > --- a/tools/testing/selftests/rcutorture/bin/mkinitrd.sh
> > > > +++ b/tools/testing/selftests/rcutorture/bin/mkinitrd.sh
> > > > @@ -39,9 +39,22 @@ 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.
> > > > while :
> > > > do
> > > > - sleep 1000000
> > > > + 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
> > >
> > > Ow. If there's no better way to do this, please do at least comment how many 'a's
> > > this is. (And why 186, exactly?)
> >
> > Yeah, that is admittedly a bit strange. The reason for 186 occurrences of
> > "a" to one-time calibration, measuring a few millisecond's worth of delay.
> >
> > > Please also consider calibrating the delay loop as you do in the C code.
> >
> > Good point. And a quick web search finds me "date '+%s%N'", which gives
> > me nanoseconds since the epoch. I probably don't want to do a 2038 to
> > myself (after all, I might still be alive then), so I should probably try
> > to make something work with "date '+%N'". Or use something like this:
> >
> > $ date '+%4N'; date '+%4N';date '+%4N'; date '+%4N'
> > 6660
> > 6685
> > 6697
> > 6710
> >
> > Ah, but that means I need to add the "date" command to my initrd, doesn't
> > it? And calculation requires either bash or the "test" command. And it
> > would be quite good to restrict this to what can be done with Bourne shell
> > built-in commands, since a big point of this is to maintain a small-sized
> > initrd. :-/
>
> Sure, and I'm not suggesting adding commands to the initrd, hence my
> mention of "If there's no better way".
>
> > So how about the following patch, which attempts to explain the situation?
>
> That would help, but please also consider consolidating with something
> like a10="a a a a a a a a a a" to make it more readable (and perhaps
> rounding up to 200 for simplicity).
How about powers of four and one factor of three for 192, as shown below?
Thanx, Paul
------------------------------------------------------------------------
commit 4f8f751961b536f77c8f82394963e8e2d26efd84
Author: Paul E. McKenney <paulmck@...ux.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@...htriplett.org>
Signed-off-by: Paul E. McKenney <paulmck@...ux.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 $a8"
+a192="$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
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