linux-kernel - Re: [External] Re: [PATCH] sched/topology: Optimized copy default topology in sched_init

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Message-ID: <8bcb77e0-b893-f309-1b81-dc665f61fe84@bytedance.com>
Date:   Wed, 13 Jul 2022 21:21:32 +0800
From:   Hao Jia <jiahao.os@...edance.com>
To:     Valentin Schneider <vschneid@...hat.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,
        bristot@...hat.com
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [External] Re: [PATCH] sched/topology: Optimized copy default
 topology in sched_init_numa()



On 2022/7/12 Valentin Schneider wrote:
> On 11/07/22 18:28, Hao Jia wrote:
>> On 2022/7/4 Valentin Schneider wrote:
>>>
>>> It's not a very hot path but I guess this lets you shave off a bit of boot
>>> time... While you're at it, you could add an early
>> Thanks for your time and suggestion.
>>>
>>>     if (nr_node_ids == 1)
>>>             return;
>>>
>>
>> This will cause the values of sched_domains_numa_levels and
>> sched_max_numa_distance to be different from before, and
>> sched_domains_numa_levels may cause the return value of
>> sched_numa_find_closest() to be different.
>> I'm not sure if it will cause problems.
>>
> 
> True, we need to be careful here, but those are all static so they get
> initialized to sensible defaults (zero / NULL pointer).
> 
> sched_numa_find_closest() will return nr_cpu_ids which make sense, so I
> think we can get away with an early return

Yes, this may affect the return value of housekeeping anycpu(), which 
doesn't seem to be a problem.

> 
>>> since !NUMA systems still go through sched_init_numa() if they have a
>>> kernel with CONFIG_NUMA (which should be most of them nowdays) and IIRC
>>> they end up with an unused NODE topology level.
>>>
>>
>> I'm confused why most !NUMA systems enable CONFIG_NUMA in the kernel?
>> Maybe for scalability?
>>
> 
> It just makes things easier on a distribution point of view - just ship a
> single kernel image everyone can use, rather than N different images for N
> different types of systems.
> 
> AFAIA having CONFIG_NUMA on an UMA (!NUMA) system isn't bad, it just adds
> more things in the sched_domain_topology during boot time which end up
> being unused.

Thank you very much for your answer.

Thinks,
Hao

>