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

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Message-ID: <20181206005147.GA3300@andrea>
Date:   Thu, 6 Dec 2018 01:51:47 +0100
From:   Andrea Parri <andrea.parri@...rulasolutions.com>
To:     "Paul E. McKenney" <paulmck@...ux.ibm.com>
Cc:     Josh Triplett <josh@...htriplett.org>, 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

> 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? ;-)

  Andrea


> +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
>