From mboxrd@z Thu Jan 1 00:00:00 1970 From: Philippe Gerum In-Reply-To: <4D9230C5.7050007@domain.hid> References: <1301412534.2109.24.camel@domain.hid> <4D922EF5.8020808@domain.hid> <1301426227.2109.35.camel@domain.hid> <4D9230C5.7050007@domain.hid> Content-Type: text/plain; charset="UTF-8" Date: Tue, 29 Mar 2011 21:27:10 +0200 Message-ID: <1301426830.2109.41.camel@domain.hid> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Subject: Re: [Xenomai-help] Priority coupling broken? List-Id: Help regarding installation and common use of Xenomai List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Gilles Chanteperdrix Cc: xenomai@xenomai.org On Tue, 2011-03-29 at 21:19 +0200, Gilles Chanteperdrix wrote: > Philippe Gerum wrote: > > On Tue, 2011-03-29 at 21:11 +0200, Gilles Chanteperdrix wrote: > >> Philippe Gerum wrote: > >>> On Tue, 2011-03-29 at 16:41 +0200, Henri Roosen wrote: > >>>> Hi, > >>>> > >>>> I have several Xenomai RT threads (prio > 0) that get ready to run all > >>>> at the same time. Priority coupling is enabled in the kernel. > >>>> > >>>> If one of them (unfortunately) makes a Linux system call, I see that > >>>> first other lower and same priority Xenomai tasks are scheduled before > >>>> the switched task is run in the Linux domain. As I understand, > >>>> priority coupling should prevent this. > >>>> > >>>> To rule out a problem in the application, this is also tested with a > >>>> simple application based on the rt_print example. In my opinion, with > >>>> priority coupling enabled this should print: > >>>> Wakeup! - I am - awake! - Me too! > >>>> But I get: > >>>> Wakeup! - I am - Me too! - awake! > >>>> So task 2 gets run before task 3 completes in the Linux domain. > >>>> > >>>> Please find attached the test application and the .config file. > >>> The fine print with priority coupling is that it stops immediately > >>> whenever the thread blocks linux-wise; this is actually why, after all > >>> this time debugging it, I'm pondering now whether I should keep this > >>> behavior/feature in 3.x. > >>> > >>> Initially, this was aimed at enforcing the right scheduling sequence > >>> with traditional RTOS APIs, specifically when it comes to create > >>> threads, so that high priority children do run prior to low priority > >>> parents (some legacy apps may expect this). But the fact is that this > >>> behavior also carries a number of uncertainties, and having the thread > >>> de-boosted when blocked by Linux is a serious one. > >> Maybe each thread could have a bit telling whether or not it should run > >> under priority coupling, this bit would be disabled at all times, except > >> during the thread creation routines, and at other times if the user > >> called xnpod_set_mode to enable it if he wants? > >> > > > > This bit exists, it is XNRPIOFF. What I'm pondering is whether this all > > makes sense to provide priority coupling without any mean to actually > > control the impact the regular kernel may have on it. > > > without the irq shield you mean :-) > No, it is not related. The issue now is with the inability to determine whether and when the kernel may cause the priority boost to drop without the user knowing about it. -- Philippe.