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

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Message-ID: <Z6caE43btbZZthiw@gpd3>
Date: Sat, 8 Feb 2025 09:47:15 +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 5/6] sched_ext: idle: Per-node idle cpumasks

On Fri, Feb 07, 2025 at 12:30:37PM -1000, Tejun Heo wrote:
> Hello,
> 
> On Fri, Feb 07, 2025 at 09:40:52PM +0100, Andrea Righi wrote:
> > +/*
> > + * cpumasks to track idle CPUs within each NUMA node.
> > + *
> > + * If SCX_OPS_BUILTIN_IDLE_PER_NODE is not enabled, a single global cpumask
> > + * from is used to track all the idle CPUs in the system.
> > + */
> > +struct idle_cpus {
> >  	cpumask_var_t cpu;
> >  	cpumask_var_t smt;
> > -} idle_masks CL_ALIGNED_IF_ONSTACK;
> > +};
> 
> Can you prefix the type name with scx_?
> 
> Unrelated to this series but I wonder whether we can replace "smt" with
> "core" in the future to become more consistent with how the terms are used
> in the kernel:
> 
>   struct scx_idle_masks {
>           cpumask_var_t   cpus;
>           cpumask_var_t   cores;
>   };
> 
> We expose "smt" name through kfuncs but we can rename that to "core" through
> compat macros later too.
> 
> > +/*
> > + * 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;
> > +	s32 cpu = -EBUSY;
> > +
> > +	if (!static_branch_maybe(CONFIG_NUMA, &scx_builtin_idle_per_node))
> > +		return pick_idle_cpu_from_node(cpus_allowed, NUMA_NO_NODE, flags);
> > +
> > +	/*
> > +	 * If an initial node is not specified, start with the current
> > +	 * node.
> > +	 */
> > +	if (node == NUMA_NO_NODE)
> > +		node = numa_node_id();
> > +
> > +	/*
> > +	 * Traverse all nodes in order of increasing distance, starting
> > +	 * from @node.
> > +	 *
> > +	 * This loop is O(N^2), with N being the amount of NUMA nodes,
> > +	 * which might be quite expensive in large NUMA systems. However,
> > +	 * this complexity comes into play only when a scheduler enables
> > +	 * SCX_OPS_BUILTIN_IDLE_PER_NODE and it's requesting an idle CPU
> > +	 * without specifying a target NUMA node, so it shouldn't be a
> > +	 * bottleneck is most cases.
> > +	 *
> > +	 * As a future optimization we may want to cache the list of hop
> > +	 * nodes in a per-node array, instead of actually traversing them
> > +	 * every time.
> > +	 */
> > +	for_each_numa_node(node, unvisited, N_POSSIBLE) {
> > +		cpu = pick_idle_cpu_from_node(cpus_allowed, node, flags);
> 
> Maybe rename pick_idle_cpu_in_node() to stay in sync with
> SCX_PICK_IDLE_IN_NODE? It's not like pick_idle_cpu_from_node() walks from
> the node, right? It just picks within the node.
> 
> > @@ -460,38 +582,50 @@ s32 scx_select_cpu_dfl(struct task_struct *p, s32 prev_cpu, u64 wake_flags, bool
> >  
> >  void scx_idle_reset_masks(void)
> >  {
> > +	int node;
> > +
> > +	if (!static_branch_maybe(CONFIG_NUMA, &scx_builtin_idle_per_node)) {
> > +		cpumask_copy(idle_cpumask(NUMA_NO_NODE)->cpu, cpu_online_mask);
> > +		cpumask_copy(idle_cpumask(NUMA_NO_NODE)->smt, cpu_online_mask);
> > +		return;
> > +	}
> > +
> >  	/*
> >  	 * Consider all online cpus idle. Should converge to the actual state
> >  	 * quickly.
> >  	 */
> > -	cpumask_copy(idle_masks.cpu, cpu_online_mask);
> > -	cpumask_copy(idle_masks.smt, cpu_online_mask);
> > -}
> > +	for_each_node(node) {
> > +		const struct cpumask *node_mask = cpumask_of_node(node);
> > +		struct cpumask *idle_cpus = idle_cpumask(node)->cpu;
> > +		struct cpumask *idle_smts = idle_cpumask(node)->smt;
> > -void scx_idle_init_masks(void)
> > -{
> > -	BUG_ON(!alloc_cpumask_var(&idle_masks.cpu, GFP_KERNEL));
> > -	BUG_ON(!alloc_cpumask_var(&idle_masks.smt, GFP_KERNEL));
> > +		cpumask_and(idle_cpus, cpu_online_mask, node_mask);
> > +		cpumask_copy(idle_smts, idle_cpus);
> > +	}
> 
> nitpick: Maybe something like the following is more symmetric with the
> global case and easier to read?
> 
>   for_each_node(node) {
>         const struct cpumask *node_mask = cpumask_of_node(node);
>         cpumask_and(idle_cpumask(node)->cpu, cpu_online_mask, node_mask);
>         cpumask_and(idle_cpumask(node)->smt, cpu_online_mask, node_mask);
>   }
> 
> >  }
> >  
> >  static void update_builtin_idle(int cpu, bool idle)
> >  {
> > -	assign_cpu(cpu, idle_masks.cpu, idle);
> > +	int node = idle_cpu_to_node(cpu);

Ok to all of the above.

> 
> minor: I wonder whether idle_cpu_to_node() name is a bit confusing - why
> does a CPU being idle have anything to do with its node mapping? If there is
> a better naming convention, great. If not, it is what it is.

Maybe scx_cpu_to_node()? At the end it's just a wrapper to cpu_to_node(),
but from the scx perspective, so if NUMA-awareness is not enabled in scx,
it'd return NUMA_NO_NODE.

Thanks,
-Andrea