Re: [PATCH v3] sched/topology: Enable topology_span_sane check only for debug builds

K Prateek Nayak
2025-02-05T07:20:52Z

Hello Naman,

On 2/3/2025 5:17 PM, Naman Jain wrote:
> From: Saurabh Sengar <ssengar@linux.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@vger.kernel.org
> Fixes: ccf74128d66c ("sched/topology: Assert non-NUMA topology masks don't (partially) overlap")
> Signed-off-by: Saurabh Sengar <ssengar@linux.microsoft.com>
> Co-developed-by: Naman Jain <namjain@linux.microsoft.com>
> Signed-off-by: Naman Jain <namjain@linux.microsoft.com>
> ---
>
> 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 | 7 +++++++
> 1 file changed, 7 insertions(+)
>
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index c49aea8c1025..b030c1a2121f 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -2359,6 +2359,13 @@ static bool topology_span_sane(struct sched_domain_topology_level *tl,
> {
> int i = cpu + 1;
>
> + /* Skip the topology sanity check for non-debug, as it is a time-consuming operatin */

s/operatin/operation/

> + if (!sched_debug()) {
> + pr_info_once("%s: Skipping topology span sanity check. Use `sched_verbose` boot parameter to enable it.\n",

This could be broken down as follows:

pr_info_once("%s: Skipping topology span sanity check."
" Use `sched_verbose` boot parameter to enable it.\n",
__func__);

Running:

grep -r -A 5 "pr_info(.*[^;,]$" kernel/

gives similar usage across kernel/*. Apart from those nits, feel
free to add:

Tested-by: K Prateek Nayak <kprateek.nayak@amd.com> # x86

if the future version does not change much.

--
Thanks and Regards,
Prateek

> + __func__);
> + return true;
> + }
> +
> /* NUMA levels are allowed to overlap */
> if (tl->flags & SDTL_OVERLAP)
> return true;
>
> base-commit: 00f3246adeeacbda0bd0b303604e46eb59c32e6e