Re: [tip:core/rcu] rcutorture: Make initrd/init execute in userspace
From: Paul E. McKenney
Date: Wed Dec 05 2018 - 20:06:25 EST
On Wed, Dec 05, 2018 at 04:58:27PM -0800, Josh Triplett wrote:
> 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? ;-)
>
> ... *facepalm*
Yeah, me as well...
> Good catch.
Thank you both!!!
Thanx, Paul