Re: [PATCH 14/31] sched_ext: Implement BPF extensible scheduler class

[Tejun Heo]

Hello, Barret.

On Fri, Dec 02, 2022 at 08:01:37AM -1000, Tejun Heo wrote:
> > you still end up grabbing both locks, but just not at the same time.
> 
> Yeah, this probably would look better than the current double lock dancing,
> especially in the finish_dispatch() path.
>
> > plus, task_rq_lock() takes the guesswork out of whether you're getting p's
> > rq lock or not.  it looks like you're using the holding_cpu to handle the
> > race where p moves cpus after you read task_rq(p) but before you lock that
> > task_rq.  maybe you can drop the whole concept of the holding_cpu?
> 
> ->holding_cpu is there to basically detect intervening dequeues, so if we
> lock them out with TASK_ON_RQ_MIGRATING, we might be able to drop it. I need
> to look into it more tho. Things get pretty subtle around there, so I could
> easily be missing something. I'll try this and let you know how it goes.

I tried both and I'm pretty ambivalent. The problem is that the
finish_dispatch() path can't use TASK_ON_RQ_MIGRATING the way the consume
path does because the dispatch path isn't starting with the task locked. The
only claim it has to the task is through p->scx.ops_state.

It can be argued that getting rid of double locking is still nice but given
that holding_cpu is needed anyway and can play the same role, I'm not sure
how attractive it is. I suppose we can go either way.

Thanks.

-- 
tejun