From mboxrd@z Thu Jan 1 00:00:00 1970 Subject: Re: [Xenomai-core] Prio-inversion on cleanup? From: Philippe Gerum In-Reply-To: <44A3ACA8.6070201@domain.hid> References: <44A1608B.3090605@domain.hid> <44A295ED.2080306@domain.hid> <44A39C3C.1080508@domain.hid> <1151576082.5291.41.camel@domain.hid> <44A3ACA8.6070201@domain.hid> Content-Type: text/plain Date: Thu, 29 Jun 2006 12:48:09 +0200 Message-Id: <1151578090.5291.49.camel@domain.hid> Mime-Version: 1.0 Content-Transfer-Encoding: 7bit Reply-To: rpm@xenomai.org List-Id: "Xenomai life and development \(bug reports, patches, discussions\)" List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Jan Kiszka Cc: xenomai-core On Thu, 2006-06-29 at 12:34 +0200, Jan Kiszka wrote: > Philippe Gerum wrote: > > On Thu, 2006-06-29 at 11:24 +0200, Jan Kiszka wrote: > >> Jan Kiszka wrote: > >>> ... > >>> The pthread is blocked on the irqbench device ioctl. On hitting ^C, > >>> close() is invoked from the main thread for that device. The pthread is > >>> woken up and obviously relaxed on some linux syscall (after being > >>> interrupted twice by the periodic timer event of a "latency -p 300 -P > >>> 50" instance). This passes the control over to the main thread while > >>> keeping the pthread prio of 99. And this prio seems to survive for the > >>> following 11 ms (full trace available on request). > >>> > >>> Any ideas what's going on? > >>> > >> Ok, I think I finally understood the issue. It seems to lie deep in the > >> POSIX user-space lib, specifically its use of standard pthread services. > >> Let me first clarify my scenario: > >> > >> A high-prio pthread of known and (theoretically) bounded workload shall > >> be started and stopped while a low-prio thread is already running. The > >> low-prio thread shall only be impacted by the real workload of the > >> high-prio one, not by its creation/destruction - at least not > >> significantly. To achieve this with the POSIX skin (actually this > >> applies to preempt-rt in theory as well), I have to create the thread > >> under SCHED_OTHER, raise its priority right before entering the > >> workload, and lower it again before leaving the thread. > >> > >> But, unfortunately, __wrap_pthread_setschedparam() depends on some > >> real_pthread functions to be called. One of them is > >> __real_pthread_setschedparam, and this one issues a linux syscall for > >> obvious reasons. When lowering the thread to SCHED_OTHER, this syscall > >> is still issued under the original priority. And here we get bitten by > >> the prio-inheritance feature of the nucleus which, in my case, lets > >> significant parts of standard Linux execute under high priority, > >> delaying my other real-time threads. > >> > >> Now I wonder how to resolve this, how to make pthread_setschedparam (a > >> rather central RT-service) really real-time safe? I would say we need > >> something like a lazy schedparam propagation to Linux which only takes > >> place when the thread enters secondary mode intentionally or no other RT > >> thread is ready. But I do not have a design for this at hand. Nasty. > >> > >> [My preferred way for every setup != CONFIG_PREEMPT_RT + CONFIG_XENOMAI > >> would still be to switch this prio-inheritance off for the root thread. > >> But this was nack'ed by Philippe several times before... ;)] > >> > > > > I nacked the proposal to _always_ switch it off. Some applications > > deeply need this. > > I think to remember asking for a CONFIG switch here. Some applications > actually benefit while others (I even think most) do not need it or even > easily screw themselves up during init/cleanup. You know, my old > concerns. :) > A dynamic switch is better there. You may want this behaviour to be settable on a thread-by-thread basis. > > > >> Side note: the native skin does not seem to suffer from this effect as > >> it only tracks the current prio at Xenomai level. > >> > > > > Switching off priority adjustment for the root thread before moving a > > SCHED_FIFO shadow to SCHED_OTHER would prevent this side-effect. We'd > > need to add a per-thread status bit to check whether we should run > > xnpod_renice_root() or not for any given thread, and switch it on/off > > from __wrap_pthread_setschedparam. > > > > This doesn't sound bad and would probably help low-prio threads also in > some other scenarios. > I'm currently implementing that at nucleus level. > Nevertheless, a syscall-less pthread_setschedparam would still be a good > idea as well, this time having the caller in mind who wishes to change > its priority without entering Linux. Reading the comment Gilles put there, it's likely not possible to have a syscall-less implementation on top of the NPTL. We need to give a chance to the NPTL to track the priority update, otherwise, pthread_getschedparam() is going to break. > > Jan > -- Philippe.