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Date:   Wed, 5 Dec 2018 16:08:09 -0800
From:   "Paul E. McKenney" <paulmck@linux.ibm.com>
To:     Josh Triplett <josh@joshtriplett.org>
Cc:     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
Reply-To: paulmck@linux.ibm.com
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 <20181204230423.GP4170@linux.ibm.com>
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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@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 $a8"
+a192="$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