Re: [tip:core/rcu] rcutorture: Make initrd/init execute in userspace

[Josh Triplett]

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@xxxxxxxxxxxxxxxx>

> 							Thanx, Paul
> 
> ------------------------------------------------------------------------
> 
> 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 $a8"
> +a192="$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
>