linux-kernel - Re: [PATCH 1/2] sched_ext: Fix ops.dequeue() semantics

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Message-ID: <aVK1a3pnCHnMWOWY@gpd4>
Date: Mon, 29 Dec 2025 18:07:55 +0100
From: Andrea Righi <arighi@...dia.com>
To: Tejun Heo <tj@...nel.org>
Cc: David Vernet <void@...ifault.com>, Changwoo Min <changwoo@...lia.com>,
	Emil Tsalapatis <emil@...alapatis.com>,
	Daniel Hodges <hodgesd@...a.com>, sched-ext@...ts.linux.dev,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/2] sched_ext: Fix ops.dequeue() semantics

Hi Tejun,

On Sun, Dec 28, 2025 at 01:38:01PM -1000, Tejun Heo wrote:
> Hello again, again.
> 
> On Sun, Dec 28, 2025 at 01:28:04PM -1000, Tejun Heo wrote:
> ...
> > So, please ignore that part. That's non-sense. I still wonder whether we can
> > create some interlocking between scx_bpf_dsq_insert() and ops.dequeue()
> > without making hot path slower. I'll think more about it.
> 
> And we can't create an interlocking between scx_bpf_dsq_insert() and
> ops.dequeue() without adding extra atomic operations in hot paths. The only
> thing shared is task rq lock and dispatch path can't do that synchronously.
> So, yeah, it looks like the best we can do is always letting the BPF sched
> know and let it figure out locking and whether the task needs to be
> dequeued from BPF side.

How about setting a flag in deq_flags to distinguish between a "dispatch"
dequeue vs a real dequeue (due to property changes or other reasons)?

We should be able to pass this information in a reliable way without any
additional synchronization in the hot paths. This would let schedulers that
use arena data structures check the flag instead of doing their own
internal lookups.

And it would also allow us to provide both semantics:
1) Catch real dequeues that need special BPF-side actions (check the flag)
2) Track all ops.enqueue()/ops.dequeue() pairs for accounting purposes
   (ignore the flag)

Thanks,
-Andrea