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

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Date: Wed, 26 Jun 2024 08:01:30 -1000
From: Tejun Heo <tj@...nel.org>
To: Peter Zijlstra <peterz@...radead.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()

Hello,

On Wed, Jun 26, 2024 at 10:23:42AM +0200, Peter Zijlstra wrote:
...
>  - cpuset
>  - cpuset-v2
>  - isolcpus boot crap
> 
> And they're all subtly different iirc, but IIRC the cpuset ones are
> simplest since the task is part of a cgroup and the cgroup cpumask is
> imposed on them and things should be fairly straight forward.
> 
> The isolcpus thing creates a pile of single CPU partitions and people
> have to manually set cpu-affinity, and here we have some hysterical
> behaviour that I would love to change but have not yet dared do --
> because I know there's people doing dodgy things because they've been
> sending 'bug' reports.
> 
> Specifically it is possible to set a cpumask that spans multiple
> partitions :-( Traditionally the behaviour was that it would place the
> task on the lowest cpu number, the current behaviour is the task it
> placed randomly on any CPU in the given mask.

This is what I was missing. I was just thinking cpuset case and as cpuset
partitions are always reflected in the task cpumasks, there isn't whole lot
to do.

...
> > While it would
> > make sense to communicate partitions to the BPF scheduler, would it make
> > sense to reject BPF scheduler based on it? ie. Assuming that the feature is
> > implemented, what would distinguish between one BPF scheduler which handles
> > partitions specially and the other which doesn't care?
> 
> Correctness? Anyway, can't you handle this in the kernel part, simply
> never allow a shared runqueue to cross a root_domain's mask and put some
> WARNs on to ensure constraints are respected etc.? Should be fairly
> simple to check prev_cpu and new_cpu are having the same root_domain for
> instance.

Yeah, I'll plug it. It might as well be just reject and ejecting BPF
schedulers when conditions are detected. The BPF scheduler doesn't have to
use the built-in DSQs and can decide to dispatch to any CPU from its BPF
queues (however that may be implemented, it can also be in userspace), so
it's a bit tricky to enforce correctness dynamically after the fact. I'll
think more on it.

Thanks.

-- 
tejun