linux-kernel - Re: [RFC PATCH 08/11] sched: Remove SD_WAKE_AFFINE flag and replace it with SD_BALANCE

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Message-ID: <20160623130433.GF8415@codeblueprint.co.uk>
Date:	Thu, 23 Jun 2016 14:04:33 +0100
From:	Matt Fleming <matt@...eblueprint.co.uk>
To:	Yuyang Du <yuyang.du@...el.com>
Cc:	peterz@...radead.org, mingo@...nel.org,
	linux-kernel@...r.kernel.org, umgwanakikbuti@...il.com,
	bsegall@...gle.com, pjt@...gle.com, morten.rasmussen@....com,
	vincent.guittot@...aro.org, dietmar.eggemann@....com
Subject: Re: [RFC PATCH 08/11] sched: Remove SD_WAKE_AFFINE flag and replace
 it with SD_BALANCE_WAKE

On Thu, 16 Jun, at 09:49:32AM, Yuyang Du wrote:
> SD_BALANCE_{FORK|EXEC|WAKE} flags are for select_task_rq() to select a
> CPU to run a new task or a waking task. SD_WAKE_AFFINE is a flag to
> try selecting the waker CPU to run the waking task.
> 
> SD_BALANCE_WAKE is not a sched_domain flag, but SD_WAKE_AFFINE is.
> Conceptually, SD_BALANCE_WAKE should be a sched_domain flag just like
> the other two, so we first make SD_BALANCE_WAKE a sched_domain flag.
> 
> Moreover, the semantic of SD_WAKE_AFFINE is included in the semantic
> of SD_BALANCE_WAKE. When in wakeup balancing, it is natual to try
> the waker CPU if the waker CPU is allowed, in that sense, we don't
> need a separate flag to specify it, not mentioning that SD_WAKE_AFFINE
> is almost enabled in every sched_domains.
> 
> With the above combined, there is no need to have SD_WAKE_AFFINE, so
> we remove and replace it with SD_BALANCE_WAKE. This can be accomplished
> without any functionality change.
> 
> Signed-off-by: Yuyang Du <yuyang.du@...el.com>
> ---
>  include/linux/sched.h   |    1 -
>  kernel/sched/core.c     |    7 +++----
>  kernel/sched/deadline.c |    2 +-
>  kernel/sched/fair.c     |    9 ++++-----
>  kernel/sched/rt.c       |    2 +-
>  5 files changed, 9 insertions(+), 12 deletions(-)
> 
> diff --git a/include/linux/sched.h b/include/linux/sched.h
> index d74e757..0803abd 100644
> --- a/include/linux/sched.h
> +++ b/include/linux/sched.h
> @@ -1014,7 +1014,6 @@ extern void wake_up_q(struct wake_q_head *head);
>  #define SD_BALANCE_EXEC		0x0004	/* Balance on exec */
>  #define SD_BALANCE_FORK		0x0008	/* Balance on fork, clone */
>  #define SD_BALANCE_WAKE		0x0010  /* Balance on wakeup */
> -#define SD_WAKE_AFFINE		0x0020	/* Wake task to waking CPU */
>  #define SD_SHARE_CPUCAPACITY	0x0080	/* Domain members share cpu power */
>  #define SD_SHARE_POWERDOMAIN	0x0100	/* Domain members share power domain */
>  #define SD_SHARE_PKG_RESOURCES	0x0200	/* Domain members share cpu pkg resources */

I'm curious - doesn't this break userspace ABI? These flags are
exported via procfs, so I would have assumed removing or changing the
value of any of these constants would be forbidden.