On 10/10/2012 03:01 PM, Gilles Chanteperdrix wrote: > On 10/10/2012 02:57 PM, Philippe Gerum wrote: >> On 10/10/2012 02:55 PM, Gilles Chanteperdrix wrote: >>> On 10/10/2012 02:41 PM, Philippe Gerum wrote: >>>> On 10/10/2012 02:33 PM, Gilles Chanteperdrix wrote: >>>>> On 10/10/2012 02:09 PM, Philippe Gerum wrote: >>>>>> On 10/10/2012 01:30 PM, Gilles Chanteperdrix wrote: >>>>>>> On 10/10/2012 12:54 PM, Philippe Gerum wrote: >>>>>>> >>>>>>>> On 10/10/2012 12:25 PM, Jan Kiszka wrote: >>>>>>>>> On 2012-10-10 12:07, Gilles Chanteperdrix wrote: >>>>>>>>>> On 10/10/2012 12:04 PM, Jan Kiszka wrote: >>>>>>>>>>> On 2012-10-10 11:23, Gilles Chanteperdrix wrote: >>>>>>>>>>>> On 10/10/2012 11:01 AM, Jan Kiszka wrote: >>>>>>>>>>>>> On 2012-10-10 10:58, Gilles Chanteperdrix wrote: >>>>>>>>>>>>>> On 10/10/2012 10:10 AM, Jan Kiszka wrote: >>>>>>>>>>>>>>> On 2012-10-10 10:04, Gilles Chanteperdrix wrote: >>>>>>>>>>>>>>>> On 10/10/2012 09:56 AM, Jan Kiszka wrote: >>>>>>>>>>>>>>>>> On 2012-10-10 09:51, Gilles Chanteperdrix wrote: >>>>>>>>>>>>>>>>>> On 10/10/2012 09:38 AM, Thierry Bultel wrote: >>>>>>>>>>>>>>>>>>> Hi Gilles, >>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>> Many thanks, >>>>>>>>>>>>>>>>>>> The first patch does not work, the second does. >>>>>>>>>>>>>>>>>>> I think the reason for 1st patch why is that in rtcan_virt, >>>>>>>>>>>>>>>>>>> we have >>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>> rtdm_lock_get_irqsave(&rtcan_recv_list_lock, lock_ctx); >>>>>>>>>>>>>>>>>>> rtdm_lock_get(&rtcan_socket_lock); >>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>> ... >>>>>>>>>>>>>>>>>>> ---> rtcan_rcv(rx_dev, &skb); >>>>>>>>>>>>>>>>>>> .... >>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>> rtdm_lock_put(&rtcan_socket_lock); >>>>>>>>>>>>>>>>>>> rtdm_lock_put_irqrestore(&rtcan_recv_list_lock, lock_ctx); >>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>> and >>>>>>>>>>>>>>>>>>> rtcan_rcv->rtcan_rcv_deliver->rtdm_sem_up(&sock->recv_sem); >>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>> thus the same re-scheduling stuff with interrupts locked. >>>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>>> Are you not not afraid of side effects with the second >>>>>>>>>>>>>>>>>>> patch, >>>>>>>>>>>>>>>>>>> since you change the overall behaviour ? >>>>>>>>>>>>>>>>>>> Won't you prefer a only locally modified rtcan_virt ? >>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>>> We should ask Jan's opinion. In any case, if we adopt the >>>>>>>>>>>>>>>>>> second patch, >>>>>>>>>>>>>>>>>> we might want to try and reduce the overhead of >>>>>>>>>>>>>>>>>> xnpod_unlock_sched. >>>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>>> We were signaling the semaphore while holding a spin lock? >>>>>>>>>>>>>>>>> That's a >>>>>>>>>>>>>>>>> clear bug. Your patch is aligning rtcan to the pattern we are >>>>>>>>>>>>>>>>> also using >>>>>>>>>>>>>>>>> in RTnet. We just need to make sure (haven't looked at the >>>>>>>>>>>>>>>>> full context >>>>>>>>>>>>>>>>> yet) that sock remains valid even after dropping the lock(s). >>>>>>>>>>>>>>>> >>>>>>>>>>>>>>>> The second patch idea was to lock the scheduler while >>>>>>>>>>>>>>>> spinlocks are >>>>>>>>>>>>>>>> held, so that posting a semaphore while holding a spin lock is >>>>>>>>>>>>>>>> no longer >>>>>>>>>>>>>>>> a bug. >>>>>>>>>>>>>>> >>>>>>>>>>>>>>> Sounds a bit hacky, >>>>>>>>>>>>>> >>>>>>>>>>>>>> Well, that is what the linux kernel does. >>>>>>>>>>>>>> >>>>>>>>>>>>>> but I think we have this pattern >>>>>>>>>>>>>>> (RTDM_EXECUTE_ATOMICALLY) >>>>>>>>>>>>>> >>>>>>>>>>>>>> RTDM_EXECUTE_ATOMICALLY is a bit of a misnomer, if you do: >>>>>>>>>>>>>> RTDM_EXECUTE_ATOMICALLY(foo(); rtdm_sem_up(); bar()); >>>>>>>>>>>>>> foo() and bar() are not executed atomically if sem_up wakes up >>>>>>>>>>>>>> another >>>>>>>>>>>>>> thread. >>>>>>>>>>>>>> >>>>>>>>>>>>>> So, I do not see how RTDM_EXECUTE_ATOMICALLY solves the issue we >>>>>>>>>>>>>> are >>>>>>>>>>>>>> talking about. >>>>>>>>>>>>> >>>>>>>>>>>>> RTDM_EXECUTE_ATOMICALLY holds the nucleus lock across the >>>>>>>>>>>>> encapsulated >>>>>>>>>>>>> code, executing it atomically as rescheduling is postponed