linux-kernel - Re: [PATCH v2] sched: sched_domain fix highest_flag

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Message-ID: <69257e98-2d93-673a-5dac-e11b6d5610be@arm.com>
Date:   Tue, 27 Oct 2020 10:57:56 +0100
From:   Dietmar Eggemann <dietmar.eggemann@....com>
To:     Xuewen Yan <xuewen.yan94@...il.com>, xuewen.yan@...soc.com,
        mingo@...hat.com, peterz@...radead.org, juri.lelli@...hat.com,
        vincent.guittot@...aro.org
Cc:     rostedt@...dmis.org, bsegall@...gle.com, mgorman@...e.de,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v2] sched: sched_domain fix highest_flag_domain function

On 27/10/2020 04:32, Xuewen Yan wrote:
> the highest_flag_domain is to search the highest sched_domain
> containing flag, but if the lower sched_domain didn't contain
> the flag, but the higher sched_domain contains the flag, the
> function will return NULL instead of the higher sched_domain.
> 
> For example:
> In MC domain : no SD_ASYM_CPUCAPACITY flag;
> In DIE domain : containing SD_ASYM_CPUCAPACITY flag;
> the "highest_flag_domain(cpu, SD_ASYM_CPUCAPACITY)" will return NULL.
> 
> Signed-off-by: Xuewen Yan <xuewen.yan94@...il.com>
> ---
>  kernel/sched/sched.h | 2 +-
>  1 file changed, 1 insertion(+), 1 deletion(-)
> 
> diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
> index 28709f6..2c7c566 100644
> --- a/kernel/sched/sched.h
> +++ b/kernel/sched/sched.h
> @@ -1427,7 +1427,7 @@ static inline struct sched_domain *highest_flag_domain(int cpu, int flag)
>  
>  	for_each_domain(cpu, sd) {
>  		if (!(sd->flags & flag))
> -			break;
> +			continue;
>  		hsd = sd;
>  	}

We distinguish between SDF_SHARED_PARENT and SDF_SHARED_CHILD SD flags.

1 SD_FLAG(*SD_ASYM_CPUCAPACITY*, SDF_SHARED_PARENT | SDF_NEEDS_GROUPS)
2 SD_FLAG(SD_SERIALIZE, SDF_SHARED_PARENT | SDF_NEEDS_GROUPS)
3 SD_FLAG(SD_OVERLAP, SDF_SHARED_PARENT | SDF_NEEDS_GROUPS)
4 SD_FLAG(SD_NUMA, SDF_SHARED_PARENT | SDF_NEEDS_GROUPS)

1 SD_FLAG(SD_BALANCE_NEWIDLE, SDF_SHARED_CHILD | SDF_NEEDS_GROUPS)
2 SD_FLAG(SD_BALANCE_EXEC, SDF_SHARED_CHILD | SDF_NEEDS_GROUPS)
3 SD_FLAG(SD_BALANCE_FORK, SDF_SHARED_CHILD | SDF_NEEDS_GROUPS)
4 SD_FLAG(SD_BALANCE_WAKE, SDF_SHARED_CHILD | SDF_NEEDS_GROUPS)
5 82 SD_FLAG(SD_WAKE_AFFINE, SDF_SHARED_CHILD)
6 SD_FLAG(SD_SHARE_CPUCAPACITY, SDF_SHARED_CHILD | SDF_NEEDS_GROUPS)
7 SD_FLAG(SD_SHARE_PKG_RESOURCES, SDF_SHARED_CHILD | SDF_NEEDS_GROUPS)
8 SD_FLAG(SD_ASYM_PACKING, SDF_SHARED_CHILD | SDF_NEEDS_GROUPS)

We call lowest_flag_domain() on SDF_SHARED_PARENT and
highest_flag_domain() on SDF_SHARED_CHILD SD flags.

1 sd = lowest_flag_domain(cpu, SD_NUMA);
2 sd = lowest_flag_domain(cpu, *SD_ASYM_CPUCAPACITY*);

1 sd = highest_flag_domain(cpu, SD_SHARE_PKG_RESOURCES);
2 sd = highest_flag_domain(cpu, SD_ASYM_PACKING);