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

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Message-ID: <xhsmha59koswk.mognet@vschneid-thinkpadt14sgen2i.remote.csb>
Date: Mon, 17 Mar 2025 11:25:47 +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>, Shrikanth Hegde <sshegde@...ux.ibm.com>
Subject: Re: [PATCH v5] sched/topology: Enable topology_span_sane check only
 for debug builds

On 17/03/25 15:27, Naman Jain wrote:
> 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;
>>   	}
>
>
> Hi Valentin,
> Splitting the warning to different lines give checkpatch error with 
> --strict option. --fix option suggests we keep it like we have it 
> originally(single line). Please let me know if you feel we can change it 
> to something else or if you are fine with picking the change for next 
> iteration. Thanks again.
>

Hah, didn't know that was in checkpatch :-) As I said before that really
was more of a nitpick, consider me OK with the current patch.