linux-kernel - Re: [PATCH v4] sched_ext: Trigger ops.update_idle() from pick_task

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Message-ID: <Zw7o0ot9Utomq9qa@gpd3>
Date: Wed, 16 Oct 2024 00:12:34 +0200
From: Andrea Righi <andrea.righi@...ux.dev>
To: Tejun Heo <tj@...nel.org>
Cc: David Vernet <void@...ifault.com>,
	Peter Zijlstra <peterz@...radead.org>,
	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>,
	linux-kernel@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [PATCH v4] sched_ext: Trigger ops.update_idle() from
 pick_task_idle()

On Tue, Oct 15, 2024 at 05:31:09PM +0200, Andrea Righi wrote:
...
> > - For the purpose of determining whether a CPU is idle for e.g. task
> >   placement from ops.select_cpu(). The CPU *should* be considered idle in
> >   this polling state.
> > 
> > Overall, it feels a bit contrived to generate update_idle() events
> > consecutively for this. If a scheduler wants to poll in idle state, can't it
> > do something like the following?
> > 
> > - Trigger kick from update_idle(cpu, true) and remember that the CPU is in
> >   the polling state.
> > 
> > - Keep kicking from ops.dispatch() until polling state is cleared.
> > 
> > As what kick() guarnatees is at least one dispatch event after kicking, this
> > is guaranteed to be correct and the control flow, while a bit more
> > complicated, makes sense - it triggers dispatch on idle transition and keeps
> > dispatching in the idle state.
> > 
> > What do you think?
> 
> That seems to work in theory, I'll run some tests to confirm that it
> also works in practice. :)
> 
> It looks definitely nicer than triggering multiple ops.update_idle()
> from the kernel and we can maintain the proper semantic of triggering
> update_idle() only on actual idle state changes.

For the record, this seems to be working for now. Here is the
implementation of the idea proposed by Tejun:
https://github.com/sched-ext/scx/commit/40ccca8cbef8fc73e16bfb789c7565326b3cca62

Therefore, we can ignore the kernel patch.

Thanks for the help on this!
-Andrea