linux-kernel - [tip: sched/core] sched/fair: Add comments for group_type and balancing at SD

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Message-ID: <157409896112.12247.8307010883378296448.tip-bot2@tip-bot2>
Date:   Mon, 18 Nov 2019 17:42:41 -0000
From:   "tip-bot2 for Vincent Guittot" <tip-bot2@...utronix.de>
To:     linux-tip-commits@...r.kernel.org
Cc:     Mel Gorman <mgorman@...e.de>,
        Vincent Guittot <vincent.guittot@...aro.org>,
        Valentin Schneider <valentin.schneider@....com>,
        Ben Segall <bsegall@...gle.com>,
        Dietmar Eggemann <dietmar.eggemann@....com>,
        Juri Lelli <juri.lelli@...hat.com>,
        Linus Torvalds <torvalds@...ux-foundation.org>,
        Mike Galbraith <efault@....de>,
        Peter Zijlstra <peterz@...radead.org>,
        Steven Rostedt <rostedt@...dmis.org>,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...nel.org>, Borislav Petkov <bp@...en8.de>,
        linux-kernel@...r.kernel.org
Subject: [tip: sched/core] sched/fair: Add comments for group_type and
 balancing at SD_NUMA level

The following commit has been merged into the sched/core branch of tip:

Commit-ID:     a9723389cc759c891d481de271ac73eeaa123bcb
Gitweb:        https://git.kernel.org/tip/a9723389cc759c891d481de271ac73eeaa123bcb
Author:        Vincent Guittot <vincent.guittot@...aro.org>
AuthorDate:    Tue, 12 Nov 2019 15:50:43 +01:00
Committer:     Ingo Molnar <mingo@...nel.org>
CommitterDate: Mon, 18 Nov 2019 14:33:12 +01:00

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@...e.de>
Signed-off-by: Vincent Guittot <vincent.guittot@...aro.org>
Acked-by: Valentin Schneider <valentin.schneider@....com>
Cc: Ben Segall <bsegall@...gle.com>
Cc: Dietmar Eggemann <dietmar.eggemann@....com>
Cc: Juri Lelli <juri.lelli@...hat.com>
Cc: Linus Torvalds <torvalds@...ux-foundation.org>
Cc: Mike Galbraith <efault@....de>
Cc: Peter Zijlstra <peterz@...radead.org>
Cc: Steven Rostedt <rostedt@...dmis.org>
Cc: Thomas Gleixner <tglx@...utronix.de>
Link: https://lkml.kernel.org/r/1573570243-1903-1-git-send-email-vincent.guittot@linaro.org
Signed-off-by: Ingo Molnar <mingo@...nel.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 2fc08e7..1f93d96 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) {