From mboxrd@z Thu Jan 1 00:00:00 1970 Message-ID: <4CD2C8E2.9090608@domain.hid> Date: Thu, 04 Nov 2010 15:53:22 +0100 From: Anders Blomdell MIME-Version: 1.0 References: <4CC82C8D.3080808@domain.hid> <4CD13A70.8040702@domain.hid> <4CD14B1E.4000707@domain.hid> <4CD14C92.90901@domain.hid> <4CD14DBC.3060505@domain.hid> <4CD1509A.3000908@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> In-Reply-To: <4CD2C50F.1090604@domain.hid> Content-Type: text/plain; charset=UTF-8; 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: Jan Kiszka Cc: "xenomai@xenomai.org" Jan Kiszka wrote: > Am 04.11.2010 14:18, Anders Blomdell wrote: >> Gilles Chanteperdrix wrote: >>> Jan Kiszka wrote: >>>> Am 04.11.2010 10:26, Jan Kiszka wrote: >>>>> Am 04.11.2010 10:16, Gilles Chanteperdrix wrote: >>>>>> Jan Kiszka wrote: >>>>>>> Take a step back and look at the root cause for this issue again. Unlocked >>>>>>> >>>>>>> if need-resched >>>>>>> __xnpod_schedule >>>>>>> >>>>>>> is inherently racy and will always be (not only for the remote >>>>>>> reschedule case BTW). >>>>>> Ok, let us examine what may happen with this code if we only set the >>>>>> XNRESCHED bit on the local cpu. First, other bits than XNRESCHED do not >>>>>> matter, because they can not change under our feet. So, we have two >>>>>> cases for this race: >>>>>> 1- we see the XNRESCHED bit, but it has been cleared once nklock is >>>>>> locked in __xnpod_schedule. >>>>>> 2- we do not see the XNRESCHED bit, but it get set right after we test it. >>>>>> >>>>>> 1 is not a problem. >>>>> Yes, as long as we remove the debug check from the scheduler code (or >>>>> fix it somehow). The scheduling code already catches this race. >>>>> >>>>>> 2 is not a problem, because anything which sets the XNRESCHED (it may >>>>>> only be an interrupt in fact) bit will cause xnpod_schedule to be called >>>>>> right after that. >>>>>> >>>>>> So no, no race here provided that we only set the XNRESCHED bit on the >>>>>> local cpu. >>>>>> >>>>>> So we either have to accept this and remove the >>>>>>> debugging check from the scheduler or push the check back to >>>>>>> __xnpod_schedule where it once came from. When this it cleaned up, we >>>>>>> can look into the remote resched protocol again. >>>>>> The problem of the debug check is that it checks whether the scheduler >>>>>> state is modified without the XNRESCHED bit being set. And this is the >>>>>> problem, because yes, in that case, we have a race: the scheduler state >>>>>> may be modified before the XNRESCHED bit is set by an IPI. >>>>>> >>>>>> If we want to fix the debug check, we have to have a special bit, on in >>>>>> the sched->status flag, only for the purpose of debugging. Or remove the >>>>>> debug check. >>>>> Exactly my point. Is there any benefit in keeping the debug check? The >>>>> code to make it work may end up as "complex" as the logic it verifies, >>>>> at least that's my current feeling. >>>>> >>>> This would be the radical approach of removing the check (and cleaning >>>> up some bits). If it's acceptable, I would split it up properly. >>> This debug check saved our asses when debugging SMP issues, and I >>> suspect it may help debugging skin issues. So, I think we should try and >>> keep it. >>> >>> >>> 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. Any reason why the two changes below would fail (I need to get things working real soon now). > >> 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 only thing I can think of are atomic set/clear on an independent variable. /Anders