Re: sched_ext/for-6.11: cpu validity check in ops_cpu_valid

[Vishal Chourasia <vishalc@linux.ibm.com>]

On Tue, Jul 16, 2024 at 11:44:51AM -1000, Tejun Heo wrote:
> Hello, Vishal.
> 
> On Tue, Jul 16, 2024 at 12:19:16PM +0530, Vishal Chourasia wrote:
> ...
> > However, the case of the BPF scheduler is different; we shouldn't need
> > to handle corner cases but instead immediately flag such cases.
> 
> I'm not convinced of this. There's a tension here and I don't think either
> end of the spectrum is the right solution. Please see below.
> 
> > Consider this: if a BPF scheduler is returning a non-present CPU in
> > select_cpu, the corresponding task will get scheduled on a CPU (using
> > the fallback mechanism) that may not be the best placement, causing
> > inconsistent behavior. And there will be no red flags reported making it
> > difficult to catch. My point is that sched_ext should be much stricter
> > towards the BPF scheduler.
> 
> While flagging any deviation as failure and aborting sounds simple and clean
> on the surface, I don't think it's that clear cut. There already are edge
> conditions where ext or core scheduler code overrides sched_class decisions
> and it's not straightforward to get synchronization against e.g. CPU hotplug
> watertight from the BPF scheduler. So, we can end up with aborting a
> scheduler once in a blue moon for a condition which can only occur during
> hotplug and be easily worked around without any noticeable impact. I don't
> think that's what we want.
> 
> That's not to say that the current situation is great because, as you
> pointed out, it's possible to be systematically buggy and fly under the
> radar, although I have to say that I've never seen this particular part
> being a problem but YMMV.
> 
> Currently, error handling is binary. Either it's all okay or the scheduler
> dies, but I think things like select_cpu() returning an offline CPU likely
> needs a bit more nuance. ie. If it happens once around CPU hotplug, who
> cares? But if a scheduler is consistently returning an invalid CPU, that
> certainly is a problem and it may not be easy to notice. One way to go about
> it could be collecting stats for these events and let the BPF scheduler
> decide what to do about them.
> 
> Thanks.
> 
> -- 
> tejun
Thanks for the replies.

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