linux-kernel - Re: [PATCH 5/6] sched_ext: idle: Per-node idle cpumasks

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Message-ID: <Z6sddk2otmAVrfcb@gpd3>
Date: Tue, 11 Feb 2025 10:50:46 +0100
From: Andrea Righi <arighi@...dia.com>
To: Yury Norov <yury.norov@...il.com>
Cc: Tejun Heo <tj@...nel.org>, 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 5/6] sched_ext: idle: Per-node idle cpumasks

On Tue, Feb 11, 2025 at 08:41:45AM +0100, Andrea Righi wrote:
> On Tue, Feb 11, 2025 at 08:32:51AM +0100, Andrea Righi wrote:
> > On Mon, Feb 10, 2025 at 11:57:42AM -0500, Yury Norov wrote:
> > ...
> > > > > +/*
> > > > > + * Find the best idle CPU in the system, relative to @node.
> > > > > + */
> > > > > +s32 scx_pick_idle_cpu(const struct cpumask *cpus_allowed, int node, u64 flags)
> > > > > +{
> > > > > +	nodemask_t unvisited = NODE_MASK_ALL;
> > > 
> > > This should be a NODEMASK_ALLOC(). We don't want to eat up too much of the
> > > stack, right?
> > 
> > Ok, and if I want to initialize unvisited to all online nodes, is there a
> > better than doing:
> > 
> >   nodemask_clear(*unvisited);
> >   nodemask_or(*unvisited, *unvisited, node_states[N_ONLINE]);
> > 
> > We don't have nodemask_copy() right?
> 
> Sorry, and with that I mean nodes_clear() / nodes_or() / nodes_copy().

Also, it might be problematic to use NODEMASK_ALLOC() here, since we're
potentially holding raw spinlocks. Maybe we could use per-cpu nodemask_t,
but then we need to preempt_disable() the entire loop, since
scx_pick_idle_cpu() can be be called potentially from any context.

Considering that the maximum value for NODE_SHIFT is 10 with CONFIG_MAXSMP,
nodemask_t should be 128 bytes at most, that doesn't seem too bad... Maybe
we can accept to have it on the stack in this case?

Thanks,
-Andrea