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

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Message-ID: <16004c77-de73-4208-9a10-feee7225e738@linux.microsoft.com>
Date: Wed, 12 Mar 2025 12:37:43 +0530
From: Naman Jain <namjain@...ux.microsoft.com>
To: Valentin Schneider <vschneid@...hat.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>,
 Shrikanth Hegde <sshegde@...ux.ibm.com>
Subject: Re: [PATCH v5] sched/topology: Enable topology_span_sane check only
 for debug builds



On 3/11/2025 9:02 PM, Valentin Schneider wrote:
> 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;
>   	}


Thanks for reviewing. I'll wait a couple more days to push the next
version.

Thanks,
Naman