linux-kernel - Re: [PATCH 6/6] sched_ext: idle: Introduce node-aware idle cpu kfunc helpers

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Message-ID: <Z6hjI3ul5E0BBtjp@gpd3>
Date: Sun, 9 Feb 2025 09:11:15 +0100
From: Andrea Righi <arighi@...dia.com>
To: Tejun Heo <tj@...nel.org>
Cc: David Vernet <void@...ifault.com>, Changwoo Min <changwoo@...lia.com>,
	Ingo Molnar <mingo@...hat.com>,
	Peter Zijlstra <peterz@...radead.org>,
	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>, Ian May <ianm@...dia.com>,
	bpf@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH 6/6] sched_ext: idle: Introduce node-aware idle cpu kfunc
 helpers

On Sat, Feb 08, 2025 at 08:31:27PM -1000, Tejun Heo wrote:
> Hello,
> 
> On Sat, Feb 08, 2025 at 10:19:38AM +0100, Andrea Righi wrote:
> ...
> > > This is contingent on scx_builtin_idle_per_node, right? It's confusing for
> > > CPU -> node mapping function to return NUMA_NO_NODE depending on an ops
> > > flag. Shouldn't this be a generic mapping function?
> > 
> > The idea is that BPF schedulers can use this kfunc to determine the right
> > idle cpumask to use, for example a typical usage could be:
> > 
> >   int node = scx_bpf_cpu_node(prev_cpu);
> >   s32 cpu = scx_bpf_pick_idle_cpu_in_node(p->cpus_ptr, node, SCX_PICK_IDLE_IN_NODE);
> > 
> > Or:
> > 
> >   int node = scx_bpf_cpu_node(prev_cpu);
> >   const struct cpumask *idle_cpumask = scx_bpf_get_idle_cpumask_node(node);
> > 
> > When SCX_OPS_BUILTIN_IDLE_PER_NODE is disabled, we need to point to the
> > global idle cpumask, that is identified by NUMA_NO_NODE, so this is why we
> > can return NUMA_NO_NODE fro scx_bpf_cpu_node().
> > 
> > Do you think we should make this more clear / document this better. Or do
> > you think we should use a different API?
> 
> I think this is too error-prone. It'd be really easy for users to assume
> that scx_bpf_cpu_node() always returns the NUMA node for the given CPU which
> can lead to really subtle surprises. Why even allow e.g.
> scx_bpf_get_idle_cpumask_node() if IDLE_PER_NODE is not enabled?

Ok, for the kfuncs I agree that we should just trigger an scx_ops_error()
if any of the scx_*_node() are used when SCX_OPS_BUILTIN_IDLE_PER_NODE is
disabled (will change this).

About scx_cpu_node(), which is used internally, I think it's convenient to
return NUMA_NO_NODE when idle-per-node is disabled, just to avoid repeating
the same check for scx_builtin_idle_per_node everywhere, and NUMA_NO_NODE
internally always means "use the global cpumask".

Do you think this is still error-prone? Or should I try to refactor the
code to get rid of this NUMA_NO_NODE == global cpumask logic?

Thanks,
-Andrea