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McKenney" To: Josh Triplett 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 Reply-To: paulmck@linux.ibm.com References: <20181204222412.GA6874@localhost> <20181204230423.GP4170@linux.ibm.com> <20181205222523.GA14609@localhost> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181205222523.GA14609@localhost> User-Agent: Mutt/1.5.21 (2010-09-15) X-TM-AS-GCONF: 00 x-cbid: 18120600-0072-0000-0000-000003D4DD93 X-IBM-SpamModules-Scores: X-IBM-SpamModules-Versions: BY=3.00010178; HX=3.00000242; KW=3.00000007; PH=3.00000004; SC=3.00000270; SDB=6.01127552; UDB=6.00582089; IPR=6.00907686; MB=3.00024466; MTD=3.00000008; XFM=3.00000015; UTC=2018-12-06 00:08:12 X-IBM-AV-DETECTION: SAVI=unused REMOTE=unused XFE=unused x-cbparentid: 18120600-0073-0000-0000-00004A586C7A Message-Id: <20181206000809.GX4170@linux.ibm.com> X-Proofpoint-Virus-Version: vendor=fsecure engine=2.50.10434:,, definitions=2018-12-05_09:,, signatures=0 X-Proofpoint-Spam-Details: rule=outbound_notspam policy=outbound score=0 priorityscore=1501 malwarescore=0 suspectscore=0 phishscore=0 bulkscore=0 spamscore=0 clxscore=1015 lowpriorityscore=0 mlxscore=0 impostorscore=0 mlxlogscore=999 adultscore=0 classifier=spam adjust=0 reason=mlx scancount=1 engine=8.0.1-1810050000 definitions=main-1812050211 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 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? 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