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Subject: Re: [PATCH v3 5/7] sched/topology: Add more flags to the SD
 degeneration mask
To:     Valentin Schneider <valentin.schneider@arm.com>,
        linux-kernel@vger.kernel.org
Cc:     mingo@kernel.org, peterz@infradead.org, vincent.guittot@linaro.org,
        morten.rasmussen@arm.com
References: <20200701190656.10126-1-valentin.schneider@arm.com>
 <20200701190656.10126-6-valentin.schneider@arm.com>
From:   Dietmar Eggemann <dietmar.eggemann@arm.com>
Message-ID: <3406fe6b-405b-d542-6459-732f4ae890f6@arm.com>
Date:   Thu, 2 Jul 2020 20:28:38 +0200
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On 01/07/2020 21:06, Valentin Schneider wrote:
> I don't think it is going to change much in practice, but we were missing
> those:
> 
> o SD_BALANCE_WAKE: Used just like the other SD_BALANCE_* flags, so also
>   needs > 1 group.
> o SD_ASYM_PACKING: Hinges on load balancing (periodic / wakeup), thus needs
>   > 1 group to happen
> o SD_OVERLAP: Describes domains with overlapping groups; can't have
>   overlaps with a single group.
> 
> SD_PREFER_SIBLING is as always the odd one out: we currently consider it
> in sd_parent_degenerate() but not in sd_degenerate(). It too hinges on load
> balancing, and thus won't have any effect when set on a domain with a
> single group. Add it to the sd_degenerate() groups mask.
> 
> Signed-off-by: Valentin Schneider <valentin.schneider@arm.com>
> ---
>  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 6047d491abe9..fe393b295444 100644
> --- a/kernel/sched/topology.c
> +++ b/kernel/sched/topology.c
> @@ -161,11 +161,15 @@ static int sd_degenerate(struct sched_domain *sd)
>  
>  	/* Following flags need at least 2 groups */
>  	if (sd->flags & (SD_BALANCE_NEWIDLE |
> +			 SD_BALANCE_WAKE |
>  			 SD_BALANCE_FORK |
>  			 SD_BALANCE_EXEC |
> +			 SD_ASYM_PACKING |
>  			 SD_SHARE_CPUCAPACITY |
>  			 SD_ASYM_CPUCAPACITY |
> -			 SD_SHARE_PKG_RESOURCES)) {
> +			 SD_SHARE_PKG_RESOURCES |
> +			 SD_OVERLAP |
> +			 SD_PREFER_SIBLING)) {

Why do you add SD_PREFER_SIBLING here? It doesn't have SDF_NEEDS_GROUPS
set in [PATCH v3 6/7].

$ cat ./include/linux/sched/sd_flags.h | grep ^SD_FLAG
SD_FLAG(SD_BALANCE_NEWIDLE,     0, SDF_SHARED_CHILD | SDF_NEEDS_GROUPS)
SD_FLAG(SD_BALANCE_EXEC,        1, SDF_SHARED_CHILD | SDF_NEEDS_GROUPS)
SD_FLAG(SD_BALANCE_FORK,        2, SDF_SHARED_CHILD | SDF_NEEDS_GROUPS)
SD_FLAG(SD_BALANCE_WAKE,        3, SDF_SHARED_CHILD | SDF_NEEDS_GROUPS)
SD_FLAG(SD_WAKE_AFFINE,         4, SDF_SHARED_CHILD)
SD_FLAG(SD_ASYM_CPUCAPACITY,    5, SDF_SHARED_PARENT | SDF_NEEDS_GROUPS)
SD_FLAG(SD_SHARE_CPUCAPACITY,   6, SDF_SHARED_CHILD | SDF_NEEDS_GROUPS)
SD_FLAG(SD_SHARE_PKG_RESOURCES, 7, SDF_SHARED_CHILD | SDF_NEEDS_GROUPS)
SD_FLAG(SD_SERIALIZE,           8, SDF_SHARED_PARENT)
SD_FLAG(SD_ASYM_PACKING,        9, SDF_SHARED_CHILD | SDF_NEEDS_GROUPS)
SD_FLAG(SD_PREFER_SIBLING,      10, 0)                           <-- !!!
SD_FLAG(SD_OVERLAP,             11, SDF_SHARED_PARENT |SDF_NEEDS_GROUPS)
SD_FLAG(SD_NUMA,                12, SDF_SHARED_PARENT)