linux-kernel - Re: [PATCH] sched/topology: Enable topology_span

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Message-ID: <xhsmhsesmu62e.mognet@vschneid-thinkpadt14sgen2i.remote.csb>
Date: Wed, 23 Oct 2024 18:39:37 +0200
From: Valentin Schneider <vschneid@...hat.com>
To: Saurabh Sengar <ssengar@...ux.microsoft.com>, mingo@...hat.com,
 peterz@...radead.org, juri.lelli@...hat.com, vincent.guittot@...aro.org,
 dietmar.eggemann@....com, rostedt@...dmis.org, bsegall@...gle.com,
 mgorman@...e.de, linux-kernel@...r.kernel.org
Cc: stable@...r.kernel.org, ssengar@...rosoft.com, srivatsa@...il.mit.edu
Subject: Re: [PATCH] sched/topology: Enable topology_span_sane check only
 for debug builds

On 22/10/24 10:57, Saurabh Sengar wrote:
> On a x86 system under test with 1780 CPUs, topology_span_sane() takes
> around 8 seconds cumulatively for all the iterations. It is an expensive
> operation which does the sanity of non-NUMA topology masks.
>
> CPU topology is not something which changes very frequently hence make
> this check optional for the systems where the topology is trusted and
> need faster bootup.
>
> Restrict this to SCHED_DEBUG builds so that this penalty can be avoided
> for the systems who wants to avoid it.
>
> Fixes: ccf74128d66c ("sched/topology: Assert non-NUMA topology masks don't (partially) overlap")
> Signed-off-by: Saurabh Sengar <ssengar@...ux.microsoft.com>

Please see:
http://lore.kernel.org/r/20241010155111.230674-1-steve.wahl@hpe.com

Also note that most distros ship with CONFIG_SCHED_DEBUG=y, so while I'm
not 100% against it this would at the very least need to be gated behind
e.g. the sched_verbose cmdline argument to be useful.

But before that I'd like the "just run it once" option to be explored
first.