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sched/fair: Add comments for group_type and balancing at SD_NUMA level

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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>
This commit is contained in:
Vincent Guittot 2019-11-12 15:50:43 +01:00 committed by Ingo Molnar
parent 3318544b72
commit a9723389cc
1 changed files with 31 additions and 4 deletions
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35
kernel/sched/fair.c
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@ -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) {