linux-kernel - Re: [PATCH v3 05/13] sched: Introduce SD_ASYM_CPUCAPACITY sched

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Message-ID: <20160815105459.GS6879@twins.programming.kicks-ass.net>
Date:	Mon, 15 Aug 2016 12:54:59 +0200
From:	Peter Zijlstra <peterz@...radead.org>
To:	Morten Rasmussen <morten.rasmussen@....com>
Cc:	mingo@...hat.com, dietmar.eggemann@....com, yuyang.du@...el.com,
	vincent.guittot@...aro.org, mgalbraith@...e.de,
	sgurrappadi@...dia.com, freedom.tan@...iatek.com,
	keita.kobayashi.ym@...esas.com, linux-kernel@...r.kernel.org
Subject: Re: [PATCH v3 05/13] sched: Introduce SD_ASYM_CPUCAPACITY
 sched_domain topology flag

On Mon, Jul 25, 2016 at 02:34:22PM +0100, Morten Rasmussen wrote:
> @@ -6336,14 +6338,16 @@ static int sched_domains_curr_level;
>   * SD_NUMA                - describes NUMA topologies
>   * SD_SHARE_POWERDOMAIN   - describes shared power domain
>   *
> - * Odd one out:
> + * Odd ones out:
>   * SD_ASYM_PACKING        - describes SMT quirks
> + * SD_ASYM_CPUCAPACITY    - describes mixed capacity topologies
>   */

So I'm not sure the new CPUCAPACITY is 'odd'.

That said, the comment is very terse and doesn't explain why PACKING is
odd.

IIRC the distinction is that the 'normal' ones only describe topology,
while the ASYM_PACKING one also prescribes behaviour. It is odd in the
way that it doesn't only describe things.

This ASYM_CPUCAPACITY otoh is purely descriptive, it doesn't prescribe
how to deal with it.

Does something like so  clarify things?

--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6355,13 +6355,19 @@ static int sched_domains_curr_level;
 /*
  * SD_flags allowed in topology descriptions.
  *
- * SD_SHARE_CPUCAPACITY      - describes SMT topologies
- * SD_SHARE_PKG_RESOURCES - describes shared caches
- * SD_NUMA                - describes NUMA topologies
- * SD_SHARE_POWERDOMAIN   - describes shared power domain
+ * These flags are purely descriptive of the topology and do not prescribe
+ * behaviour. Behaviour is artificial and mapped in the below sd_init()
+ * function:
  *
- * Odd one out:
- * SD_ASYM_PACKING        - describes SMT quirks
+ *   SD_SHARE_CPUCAPACITY   - describes SMT topologies
+ *   SD_SHARE_PKG_RESOURCES - describes shared caches
+ *   SD_NUMA                - describes NUMA topologies
+ *   SD_SHARE_POWERDOMAIN   - describes shared power domain
+ *
+ * Odd one out, which beside describing the topology has a quirk also
+ * prescribes the desired behaviour that goes along with it:
+ *
+ *   SD_ASYM_PACKING        - describes SMT quirks
  */
 #define TOPOLOGY_SD_FLAGS		\
 	(SD_SHARE_CPUCAPACITY |		\