until >>>>>>>>>>>>> the end >>>>>>>>>>>>> of the block. >>>>>>>>>>>> >>>>>>>>>>>> Err... no. Absolutely not. >>>>>>>>>>> >>>>>>>>>>> Err... absolutely right. >>>>>>>>>>> >>>>>>>>>>> The good news is: we don't need to worry about such kind of >>>>>>>>>>> locking. In >>>>>>>>>>> rtcan_raw_recvmsg, the socket is locked via the RTDM context as we >>>>>>>>>>> are >>>>>>>>>>> in a handler. So it won't disappear when we drop the lock, and your >>>>>>>>>>> first patch is fine. >>>>>>>>>> >>>>>>>>>> Which one? The first one does not seem to work because the rtdm locks >>>>>>>>>> seem to be nested. The second one would probably need to find a way >>>>>>>>>> to >>>>>>>>>> reduce the overhead of xnpod_unlock_sched(). What can be done, >>>>>>>>>> however, >>>>>>>>>> is adding a call to xnpod_lock_sched()/xnpod_unlock_sched() in >>>>>>>>>> RTDM_EXECUTE_ATOMICALLY. >>>>>>>>> >>>>>>>>> Oh, I'm seeing the locking forest in rtcan now. I suppose rtcan wasn't >>>>>>>>> used much on SMP so far. That looks indeed unresolvable without a >>>>>>>>> semantical change to rtdm_lock/unlock. >>>>>>>>> >>>>>>>>> But then we really need something as light-weight as >>>>>>>>> preempt_enable/disable. >>>>>>>>> >>>>>>>> This is not as lightweight as it might be given that we pair a flag and >>>>>>>> a counter to achieve this (which saves one data reference in >>>>>>>> xnpod_schedule() though), but this is a start: >>>>>>>> >>>>>>>> http://git.xenomai.org/?p=xenomai-2.6.git;a=commit;h=aed4dfce9967e45ef7e8a8da4b6c90267ea81497 >>>>>>>> >>>>>>>> So, I'm setting __xnpod_lock_sched() and __xnpod_unlock_sched() in >>>>>>>> stone >>>>>>>> in the nucleus API to manipulate the sched locking counter from a >>>>>>>> context where the nucleus lock is already held, so that RTDM can rely >>>>>>>> on >>>>>>>> this for RTDM_EXECUTE_ATOMICALLY(). >>>>>>> >>>>>>> >>>>>>> The problem of xnpod_unlock_sched from my point of view is this section >>>>>>> of code: >>>>>>> >>>>>>> xnsched_set_self_resched(curr->sched); >>>>>>> xnpod_schedule(); >>>>>>> >>>>>>> It means that we will go to the full blown __xnpod_schedule when >>>>>>> unlocking the top-most spinlock. >>>>>>> >>>>>>> I guess what I meant is that we should have a scheduler bit that is >>>>>>> simply tested in xnpod_schedule, but then we loose the ability for the >>>>>>> threads with the scheduler locked to suspend themselves. So, maybe we >>>>>>> should define another xnpod_lock/unlock pair, maybe something like >>>>>>> xnpod_preempt_disablle()/xnpod_preempt_enabled(). >>>>>>> >>>>>>> >>>>>> >>>>>> xnpod_lock_sched() really means xnpod_preempt_disable() in RTOS parlance. >>>>> >>>>> Yes, but the reason why we have to go to __xnpod_schedule in >>>>> xnpod_unlock_sched() is to allow thread to be suspended with the >>>>> scheduler locked (you added that at some point for the vxworks emulator, >>>>> if I remember correctly). But without that need, we can put the XNLOCK >>>>> bit in the sched->status structure, and simply test that bit with all >>>>> the others in xnpod_schedule(), and forget about the need to call >>>>> xnsched_set_self_resched() in xnpod_unlock_sched(). That is all I meant. >>>>> >>>> >>>> You may have to reschedule after an IRQ has interrupted the lock owner >>>> inside the locked section, in which case you have to make sure to >>>> reschedule when dropping the sched lock. >>> >>> Well, in that case the resched bit will have been set already by the irq >>> handler calling xnpod_resume_thread, or other service, you do not have >>> to force it. >>> >> >> Yes, and this is why you have to call __xnpod_schedule() regardless. >> > no, xnpod_schedule is enough, it will only call __xnpod_schedule() if > the resched bit is set. >
We are not discussing about the same issue, I'm afraid. We can't optimize the schedlock path via a scheduler flag since we have to care about lock nesting. Since the only sane option there is to hold such counter in the current thread context, the optimization is 100% in the way we access this information. -- Philippe. _______________________________________________ Xenomai mailing list [email protected] http://www.xenomai.org/mailman/listinfo/xenomai
