From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <4CD3DAC5.6000400@domain.hid> Date: Fri, 05 Nov 2010 11:21:57 +0100 From: Anders Blomdell MIME-Version: 1.0 References: <4CC82C8D.3080808@domain.hid> <4CD152F3.4080203@domain.hid> <4CD16654.6080704@domain.hid> <4CD18782.7090607@domain.hid> <4CD191EE.7000604@domain.hid> <4CD1936E.50203@domain.hid> <4CD1BA29.9000303@domain.hid> <1288816871.1842.84.camel@domain.hid> <4CD1DC1B.8060407@domain.hid> <4CD1DE12.5010309@domain.hid> <4CD1E890.5010702@domain.hid> <4CD1EC2F.4040603@domain.hid> <4CD1ED16.8030103@domain.hid> <4CD1EDA8.10007@domain.hid> <4CD1F33C.5070208@domain.hid> <4CD1F3F5.5080505@domain.hid> <4CD1F4FE.9020908@domain.hid> <4CD1F69B.9070100@domain.hid> <4CD1F906.1070703@domain.hid> <4CD1FABD.1080301@domain.hid> <4CD2612C.2070507@domain.hid> <4CD279F7.7070502@domain.hid> <4CD27C46.8010302@domain.hid> <4CD27DC2.7060607@domain.hid> <4CD2A96B.3080001@domain.hid> <4CD2B2A7.9010900@domain.hid> <4CD2C50F.1090604@domain.hid> <4CD32E76.3080004@domain.hid> <4CD33F0C.1050403@domain.hid> <4CD340AA.60002@domain.hid> <4CD34355.5020304@domain.hid> <4CD35DC7.1000507@domain.hid> In-Reply-To: <4CD35DC7.1000507@domain.hid> Content-Type: text/plain; charset=ISO-8859-1; format=flowed Content-Transfer-Encoding: 7bit Subject: Re: [Xenomai-core] Potential problem with rt_eepro100 List-Id: Xenomai life and development List-Unsubscribe: , List-Archive: List-Post: List-Help: List-Subscribe: , To: Gilles Chanteperdrix Cc: Jan Kiszka , "xenomai@xenomai.org" Gilles Chanteperdrix wrote: > Jan Kiszka wrote: >> Am 05.11.2010 00:24, Gilles Chanteperdrix wrote: >>> Jan Kiszka wrote: >>>> Am 04.11.2010 23:06, Gilles Chanteperdrix wrote: >>>>> Jan Kiszka wrote: >>>>>>>> At first sight, here you are more breaking things than cleaning them. >>>>>>> Still, it has the SMP record for my test program, still runs with ftrace >>>>>>> on (after 2 hours, where it previously failed after maximum 23 minutes). >>>>>> My version was indeed still buggy, I'm reworking it ATM. >>>>>> >>>>>>> If I get the gist of Jan's changes, they are (using the IPI to transfer >>>>>>> one bit of information: your cpu needs to reschedule): >>>>>>> >>>>>>> xnsched_set_resched: >>>>>>> - setbits((__sched__)->status, XNRESCHED); >>>>>>> >>>>>>> xnpod_schedule_handler: >>>>>>> + xnsched_set_resched(sched); >>>>>>> >>>>>>> If you (we?) decide to keep the debug checks, under what circumstances >>>>>>> would the current check trigger (in laymans language, that I'll be able >>>>>>> to understand)? >>>>>> That's actually what /me is wondering as well. I do not see yet how you >>>>>> can reliably detect a missed reschedule reliably (that was the purpose >>>>>> of the debug check) given the racy nature between signaling resched and >>>>>> processing the resched hints. >>>>> The purpose of the debugging change is to detect a change of the >>>>> scheduler state which was not followed by setting the XNRESCHED bit. >>>> But that is nucleus business, nothing skins can screw up (as long as >>>> they do not misuse APIs). >>> Yes, but it happens that we modify the nucleus from time to time. >>> >>>>> Getting it to work is relatively simple: we add a "scheduler change set >>>>> remotely" bit to the sched structure which is NOT in the status bit, set >>>>> this bit when changing a remote sched (under nklock). In the debug check >>>>> code, if the scheduler state changed, and the XNRESCHED bit is not set, >>>>> only consider this a but if this new bit is not set. All this is >>>>> compiled out if the debug is not enabled. >>>> I still see no benefit in this check. Where to you want to place the bit >>>> set? Aren't that just the same locations where >>>> xnsched_set_[self_]resched already is today? >>> Well no, that would be another bit in the sched structure which would >>> allow us to manipulate the status bits from the local cpu. That >>> supplementary bit would only be changed from a distant CPU, and serve to >>> detect the race which causes the false positive. The resched bits are >>> set on the local cpu to get xnpod_schedule to trigger a rescheduling on >>> the distance cpu. That bit would be set on the remote cpu's sched. Only >>> when debugging is enabled. >>> >>>> But maybe you can provide some motivating bug scenarios, real ones of >>>> the past or realistic ones of the future. >>> Of course. The bug is anything which changes the scheduler state but >>> does not set the XNRESCHED bit. This happened when we started the SMP >>> port. New scheduling policies would be good candidates for a revival of >>> this bug. >>> >> You don't gain any worthwhile check if you cannot make the >> instrumentation required for a stable detection simpler than the proper >> problem solution itself. And this is what I'm still skeptical of. > > The solution is simple, but finding the problem without the > instrumentation is way harder than with the instrumentation, so the > instrumentation is worth something. > > Reproducing the false positive is surprisingly easy with a simple > dual-cpu semaphore ping-pong test. So, here is the (tested) patch, > using a ridiculous long variable name to illustrate what I was > thinking about: > > diff --git a/include/nucleus/sched.h b/include/nucleus/sched.h > index 8888cf4..454b8e8 100644 > --- a/include/nucleus/sched.h > +++ b/include/nucleus/sched.h > @@ -108,6 +108,9 @@ typedef struct xnsched { > struct xnthread *gktarget; > #endif > > +#ifdef CONFIG_XENO_OPT_DEBUG_NUCLEUS > + int debug_resched_from_remote; > +#endif > } xnsched_t; > > union xnsched_policy_param; > @@ -185,6 +188,8 @@ static inline int xnsched_resched_p(struct xnsched *sched) > xnsched_t *current_sched = xnpod_current_sched(); \ > __setbits(current_sched->status, XNRESCHED); \ > if (current_sched != (__sched__)) { \ > + if (XENO_DEBUG(NUCLEUS)) \ > + __sched__->debug_resched_from_remote = 1; \ > xnarch_cpu_set(xnsched_cpu(__sched__), current_sched->resched); \ > } \ > } while (0) > diff --git a/ksrc/nucleus/pod.c b/ksrc/nucleus/pod.c > index 4cb707a..50b0f49 100644 > --- a/ksrc/nucleus/pod.c > +++ b/ksrc/nucleus/pod.c > @@ -2177,6 +2177,10 @@ static inline int __xnpod_test_resched(struct xnsched *sched) > xnarch_cpus_clear(sched->resched); > } > #endif > + if (XENO_DEBUG(NUCLEUS) && sched->debug_resched_from_remote) { > + sched->debug_resched_from_remote = 0; > + resched = 1; > + } > clrbits(sched->status, XNRESCHED); > return resched; > } > > > I am still uncertain. Will only work if all is done under nklock, otherwise two almost simultaneous xnsched_resched_p from different cpus, might lead to one of the ipi wakeups sees the 0 written due to handling the first ipi interrupt. /Anders