[PATCH v2] sched/fair: add comments for group_type and balancing at SD_NUMA level

[Ingo Molnar <mingo@kernel.org>]

* Valentin Schneider <valentin.schneider@arm.com> wrote:

> Hi Vincent,
> 
> On 12/11/2019 14:50, Vincent Guittot wrote:
> > Add comments to describe each state of goup_type and to add some details
> > about the load balance at NUMA level.
> > 
> > Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
> 
> Suggestions/nits below. There's a bit of duplication with existing
> comments (e.g. the nice blob atop sg_imbalanced()), but I think it can't
> hurt to have the few extra lines you're introducing.
> 
> ---
> diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
> index bfdcaf91b325..ec93ebd02352 100644
> --- a/kernel/sched/fair.c
> +++ b/kernel/sched/fair.c
> @@ -6955,28 +6955,26 @@ enum fbq_type { regular, remote, all };
>   * group. see update_sd_pick_busiest().
>   */
>  enum group_type {
> -	/*
> -	 * The group has spare capacity that can be used to process more work.
> -	 */
> +	/* The group isn't significantly pressured and can be used to process more work */
>  	group_has_spare = 0,
>  	/*
>  	 * The group is fully used and the tasks don't compete for more CPU
> -	 * cycles. Nevetheless, some tasks might wait before running.
> +	 * cycles. Nevertheless, some tasks might wait before running.
>  	 */
>  	group_fully_busy,
>  	/*
> -	 * One task doesn't fit with CPU's capacity and must be migrated on a
> -	 * more powerful CPU.
> +	 * (SD_ASYM_CPUCAPACITY only) One task doesn't fit on its CPU's
> +	 * capacity and must be migrated to a CPU of higher capacity.
>  	 */
>  	group_misfit_task,
>  	/*
> -	 * One local CPU with higher capacity is available and task should be
> -	 * migrated on it instead on current CPU.
> +	 * (SD_ASYM_PACKING only) One local CPU with higher capacity is
> +	 * available and task should be migrated to it.
>  	 */
>  	group_asym_packing,
>  	/*
> -	 * The tasks affinity prevents the scheduler to balance the load across
> -	 * the system.
> +	 * The tasks affinity previously prevented the scheduler from balancing
> +	 * load across the system.
>  	 */
>  	group_imbalanced,

Thanks - I did a few more fixes and updates to the comments, this is how 
it ended up looking like (full patch below):

/*
 * 'group_type' describes the group of CPUs at the moment of load balancing.
 *
 * The enum is ordered by pulling priority, with the group with lowest priority
 * first so the group_type can simply be compared when selecting the busiest
 * group. See update_sd_pick_busiest().
 */
enum group_type {
	/* The group has spare capacity that can be used to run more tasks.  */
	group_has_spare = 0,
	/*
	 * The group is fully used and the tasks don't compete for more CPU
	 * cycles. Nevertheless, some tasks might wait before running.
	 */
	group_fully_busy,
	/*
	 * SD_ASYM_CPUCAPACITY only: One task doesn't fit with CPU's capacity
	 * and must be migrated to a more powerful CPU.
	 */
	group_misfit_task,
	/*
	 * SD_ASYM_PACKING only: One local CPU with higher capacity is available,
	 * and the task should be migrated to it instead of running on the
	 * current CPU.
	 */
	group_asym_packing,
	/*
	 * The tasks' affinity constraints previously prevented the scheduler
	 * from balancing the load across the system.
	 */
	group_imbalanced,
	/*
	 * The CPU is overloaded and can't provide expected CPU cycles to all
	 * tasks.
	 */
	group_overloaded
};

I also added your Acked-by, which I think was implicit? :)

Thanks,

	Ingo

=====>
From: Vincent Guittot <vincent.guittot@linaro.org>
Date: Tue, 12 Nov 2019 15:50:43 +0100
Subject: [PATCH] sched/fair: Add comments for group_type and balancing at SD_NUMA level

Add comments to describe each state of goup_type and to add some details
about the load balance at NUMA level.

[ Valentin Schneider: Updates to the comments. ]
[ mingo: Other updates to the comments. ]

Reported-by: Mel Gorman <mgorman@suse.de>
Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Acked-by: Valentin Schneider <valentin.schneider@arm.com>
Cc: Ben Segall <bsegall@google.com>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Mike Galbraith <efault@gmx.de>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/1573570243-1903-1-git-send-email-vincent.guittot@linaro.org
Signed-off-by: Ingo Molnar <mingo@kernel.org>
---
 kernel/sched/fair.c | 35 +++++++++++++++++++++++++++++++----
 1 file changed, 31 insertions(+), 4 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 2fc08e7d9cd6..1f93d96dd06b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6980,17 +6980,40 @@ static unsigned long __read_mostly max_load_balance_interval = HZ/10;
 enum fbq_type { regular, remote, all };
 
 /*
- * group_type describes the group of CPUs at the moment of the load balance.
+ * 'group_type' describes the group of CPUs at the moment of load balancing.
+ *
  * The enum is ordered by pulling priority, with the group with lowest priority
- * first so the groupe_type can be simply compared when selecting the busiest
- * group. see update_sd_pick_busiest().
+ * first so the group_type can simply be compared when selecting the busiest
+ * group. See update_sd_pick_busiest().
  */
 enum group_type {
+	/* The group has spare capacity that can be used to run more tasks.  */
 	group_has_spare = 0,
+	/*
+	 * The group is fully used and the tasks don't compete for more CPU
+	 * cycles. Nevertheless, some tasks might wait before running.
+	 */
 	group_fully_busy,
+	/*
+	 * SD_ASYM_CPUCAPACITY only: One task doesn't fit with CPU's capacity
+	 * and must be migrated to a more powerful CPU.
+	 */
 	group_misfit_task,
+	/*
+	 * SD_ASYM_PACKING only: One local CPU with higher capacity is available,
+	 * and the task should be migrated to it instead of running on the
+	 * current CPU.
+	 */
 	group_asym_packing,
+	/*
+	 * The tasks' affinity constraints previously prevented the scheduler
+	 * from balancing the load across the system.
+	 */
 	group_imbalanced,
+	/*
+	 * The CPU is overloaded and can't provide expected CPU cycles to all
+	 * tasks.
+	 */
 	group_overloaded
 };
 
@@ -8589,7 +8612,11 @@ static inline void calculate_imbalance(struct lb_env *env, struct sd_lb_stats *s
 
 	/*
 	 * Try to use spare capacity of local group without overloading it or
-	 * emptying busiest
+	 * emptying busiest.
+	 * XXX Spreading tasks across NUMA nodes is not always the best policy
+	 * and special care should be taken for SD_NUMA domain level before
+	 * spreading the tasks. For now, load_balance() fully relies on
+	 * NUMA_BALANCING and fbq_classify_group/rq to override the decision.
 	 */
 	if (local->group_type == group_has_spare) {
 		if (busiest->group_type > group_fully_busy) {