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[209.132.180.67]) by mx.google.com with ESMTP id u6si21726111plz.220.2018.12.05.16.29.37; Wed, 05 Dec 2018 16:29:54 -0800 (PST) Received-SPF: pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) client-ip=209.132.180.67; Authentication-Results: mx.google.com; spf=pass (google.com: best guess record for domain of linux-kernel-owner@vger.kernel.org designates 209.132.180.67 as permitted sender) smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728870AbeLFA1s (ORCPT + 99 others); Wed, 5 Dec 2018 19:27:48 -0500 Received: from relay6-d.mail.gandi.net ([217.70.183.198]:46445 "EHLO relay6-d.mail.gandi.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727514AbeLFA1s (ORCPT ); Wed, 5 Dec 2018 19:27:48 -0500 X-Originating-IP: 134.134.139.73 Received: from localhost (jfdmzpr04-ext.jf.intel.com [134.134.139.73]) (Authenticated sender: josh@joshtriplett.org) by relay6-d.mail.gandi.net (Postfix) with ESMTPSA id 2807DC0002; Thu, 6 Dec 2018 00:27:42 +0000 (UTC) Date: Wed, 5 Dec 2018 16:27:38 -0800 From: Josh Triplett To: "Paul E. McKenney" Cc: tglx@linutronix.de, mingo@kernel.org, hpa@zytor.com, linux-kernel@vger.kernel.org, linux-tip-commits@vger.kernel.org Subject: Re: [tip:core/rcu] rcutorture: Make initrd/init execute in userspace Message-ID: <20181206002738.GA23990@localhost> References: <20181204222412.GA6874@localhost> <20181204230423.GP4170@linux.ibm.com> <20181205222523.GA14609@localhost> <20181206000809.GX4170@linux.ibm.com> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181206000809.GX4170@linux.ibm.com> User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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 > Thanx, Paul > > ------------------------------------------------------------------------ > > commit 4f8f751961b536f77c8f82394963e8e2d26efd84 > Author: Paul E. McKenney > 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 > Signed-off-by: Paul E. McKenney > > 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 >