Philippe Gerum wrote: > On Thu, 2006-06-29 at 12:48 +0200, Philippe Gerum wrote: >> 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. >> > > Actually, we could have both, dynamic and static switches, just like the > interrupt shield. >
Hurray! (Otherwise, I would have kicked off a discussion about the default setting ;-)) >>>>> 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. >> Yep, saw that. I would start thinking about what we gain by wrapping that service as well (e.g. by mirroring the state in a per-__thread variable). Would likely work as long as the thread stays in primary mode, but we still need to propagate the setting when entering secondary mode. Maybe some soft-IRQ to Linux can help here. It could apply the scheduler change on Linux re-entry. Well, I'm probably overseeing tons of pitfalls, or all this becomes terribly complicated. Jan
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