linux-kernel - Re: [PATCH v2 4/9] sched/topology: Kill SD_LOAD

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Message-ID: <jhjtv2ko8gg.mognet@arm.com>
Date:   Thu, 19 Mar 2020 12:06:07 +0000
From:   Valentin Schneider <valentin.schneider@....com>
To:     Dietmar Eggemann <dietmar.eggemann@....com>
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        peterz@...radead.org, vincent.guittot@...aro.org
Subject: Re: [PATCH v2 4/9] sched/topology: Kill SD_LOAD_BALANCE


On Thu, Mar 19 2020, Dietmar Eggemann wrote:
> On 11.03.20 19:15, Valentin Schneider wrote:
>> That flag is set unconditionally in sd_init(), and no one checks for it
>> anymore. Remove it.
>
> Why not merge 3/9 and 4/9 ?
>
> [...]
>
>> diff --git a/include/linux/sched/topology.h b/include/linux/sched/topology.h
>> index f341163fedc9..8de2f9744569 100644
>> --- a/include/linux/sched/topology.h
>> +++ b/include/linux/sched/topology.h
>> @@ -11,21 +11,20 @@
>>   */
>>  #ifdef CONFIG_SMP
>>
>> -#define SD_LOAD_BALANCE		0x0001	/* Do load balancing on this domain. */
>> -#define SD_BALANCE_NEWIDLE	0x0002	/* Balance when about to become idle */
>
> [...]
>
>> -#define SD_OVERLAP		0x2000	/* sched_domains of this level overlap */
>> -#define SD_NUMA			0x4000	/* cross-node balancing */
>> +#define SD_BALANCE_NEWIDLE	0x0001	/* Balance when about to become idle */
>
> IMHO, changing the values of the remaining SD flags will break lots of
> userspace scripts.
>

True, and that includes some of my own scripts. That's also part of why
I have this 3/9 and 4/9 split: 4/9 is the externally visible part. If
deemed necessary, we could keep the definition of SD_LOAD_BALANCE but
kill all of its uses.

Alternatively, I was thinking that we could leverage [1] to make
/proc/sys/kernel/sched_domain/cpu*/domain*/flags print
e.g. comma-separated flag names rather than flag values. That way the
userland scripts would no longer have to contain some
{flag_value : flag_name} translation.

[1]: https://lkml.kernel.org/r/20200311183320.19186-1-valentin.schneider@arm.com

> [...]