Messages in this thread |  | | Date | Mon, 3 Feb 2020 17:17:46 +0000 | From | Qais Yousef <> | Subject | Re: [PATCH v2] sched: rt: Make RT capacity aware |
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On 02/03/20 11:14, Steven Rostedt wrote: > On Mon, 3 Feb 2020 14:27:14 +0000 > Qais Yousef <qais.yousef@arm.com> wrote: > > > I don't see one right answer here. The current mechanism could certainly do > > better; but it's not clear what better means without delving into system > > specific details. I am open to any suggestions to improve it. > > The way I see this is that if there's no big cores available but little > cores are, and the RT task has those cores in its affinity mask then > the task most definitely should consider moving to the little core. The > cpu_find() should return them!
I almost agree. I think the cpupri_find() could certainly do better if the task is already running on a little core. It can fallback to the next best little core if no bigger core is available.
I already started looking at pushing a patch to do that.
I'm torn about pushing a task already on a big core to a little core if it says it wants it (down migration).
I guess since most tasks are fifo by default then one will starve if the other one is a long running task (assuming same priority). But long running RT tasks are not the common case, hence I wanted to hear about what use case this logic hurts. I expect by default the big cores not to be over subscribed. Based on some profiles I did at least running real Android apps I didn't see the RT tasks were overwhelming the system.
In my view, the performance dip of sharing the big core would be less than migrating the task to a little core momentarily then bring it back in to the big core.
Because the following 2 big ifs must be satisfied first to starve an RT task:
1. We need all the big cores to be overloaded first. 2. The RT tasks on all the big cores are CPU hoggers (however we want to define this)
And I think this needs more investigation.
> > But, what we can do is to mark the little core that's running an RT > task on a it that prefers bigger cores, as "rt-overloaded". This will > add this core into the being looked at when another core schedules out > an RT task. When that happens, the RT task on the little core will get > pulled back to the big core.
I didn't think of using the rt-overloaded flag in this way. That would be interesting to try.
> > Here's what I propose. > > 1. Scheduling of an RT task that wants big cores, but has little cores > in its affinity. > > 2. Calls cpu_find() which will look to place it first on a big core, if > there's a core that is running a task that is lower priority than > itself. > > 3. If all the big cores have RT tasks it can not preempt, look to find > a little core.
I agree with the above.
> > 4. If a little core is returned, and we schedule an RT task that > prefers big cores on it, we mark it overloaded. > > 5. An RT task on a big core schedules out. Start looking at the RT > overloaded run queues. > > 6. See that there's an RT task on the little core, and migrate it over.
I think the above should depend on the fitness of the cpu we currently run on. I think we shouldn't down migrate, or at least investigate better down migration makes more sense than keeping tasks running on the correct CPU where they are.
> Note, this will require a bit more logic as the overloaded code wasn't > designed for migration of running tasks, but that could be added.
I'm wary of overloading the meaning of rt.overloaded. Maybe I can convert it to a bitmap so that we can encode the reason.
Let me see how complicated to write something up.
Thanks!
-- Qais Yousef
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