Messages in this thread | | | Date | Tue, 24 Apr 2012 09:21:21 +0200 | From | Juri Lelli <> | Subject | Re: [PATCH 05/16] sched: SCHED_DEADLINE policy implementation. |
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On 04/24/2012 12:13 AM, Tommaso Cucinotta wrote: > Il 23/04/2012 17:41, Juri Lelli ha scritto: >> The user could call __setparam_dl on a throttled task through >> __sched_setscheduler. > > in case it can be related: a scenario that used to break isolation > (in the old aquosa crap): 1) create a deadline task 2) (actively) > wait till it's just about to be throttled 3) remove reservation > (i.e., return the task to the normal system policy and destroy > reservation info in the kernel) 4) reserve it again >
Yes, this is very similar to what I thought just after I've sent the email (ouch! :-)).
> Assuming the borderline condition of a nearly fully saturated system, > if 3)-4) manage to happen sufficiently close to each other and right > after 2), now the task budget is refilled with a deadline which is > where it should not be, according to the admission control rules. In > other words, we may break guarantees of other tasks by a properly > misbehaving task. Something relevant when considering misbehaviour > and admission control from a security perspective [1]. >
Thanks for the ref., I'll read it!
> At that time, I was persuaded that the right way to avoid this would > be to avoid to free system cpu bw immediately when a reservation is > destroyed, but rather wait till its current abs deadline, then "free" > the bandwidth. A new task trying to re-create the reservation too > early, i.e., at step 4) above, would be rejected by the system as it > would still see a fully occupied cpu bw. Never implemented of course > :-)... >
A kind of "two steps" approach. It would work, I just have to think how to implement it (and let the system survive ;-)). Then create some bench to test it.
> And also, from a security perspective, a misbehaving (sched_other) > task might thrash the system with useless nansosleeps forcing the OS > to continuously schedule/deschedule it. Equivalently, with a deadline > scheduler, you could try to set a very small period/deadline. That's > why in [1], among the configurable variables, there was a minimum > allowed reservation period. >
Yes, this should be easily controlled at admission time.
> Nothing really urgent, just something you might want to keep in mind > for the future, I thought. >
Well, depends on how much effort will this turn to require. I personally would prefer to be able to come out with a new release ASAP. Just to continue the discussion with the most of the comments addressed and a more updated code (I also have a mainline version of the patchset quite ready).
Thanks a lot,
- Juri
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