linux-kernel - Re: [PATCH 1/6] sched/topology: introduce for_each_numa_hop_node() / sched_numa_hop

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Message-ID: <Z2LypYxoVHwM6rbe@slm.duckdns.org>
Date: Wed, 18 Dec 2024 06:04:53 -1000
From: Tejun Heo <tj@...nel.org>
To: Andrea Righi <arighi@...dia.com>
Cc: David Vernet <void@...ifault.com>, Changwoo Min <changwoo@...lia.com>,
	Yury Norov <yury.norov@...il.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>,
	linux-kernel@...r.kernel.org
Subject: Re: [PATCH 1/6] sched/topology: introduce for_each_numa_hop_node() /
 sched_numa_hop_node()

Hello,

On Wed, Dec 18, 2024 at 11:23:40AM +0100, Andrea Righi wrote:
...
> > So, this would work but given that there is nothing dynamic about this
> > ordering, would it make more sense to build the ordering and store it
> > per-node? Then, the iteration just becomes walking that array.
> 
> I've also considered doing that. I don't know if it'd work with offline
> nodes, but maybe we can just check node_online(node) at each iteration and
> skip those that are not online.

Yeah, there can be e.g. for_each_possible_node_by_dist() where nodes with
unknown distances (offline ones?) are put at the end and then there's also
for_each_online_node_by_dist() which filters out offline ones, and the
ordering can be updated from a CPU hotplug callback. The ordering can be
probably put in an rcu protected array? I'm not sure what's the
synchronization convention around node on/offlining. Is that protected
together with CPU on/offlining?

Given that there usually aren't that many nodes, the current implementation
is probably fine too, so please feel free to ignore this suggestion for now
too.

Thanks.

-- 
tejun