linux-kernel - Re: [PATCH v5 1/4] smpboot: introduce SDTL_INIT() helper to tidy sched topology setup

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Message-ID: <0305d98a-0300-429a-adc2-39fd9b3af876@amd.com>
Date: Mon, 14 Jul 2025 09:33:42 +0530
From: K Prateek Nayak <kprateek.nayak@....com>
To: Peter Zijlstra <peterz@...radead.org>
Cc: Li Chen <me@...ux.beauty>, Ingo Molnar <mingo@...hat.com>,
 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>, Valentin Schneider <vschneid@...hat.com>,
 Li Chen <chenl311@...natelecom.cn>, Swapnil Sapkal <swapnil.sapkal@....com>,
 linux-kernel@...r.kernel.org
Subject: Re: [PATCH v5 1/4] smpboot: introduce SDTL_INIT() helper to tidy
 sched topology setup

(trimming the cc to only kernel/sched folks to reduce the noise)

On 7/11/2025 6:36 PM, Peter Zijlstra wrote:
> On Fri, Jul 11, 2025 at 11:20:30AM +0530, K Prateek Nayak wrote:
>> On 7/10/2025 4:27 PM, Li Chen wrote:
>>>  	/*
>>>  	 * .. and append 'j' levels of NUMA goodness.
>>>  	 */
>>>  	for (j = 1; j < nr_levels; i++, j++) {
>>> -		tl[i] = (struct sched_domain_topology_level){
>>> -			.mask = sd_numa_mask,
>>> -			.sd_flags = cpu_numa_flags,
>>> -			.flags = SDTL_OVERLAP,
>>> -			.numa_level = j,
>>> -			SD_INIT_NAME(NUMA)
>>> -		};
>>> +		tl[i] = SDTL_INIT(sd_numa_mask, cpu_numa_flags, NUMA);
>>> +		tl[i].numa_level = j;
>>> +		tl[i].flags = SDTL_OVERLAP;
>>
>> Tangential discussion: I was looking at this and was wondering why we
>> need a "tl->flags" when there is already sd_flags() function and we can
>> simply add SD_OVERLAP to sd_numa_flags().
>>
>> I think "tl->flags" was needed when the idea of overlap domains was
>> added in commit e3589f6c81e4 ("sched: Allow for overlapping sched_domain
>> spans") when it depended on "FORCE_SD_OVERLAP" sched_feat() which
>> allowed toggling this off but that was done away with in commit
>> af85596c74de ("sched/topology: Remove FORCE_SD_OVERLAP") so perhaps we
>> can get rid of it now?
>>
>> Relying on SD_NUMA should be enough currently. Peter, Valentin, what do
>> you think of something like below?
> 
> I think you're right. SD_NUMA appears to be the one and only case that
> also has SDTL_OVERLAP which then results in setting SD_OVERLAP, making
> SD_NUMA and SD_OVERLAP equivalent and SDTL_OVERLAP redundant.
> 
> I'll presume you're okay with me adding your SoB to things, and I'll
> push out all 5 patches to queue/sched/core to let the robots have a go
> at things.

Works for me! If you need a formal commit message:

Support for overlapping domains added in commit e3589f6c81e4 ("sched:
Allow for overlapping sched_domain spans") also allowed forcefully
setting SD_OVERLAP for !NUMA domains via FORCE_SD_OVERLAP sched_feat().

Since NUMA domains had to be presumed overlapping to ensure correct
behavior, "sched_domain_topology_level::flags" was introduced. NUMA
domains added the SDTL_OVERLAP flag would ensure SD_OVERLAP was always
added during build_sched_domains() for these domains, even when
FORCE_SD_OVERLAP was off.

Condition for adding the SD_OVERLAP flag at the aforementioned commit
was as follows:

    if (tl->flags & SDTL_OVERLAP || sched_feat(FORCE_SD_OVERLAP))
            sd->flags |= SD_OVERLAP; 

The FORCE_SD_OVERLAP debug feature was removed in commit af85596c74de
("sched/topology: Remove FORCE_SD_OVERLAP") which left the NUMA domains
as the exclusive users of SDTL_OVERLAP, SD_OVERLAP, and SD_NUMA flags.

Get rid of SDTL_OVERLAP and SD_OVERLAP as they have become redundant
and instead rely on SD_NUMA to detect the only overlapping domain
currently supported. Since SDTL_OVERLAP was the only user of
"tl->flags", get rid of "sched_domain_topology_level::flags" too.

Signed-off-by: K Prateek Nayak <kprateek.nayak@....com>
---

P.S. Are we still considering the following for v6.16 cycle?
https://lore.kernel.org/lkml/20250709161917.14298-1-kprateek.nayak@amd.com/

If not, I can rebase it on top of queue:sched/core and send it out with
the conflicts resolved to save you a couple of edits :)

-- 
Thanks and Regards,
Prateek