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

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Date:   Wed, 5 Dec 2018 17:06:18 -0800
From:   "Paul E. McKenney" <paulmck@...ux.ibm.com>
To:     Josh Triplett <josh@...htriplett.org>
Cc:     Andrea Parri <andrea.parri@...rulasolutions.com>,
        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 04:58:27PM -0800, Josh Triplett wrote:
> On Thu, Dec 06, 2018 at 01:51:47AM +0100, Andrea Parri wrote:
> > > commit 4f8f751961b536f77c8f82394963e8e2d26efd84
> > > Author: Paul E. McKenney <paulmck@...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"
> > 
> > Mmh, are you sure you don't want s/a8/a16/ here? ;-)
> 
> ... *facepalm*

Yeah, me as well...

> Good catch.

Thank you both!!!

							Thanx, Paul