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Message-ID: <20191118133457.GB66833@gmail.com>
Date: Mon, 18 Nov 2019 14:34:57 +0100
From: Ingo Molnar <mingo@...nel.org>
To: Valentin Schneider <valentin.schneider@....com>
Cc: Vincent Guittot <vincent.guittot@...aro.org>,
linux-kernel@...r.kernel.org, mingo@...hat.com,
peterz@...radead.org, dietmar.eggemann@....com,
juri.lelli@...hat.com, rostedt@...dmis.org, mgorman@...e.de,
bsegall@...gle.com
Subject: [PATCH v2] sched/fair: add comments for group_type and balancing at
SD_NUMA level
* Valentin Schneider <valentin.schneider@....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@...aro.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@...aro.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@...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 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) {
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