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

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Date:   Wed, 5 Dec 2018 14:25:24 -0800
From:   Josh Triplett <josh@...htriplett.org>
To:     "Paul E. McKenney" <paulmck@...ux.ibm.com>
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 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).

- Josh