linux-kernel - Re: [PATCH 09/10] sched_ext: idle: Get rid of the scx_selcpu_topo

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Message-ID: <Z2p3sSZJRCIfS9jA@gpd3>
Date: Tue, 24 Dec 2024 09:58:25 +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>, bpf@...r.kernel.org,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 09/10] sched_ext: idle: Get rid of the
 scx_selcpu_topo_numa logic

On Mon, Dec 23, 2024 at 03:39:56PM -0800, Yury Norov wrote:
> On Fri, Dec 20, 2024 at 04:11:41PM +0100, Andrea Righi wrote:
> > With the introduction of separate per-NUMA node cpumasks, we
> > automatically track idle CPUs within each NUMA node.
> > 
> > This makes the special logic for determining idle CPUs in each NUMA node
> > redundant and unnecessary, so we can get rid of it.
> 
> But it looks like you do more than that... 
> 
> > Signed-off-by: Andrea Righi <arighi@...dia.com>
> > ---
> >  kernel/sched/ext_idle.c | 93 ++++++++++-------------------------------
> >  1 file changed, 23 insertions(+), 70 deletions(-)
> > 
> > diff --git a/kernel/sched/ext_idle.c b/kernel/sched/ext_idle.c
> > index 013deaa08f12..b36e93da1b75 100644
> > --- a/kernel/sched/ext_idle.c
> > +++ b/kernel/sched/ext_idle.c
> > @@ -82,7 +82,6 @@ static void idle_masks_init(void)
> >  }
> >  
> >  static DEFINE_STATIC_KEY_FALSE(scx_selcpu_topo_llc);
> > -static DEFINE_STATIC_KEY_FALSE(scx_selcpu_topo_numa);
> >  
> >  /*
> >   * Return the node id associated to a target idle CPU (used to determine
> > @@ -259,25 +258,6 @@ static unsigned int numa_weight(s32 cpu)
> >  	return sg->group_weight;
> >  }
> >  
> > -/*
> > - * Return the cpumask representing the NUMA domain of @cpu (or NULL if the NUMA
> > - * domain is not defined).
> > - */
> > -static struct cpumask *numa_span(s32 cpu)
> > -{
> > -	struct sched_domain *sd;
> > -	struct sched_group *sg;
> > -
> > -	sd = rcu_dereference(per_cpu(sd_numa, cpu));
> > -	if (!sd)
> > -		return NULL;
> > -	sg = sd->groups;
> > -	if (!sg)
> > -		return NULL;
> > -
> > -	return sched_group_span(sg);
> 
> I didn't find llc_span() and node_span() in vanilla kernel. Does this series
> have prerequisites?

This patch set is based on the sched_ext/for-6.14 branch:
https://git.kernel.org/pub/scm/linux/kernel/git/tj/sched_ext.git/

I put sched_ext/for-6.14 in the cover email, maybe it wasn't very clear.
I should have mentioned the git repo in the email.

> 
> > -}
> > -
> >  /*
> >   * Return true if the LLC domains do not perfectly overlap with the NUMA
> >   * domains, false otherwise.
> > @@ -329,7 +309,7 @@ static bool llc_numa_mismatch(void)
> >   */
> >  static void update_selcpu_topology(struct sched_ext_ops *ops)
> >  {
> > -	bool enable_llc = false, enable_numa = false;
> > +	bool enable_llc = false;
> >  	unsigned int nr_cpus;
> >  	s32 cpu = cpumask_first(cpu_online_mask);
> >  
> > @@ -348,41 +328,34 @@ static void update_selcpu_topology(struct sched_ext_ops *ops)
> >  	if (nr_cpus > 0) {
> >  		if (nr_cpus < num_online_cpus())
> >  			enable_llc = true;
> > +		/*
> > +		 * No need to enable LLC optimization if the LLC domains are
> > +		 * perfectly overlapping with the NUMA domains when per-node
> > +		 * cpumasks are enabled.
> > +		 */
> > +		if ((ops->flags & SCX_OPS_BUILTIN_IDLE_PER_NODE) &&
> > +		    !llc_numa_mismatch())
> > +			enable_llc = false;
> 
> This doesn't sound like redundancy removal. I may be wrong, but this
> looks like a sort of optimization. If so, it deserves to be a separate
> patch.

So, the initial idea was to replace the current NUMA awareness logic with
the per-node cpumasks.

But in fact, we're doing this change:

 - before:
   - NUMA-awareness logic implicitly enabled if the node CPUs don't overlap
     with LLC CPUs (as it would be redundant)

 - after :
   - NUMA-awareness logic explicitly enabled when the scx scheduler sets
     SCX_OPS_BUILTIN_IDLE_PER_NODE in .flags (and in this case implicitly
     disable LLC awareness if the node/llc CPUs are overlapping)

Maybe a better approach would be to keep the old NUMA/LLC logic exactly as
it is in sched_ext/for-6.14 if SCX_OPS_BUILTIN_IDLE_PER_NODE is not
specified, otherwise use the new logic (and implicitly disable
scx_selcpu_topo_numa).

In this way this "removal" patch would only implement the logic to disable
scx_selcpu_topo_numa when SCX_OPS_BUILTIN_IDLE_PER_NODE is used.

-Andrea