From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-8.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SIGNED_OFF_BY,SPF_PASS,URIBL_BLOCKED, USER_AGENT_MUTT autolearn=ham autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id AE6D8C04EB9 for ; Thu, 6 Dec 2018 00:58:36 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [209.132.180.67]) by mail.kernel.org (Postfix) with ESMTP id 76C8920892 for ; Thu, 6 Dec 2018 00:58:36 +0000 (UTC) DMARC-Filter: OpenDMARC Filter v1.3.2 mail.kernel.org 76C8920892 Authentication-Results: mail.kernel.org; dmarc=none (p=none dis=none) header.from=joshtriplett.org Authentication-Results: mail.kernel.org; spf=none smtp.mailfrom=linux-kernel-owner@vger.kernel.org Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1728731AbeLFA6f (ORCPT ); Wed, 5 Dec 2018 19:58:35 -0500 Received: from relay6-d.mail.gandi.net ([217.70.183.198]:50855 "EHLO relay6-d.mail.gandi.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1727794AbeLFA6e (ORCPT ); Wed, 5 Dec 2018 19:58:34 -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 873FDC0005; Thu, 6 Dec 2018 00:58:30 +0000 (UTC) Date: Wed, 5 Dec 2018 16:58:27 -0800 From: Josh Triplett To: Andrea Parri Cc: "Paul E. McKenney" , 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: <20181206005827.GA24891@localhost> References: <20181204222412.GA6874@localhost> <20181204230423.GP4170@linux.ibm.com> <20181205222523.GA14609@localhost> <20181206000809.GX4170@linux.ibm.com> <20181206005147.GA3300@andrea> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: <20181206005147.GA3300@andrea> 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 Thu, Dec 06, 2018 at 01:51:47AM +0100, Andrea Parri wrote: > > 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" > > Mmh, are you sure you don't want s/a8/a16/ here? ;-) ... *facepalm* Good catch.