Re: [PATCH 01/10] sched/core: Drop mutex locks before proxy rescheduling

On Fri, Jul 10, 2026 at 1:47 PM Andrea Righi <[email protected]> wrote:
> Inspecting the proxy chain is not strictly required. A BPF scheduler can just
> accept a blocked donor, enqueue it on its current CPU and let the core
> proxy-exec path resolve and "virtually" migrate it as needed to the owner's CPU.
>
> The motivation for providing a scx_bpf_task_proxy_cpu() kfunc is purely for
> optimization reasons. By knowing the owner's CPU, a scheduler could make more
> informed admission decisions. For example, it can inspect the task currently
> running on the target CPU, determine whether the donor should take precedence
> over the running task and kick that CPU to trigger a preemption. Without this
> information, the scheduler can only enqueue the donor and wait until the task
> running on the owner's CPU releases it (potentially adding up to one time slice
> of latency).

So... I suspect I'm missing a subtlety of scx, but in the non-scx
case, if a blocked task is important enough to be selected on the
current cpu, the find_proxy_task() logic will walk the chain and do
the proxy-migration if needed and mark resched, so then that cpu can
evaluate which of the currently running task or the migrated donor is
important enough to run.

I worry the difficulty (with my poor understanding of the
optimization) is that it seems like its evaluting the preemption on
admission (I assume this means the point when we keep the blocked_on
task on the rq? I may be totally off base here), is that if a very
important task was breifly running at that moment, you might not allow
the blocked_on task to be enqueued. And at that point the donor is
effectively sleeping and no proxying can happen until it wakes (which
may be only when the lock owner eventually runs and releases the
lock).  In this way the optimization might make a call in that instant
that results in *many* time slices of latency (particularly if there
are lots other unimportant tasks on that cpu).

By letting the proxy logic handle the proxy migration and
rescheduling, the blocked_on tasks are just in the same pool of
selectable tasks on that cpu and the scheduler on that cpu gets to
decide what is the most important thing to run next.

Now, I can see the benefit of potentially saving the resched kick on
the target cpu when we do a proxy-migration - it is an interesting
idea I should think more on to see how we might do that better in the
core logic.

> That said, I don't see a correctness dependency on exposing this information.
> If the latency-policy use case is not strong enough to justify the API, I'm
> happy to drop the query kfuncs and this preparatory lock-scope change for now,
> the blocked-donor admission support can stand on its own.

Yeah. If that's the case I wonder if it might be worth waiting a bit
on these optimizations until after the rest of the core proxy logic
makes it upstream and settles a bit?

It might also help make sure the demand for this
optimization/interface is strong before we start adding interfaces
prematurely.

Really, I'd like it to all be transparent to the class schedulers, but
given the complexities, just having the ability for the scx bpf
schedulers to disable keeping blocked_on tasks on the rq (effectively
disabling scx proxy donation) seems like a good initial step.

Again, I am really excited about your work here! I've sadly really not
had any time since Dec to look into sched_ext details, and I've had to
tell a few folks who are interested in both sched_ext and proxy_exec
that they should turn proxy_exec off if they want to use sched_ext.
So I'm very eager to have a better answer there!

Thanks for all your great work here!
-john