linux-kernel - Re: [PATCH 2/3] sched_ext: Introduce per-NUMA idle cpumasks

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Message-ID: <Z0-kovS-Ba9CaP9J@yury-ThinkPad>
Date: Tue, 3 Dec 2024 16:38:58 -0800
From: Yury Norov <yury.norov@...il.com>
To: Tejun Heo <tj@...nel.org>
Cc: Andrea Righi <arighi@...dia.com>, David Vernet <void@...ifault.com>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 2/3] sched_ext: Introduce per-NUMA idle cpumasks

On Tue, Dec 03, 2024 at 02:04:15PM -1000, Tejun Heo wrote:
> Hello,
> 
> On Tue, Dec 03, 2024 at 04:36:11PM +0100, Andrea Righi wrote:
> ...
> > Probably a better way to solve this issue is to introduce new kfunc's to
> > explicitly select specific per-NUMA cpumask and modify the scx
> > schedulers to transition to this new API, for example:
> > 
> >   const struct cpumask *scx_bpf_get_idle_numa_cpumask(int node)
> >   const struct cpumask *scx_bpf_get_idle_numa_smtmask(int node)
> 
> Yeah, I don't think we want to break backward compat here. Can we introduce
> a flag to switch between node-aware and flattened logic and trigger ops
> error if the wrong flavor is used? Then, we can deprecate and drop the old
> behavior after a few releases. Also, I think it can be named
> scx_bpf_get_idle_cpumask_node().
> 
> > +static struct cpumask *get_idle_cpumask(int cpu)
> > +{
> > +	int node = cpu_to_node(cpu);
> > +
> > +	return idle_masks[node]->cpu;
> > +}
> > +
> > +static struct cpumask *get_idle_smtmask(int cpu)
> > +{
> > +	int node = cpu_to_node(cpu);
> > +
> > +	return idle_masks[node]->smt;
> > +}
> 
> Hmm... why are they keyed by cpu? Wouldn't it make more sense to key them by
> node?
> 
> > +static s32 scx_pick_idle_cpu(const struct cpumask *cpus_allowed, u64 flags)
> > +{
> > +	int start = cpu_to_node(smp_processor_id());
> > +	int node, cpu;
> > +
> > +	for_each_node_state_wrap(node, N_ONLINE, start) {
> > +		/*
> > +		 * scx_pick_idle_cpu_from_node() can be expensive and redundant
> > +		 * if none of the CPUs in the NUMA node can be used (according
> > +		 * to cpus_allowed).
> > +		 *
> > +		 * Therefore, check if the NUMA node is usable in advance to
> > +		 * save some CPU cycles.
> > +		 */
> > +		if (!cpumask_intersects(cpumask_of_node(node), cpus_allowed))
> > +			continue;
> > +		cpu = scx_pick_idle_cpu_from_node(node, cpus_allowed, flags);
> > +		if (cpu >= 0)
> > +			return cpu;
> 
> This is fine for now but it'd be ideal if the iteration is in inter-node
> distance order so that each CPU radiates from local node to the furthest
> ones.

cpumask_local_spread() does exactly that - traverses CPUs in NUMA-aware
order. Or we can use for_each_numa_hop_mask() iterator, which does the
same thing more efficiently.

Thanks,
Yury