linux-kernel - Re: [PATCH 09/39] sched: Add @reason to sched_class->rq

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Date: Tue, 25 Jun 2024 10:29:26 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: Tejun Heo <tj@...nel.org>
Cc: torvalds@...ux-foundation.org, mingo@...hat.com, juri.lelli@...hat.com,
	vincent.guittot@...aro.org, dietmar.eggemann@....com,
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	andrea.righi@...onical.com, joel@...lfernandes.org,
	linux-kernel@...r.kernel.org, bpf@...r.kernel.org,
	kernel-team@...a.com
Subject: Re: [PATCH 09/39] sched: Add @reason to
 sched_class->rq_{on|off}line()

On Mon, Jun 24, 2024 at 11:18:06AM -1000, Tejun Heo wrote:
> Hello, Peter.
> 
> On Mon, Jun 24, 2024 at 01:32:12PM +0200, Peter Zijlstra wrote:
> > On Wed, May 01, 2024 at 05:09:44AM -1000, Tejun Heo wrote:
> > > ->rq_{on|off}line are called either during CPU hotplug or cpuset partition
> > > updates. A planned BPF extensible sched_class wants to tell the BPF
> > > scheduler progs about CPU hotplug events in a way that's synchronized with
> > > rq state changes.
> > > 
> > > As the BPF scheduler progs aren't necessarily affected by cpuset partition
> > > updates, we need a way to distinguish the two types of events. Let's add an
> > > argument to tell them apart.
> > 
> > That would be a bug. Must not be able to ignore partitions.
> 
> So, first of all, this implementation was brittle in assuming CPU hotplug
> events would be called in first and broke after recent cpuset changes. In
> v7, it's replaced by hooks in sched_cpu_[de]activate(), which has the extra
> benefit of allowing the BPF hotplug methods to be sleepable.

Urgh, I suppose I should go stare at v7 then.

> Taking a step back to the sched domains. They don't translate well to
> sched_ext schedulers where task to CPU associations are often more dynamic
> (e.g. multiple CPUs sharing a task queue) and load balancing operations can
> be implemented pretty differently from CFS. The benefits of exposing sched
> domains directly to the BPF schedulers is unclear as most of relevant
> information can be obtained from userspace already.

Either which way around you want to turn it, you must not violate
partitions. If a bpf thing isn't capable of handling partitions, you
must refuse loading it when a partition exists and equally disallow
creation of partitions when it does load.

For partitions specifically, you only need the root_domain, not the full
sched_domain trees.

I'm aware you have these shared runqueues, but you don't *have* to do
that. Esp. so if the user explicitly requested partitions.