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

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Message-ID: <Z1AVx-yUY_37uMCb@gpd3>
Date: Wed, 4 Dec 2024 09:41:43 +0100
From: Andrea Righi <arighi@...dia.com>
To: Tejun Heo <tj@...nel.org>
Cc: David Vernet <void@...ifault.com>, Yury Norov <yury.norov@...il.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:
> External email: Use caution opening links or attachments
> 
> 
> 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().

I like the idea of introducing a flag. The default should be flattened
cpumask, so everything remains the same, and if a scheduler explicitly
enables SCX_OPS_NUMA_IDLE_MASK (suggestions for the name?) we can switch
to the NUMA-aware idle logic.

> 
> > +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?

I was trying to save some code, but it's definitely more clear to use
node as key and rename those get_idle_cpumask_node() /
get_idle_smtmask_node(). Will change this.

> 
> > +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.

Ok.

Thanks,
-Andrea