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 <j...@joshtriplett.org> > Thanx, Paul > > ------------------------------------------------------------------------ > > commit 4f8f751961b536f77c8f82394963e8e2d26efd84 > Author: Paul E. McKenney <paul...@linux.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 <j...@joshtriplett.org> > Signed-off-by: Paul E. McKenney <paul...@linux.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 $a8" > +a192="$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 >
