linux-kernel - Re: [PATCH v2] sched/topology: Enable topology_span

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Message-ID: <1e4c0bda-380e-5aba-984f-2a48debd7562@amd.com>
Date: Tue, 19 Nov 2024 11:54:57 +0530
From: K Prateek Nayak <kprateek.nayak@....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>,
	<vschneid@...hat.com>, <linux-kernel@...r.kernel.org>, Steve Wahl
	<steve.wahl@....com>
CC: <stable@...r.kernel.org>, <ssengar@...rosoft.com>,
	<srivatsa@...il.mit.edu>
Subject: Re: [PATCH v2] sched/topology: Enable topology_span_sane check only
 for debug builds

(+ Steve)

Hello Saurabh,
On 11/18/2024 3:09 PM, 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.

Steve too was optimizing this path. I believe his latest version can be
found at:
https://lore.kernel.org/lkml/20241031200431.182443-1-steve.wahl@hpe.com/

Does that approach help improving bootup time for you? Valentine
suggested the same approach as yours on a previous version of Steve's
optimization but Steve believed returning true can possibly have other
implication in the sched-domain building path. The thread can be found
at:
https://lore.kernel.org/lkml/Zw_k_WFeYFli87ck@swahl-home.5wahls.com/

> 
> 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 wants 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>
> ---
> [V2]
> 	- Use kernel cmdline param instead of compile time flag.
> 
>   kernel/sched/topology.c | 7 +++++++
>   1 file changed, 7 insertions(+)
> 
> diff --git a/kernel/sched/topology.c b/kernel/sched/topology.c
> index 9748a4c8d668..4ca63bff321d 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -2363,6 +2363,13 @@ 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 */
> +	if (!sched_debug_verbose) {

nit.

I think the convention in topology.c is to call "sched_debug()" and not
check "sched_debug_verbose" directly.

> +		pr_info_once("%s: Skipping topology span sanity check. Use `sched_verbose` boot parameter to enable it.\n",
> +			     __func__);
> +		return true;
> +	}
> +
>   	/* NUMA levels are allowed to overlap */
>   	if (tl->flags & SDTL_OVERLAP)
>   		return true;


-- 
Thanks and Regards,
Prateek