Date:   Wed, 5 Dec 2018 17:04:44 -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 04:27:38PM -0800, Josh Triplett wrote:
> On Wed, Dec 05, 2018 at 04:08:09PM -0800, Paul E. McKenney wrote:
> > 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?
> 
> Perfect, thanks. That's much better.
> 
> Reviewed-by: Josh Triplett <josh@...htriplett.org>

Applied, thank you!

							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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