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

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Message-ID: <Z2SNRTwDMbFtdQJf@gpd3>
Date: Thu, 19 Dec 2024 22:16:53 +0100
From: Andrea Righi <arighi@...dia.com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Yury Norov <yury.norov@...il.com>, Tejun Heo <tj@...nel.org>,
	David Vernet <void@...ifault.com>,
	Changwoo Min <changwoo@...lia.com>, Ingo Molnar <mingo@...hat.com>,
	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()

On Thu, Dec 19, 2024 at 08:52:30PM +0100, Peter Zijlstra wrote:
> On Thu, Dec 19, 2024 at 10:26:59AM -0800, Yury Norov wrote:
> > > 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.
> > 
> > I agree. The number of nodes on typical system is 1 or 2. Even if
> > it's 8, the Andrea's bubble sort will be still acceptable. So, I'm
> > OK with O(N^2) if you guys OK with it. I only would like to have
> > this choice explained in commit message.
> 
> There are systems with 100s or 1000s of nodes out there. As long as
> hitting this code path is optional I suppose that's not a problem, but
> if not, they're going to be rather upset.

Right, this code is optional, it's only hit when SCX_OPS_KEEP_BUILTIN_IDLE
is enabled (off by default) in an scx scheduler and the scheduler is asking
for any idle CPU in the system without speficying a target node.

So, it shouldn't be a big concern for now, and we can probably add
optimizations for special cases later. I'll add a comment to explain this
as well.

Thanks,
-Andrea