linux-kernel - Re: [PATCH v5] sched/topology: Enable topology_span

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Message-ID: <xhsmh34fjr3av.mognet@vschneid-thinkpadt14sgen2i.remote.csb>
Date: Tue, 11 Mar 2025 16:32:24 +0100
From: Valentin Schneider <vschneid@...hat.com>
To: Naman Jain <namjain@...ux.microsoft.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>
Cc: stable@...r.kernel.org, linux-kernel@...r.kernel.org, Steve Wahl
 <steve.wahl@....com>, Saurabh Singh Sengar <ssengar@...ux.microsoft.com>,
 srivatsa@...il.mit.edu, K Prateek Nayak <kprateek.nayak@....com>, Michael
 Kelley <mhklinux@...look.com>, Naman Jain <namjain@...ux.microsoft.com>,
 Shrikanth Hegde <sshegde@...ux.ibm.com>
Subject: Re: [PATCH v5] sched/topology: Enable topology_span_sane check only
 for debug builds

On 10/03/25 10:55, Naman Jain wrote:
> From: Saurabh Sengar <ssengar@...ux.microsoft.com>
>
> 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_verbose kernel cmdline option so that this penalty
> can be avoided for the systems who want to avoid it.
>
> Cc: stable@...r.kernel.org
> Fixes: ccf74128d66c ("sched/topology: Assert non-NUMA topology masks don't (partially) overlap")
> Signed-off-by: Saurabh Sengar <ssengar@...ux.microsoft.com>
> Co-developed-by: Naman Jain <namjain@...ux.microsoft.com>
> Signed-off-by: Naman Jain <namjain@...ux.microsoft.com>
> Tested-by: K Prateek Nayak <kprateek.nayak@....com>
> ---
> Changes since v4:
> https://lore.kernel.org/all/20250306055354.52915-1-namjain@linux.microsoft.com/
>       - Rephrased print statement and moved it to sched_domain_debug.
>         (addressing Valentin's comments)
> Changes since v3:
> https://lore.kernel.org/all/20250203114738.3109-1-namjain@linux.microsoft.com/
>       - Minor typo correction in comment
>       - Added Tested-by tag from Prateek for x86
> Changes since v2:
> https://lore.kernel.org/all/1731922777-7121-1-git-send-email-ssengar@linux.microsoft.com/
>       - Use sched_debug() instead of using sched_debug_verbose
>         variable directly (addressing Prateek's comment)
>
> Changes since v1:
> https://lore.kernel.org/all/1729619853-2597-1-git-send-email-ssengar@linux.microsoft.com/
>       - Use kernel cmdline param instead of compile time flag.
>
> Adding a link to the other patch which is under review.
> https://lore.kernel.org/lkml/20241031200431.182443-1-steve.wahl@hpe.com/
> Above patch tries to optimize the topology sanity check, whereas this
> patch makes it optional. We believe both patches can coexist, as even
> with optimization, there will still be some performance overhead for
> this check.
>
> ---
>  kernel/sched/topology.c | 9 ++++++++-
>  1 file changed, 8 insertions(+), 1 deletion(-)
>
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index c49aea8c1025..d7254c47af45 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -132,8 +132,11 @@ static void sched_domain_debug(struct sched_domain *sd, int cpu)
>  {
>       int level = 0;
>
> -	if (!sched_debug_verbose)
> +	if (!sched_debug_verbose) {
> +		pr_info_once("%s: Scheduler topology debugging disabled, add 'sched_verbose' to the cmdline to enable it\n",
> +			     __func__);
>               return;
> +	}
>

Nit: I've been told not to break warnings over multiple lines so they can
be grep'ed, but given this has the "sched_domain_debug:" prefix I think we
could get away with the below.

Regardless:
Reviewed-by: Valentin Schneider <vschneid@...hat.com>

---
diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
index d7254c47af455..b4dc7c7d2c41c 100644
--- a/kernel/sched/topology.c
+++ b/kernel/sched/topology.c
@@ -133,7 +133,8 @@ static void sched_domain_debug(struct sched_domain *sd, int cpu)
 	int level = 0;
 
 	if (!sched_debug_verbose) {
-		pr_info_once("%s: Scheduler topology debugging disabled, add 'sched_verbose' to the cmdline to enable it\n",
+		pr_info_once("%s: Scheduler topology debugging disabled, "
+			     "add 'sched_verbose' to the cmdline to enable it\n",
 			     __func__);
 		return;
 	}