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Message-ID: <aLoH_5TfiTGgQsb0@slm.duckdns.org>
Date: Thu, 4 Sep 2025 11:43:27 -1000
From: Tejun Heo <tj@...nel.org>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Andrea Righi <arighi@...dia.com>, Ingo Molnar <mingo@...hat.com>,
Juri Lelli <juri.lelli@...hat.com>,
Vincent Guittot <vincent.guittot@...aro.org>,
Dietmar Eggemann <dietmar.eggemann@....com>,
Steven Rostedt <rostedt@...dmis.org>,
Ben Segall <bsegall@...gle.com>, Mel Gorman <mgorman@...e.de>,
Valentin Schneider <vschneid@...hat.com>,
Joel Fernandes <joelagnelf@...dia.com>,
David Vernet <void@...ifault.com>,
Changwoo Min <changwoo@...lia.com>, Shuah Khan <shuah@...nel.org>,
sched-ext@...ts.linux.dev, bpf@...r.kernel.org,
linux-kernel@...r.kernel.org,
Luigi De Matteis <ldematteis123@...il.com>
Subject: Re: [PATCH 07/16] sched_ext: Add a DL server for sched_ext tasks
Hello, Peter.
On Thu, Sep 04, 2025 at 10:28:58PM +0200, Peter Zijlstra wrote:
...
> RUNNABLE:
> 1) hold both source and target rq->lock.
...
> Now, assuming you have a locking order like:
>
> p->pi_lock
> rq->lock
> dsq->lock
>
> When you do something like:
>
> __schedule()
> raw_spin_lock(rq->lock);
> next = pick_next_task() -> pick_task_scx()
> raw_spin_lock(dsq->lock);
>
> Then you are, in effect, in the RUNNABLE 1) case above. You hold both
> locks. Nothing is going to move your task around while you hold that
> dsq->lock. That task is on the dsq, anybody else wanting to also do
> anything with that task, will have to first take dsq->lock.
>
> Therefore, at this point, it is perfectly fine to do:
>
> set_task_cpu(cpu_of(rq)); // move task here
>
> There is no actual concurrency. The only thing there is is
> set_task_cpu() complaining you're not following the rules -- but you
> are, it just doesn't know -- and we can fix that.
I can't convince myself this is safe. For example, when task_rq_lock()
returns, it should guarantee that the rq that the task is currently
associated with is locked and the task can't go anywhere. However, as
task_rq_lock() isn't interlocked with dsq lock, this won't hold true. I
think this will break multiple things subtly - e.g. the assumptions that
task_call_func() makes in the comment wouldn't hold anymore,
task_sched_runtime()'s test of task_on_rq_queued() would be racy, and so on.
ie. Operations protected by deq/enq pair would be fine but anything which is
protected only by task_rq_lock/unlock() would become racy, right?
Thanks.
--
tejun
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