linux-kernel - Re: [RFC PATCH 1/3] sched/topology: Split out SD

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Date:   Mon, 23 Mar 2020 14:42:34 +0100
From:   Morten Rasmussen <morten.rasmussen@....com>
To:     Valentin Schneider <valentin.schneider@....com>
Cc:     linux-kernel@...r.kernel.org, mingo@...nel.org,
        peterz@...radead.org, vincent.guittot@...aro.org,
        dietmar.eggemann@....com
Subject: Re: [RFC PATCH 1/3] sched/topology: Split out SD_* flags declaration
 to its own file

On Wed, Mar 11, 2020 at 06:33:18PM +0000, Valentin Schneider wrote:
> diff --git a/include/linux/sched/sd_flags.h b/include/linux/sched/sd_flags.h
> new file mode 100644
> index 000000000000..685bbe736945
> --- /dev/null
> +++ b/include/linux/sched/sd_flags.h
> @@ -0,0 +1,33 @@
> +// SPDX-License-Identifier: GPL-2.0
> +/*
> + * sched-domains (multiprocessor balancing) flag declarations.
> + */
> +
> +/* Balance when about to become idle */
> +SD_FLAG(SD_BALANCE_NEWIDLE,     0)
> +/* Balance on exec */
> +SD_FLAG(SD_BALANCE_EXEC,        1)
> +/* Balance on fork, clone */
> +SD_FLAG(SD_BALANCE_FORK,        2)
> +/* Balance on wakeup */
> +SD_FLAG(SD_BALANCE_WAKE,        3)
> +/* Wake task to waking CPU */
> +SD_FLAG(SD_WAKE_AFFINE,         4)

Isn't it more like: "Consider waking task on waking CPU"?

IIRC, with this flag set the wake-up can happen either near prev_cpu or
this_cpu.

> +/* Domain members have different CPU capacities */
> +SD_FLAG(SD_ASYM_CPUCAPACITY,    5)
> +/* Domain members share CPU capacity */
> +SD_FLAG(SD_SHARE_CPUCAPACITY,   6)

Perhaps add +" (SMT)" to the comment to help the uninitiated
understanding it a bit easier?

> +/* Domain members share power domain */
> +SD_FLAG(SD_SHARE_POWERDOMAIN,   7)

This flag is set only by 32-bit arm and has never had any effect. I
think it was the beginning of something years ago that hasn't
progressed. Perhaps we can remove it now?

> +/* Domain members share CPU pkg resources */
> +SD_FLAG(SD_SHARE_PKG_RESOURCES, 8)

+" (e.g. caches)" ?

> +/* Only a single load balancing instance */
> +SD_FLAG(SD_SERIALIZE,           9)
> +/* Place busy groups earlier in the domain */
> +SD_FLAG(SD_ASYM_PACKING,        10)

Place busy _tasks_ earlier in the domain?

It is a bit unclear what 'earlier' means here but since the packing
ordering can actually be defined by the architecture, we can't be much
more specific I guess.

Morten