[PATCH 6.1 095/473] sched/fair: Add EAS checks before updating root_domain::overutilized

Greg Kroah-Hartman
2024-06-06T14:13:38Z

From: Shrikanth Hegde <sshegde@linux.ibm.com>

[ Upstream commit be3a51e68f2f1b17250ce40d8872c7645b7a2991 ]

root_domain::overutilized is only used for EAS(energy aware scheduler)
to decide whether to do load balance or not. It is not used if EAS
not possible.

Currently enqueue_task_fair and task_tick_fair accesses, sometime updates
this field. In update_sd_lb_stats it is updated often. This causes cache
contention due to true sharing and burns a lot of cycles. ::overload and
::overutilized are part of the same cacheline. Updating it often invalidates
the cacheline. That causes access to ::overload to slow down due to
false sharing. Hence add EAS check before accessing/updating this field.
EAS check is optimized at compile time or it is a static branch.
Hence it shouldn't cost much.

With the patch, both enqueue_task_fair and newidle_balance don't show
up as hot routines in perf profile.

6.8-rc4:
7.18% swapper [kernel.vmlinux] [k] enqueue_task_fair
6.78% s [kernel.vmlinux] [k] newidle_balance

+patch:
0.14% swapper [kernel.vmlinux] [k] enqueue_task_fair
0.00% swapper [kernel.vmlinux] [k] newidle_balance

While at it: trace_sched_overutilized_tp expect that second argument to
be bool. So do a int to bool conversion for that.

Fixes: 2802bf3cd936 ("sched/fair: Add over-utilization/tipping point indicator")
Signed-off-by: Shrikanth Hegde <sshegde@linux.ibm.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Reviewed-by: Qais Yousef <qyousef@layalina.io>
Reviewed-by: Srikar Dronamraju <srikar@linux.ibm.com>
Reviewed-by: Vincent Guittot <vincent.guittot@linaro.org>
Link: https://lore.kernel.org/r/20240307085725.444486-2-sshegde@linux.ibm.com
Signed-off-by: Sasha Levin <sashal@kernel.org>
---
kernel/sched/fair.c | 53 +++++++++++++++++++++++++++++----------------
1 file changed, 34 insertions(+), 19 deletions(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 91c101ecfef9f..0de8354d5ad0a 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6020,22 +6020,42 @@ static inline void hrtick_update(struct rq *rq)
#ifdef CONFIG_SMP
static inline bool cpu_overutilized(int cpu)
{
- unsigned long rq_util_min = uclamp_rq_get(cpu_rq(cpu), UCLAMP_MIN);
- unsigned long rq_util_max = uclamp_rq_get(cpu_rq(cpu), UCLAMP_MAX);
+ unsigned long rq_util_min, rq_util_max;
+
+ if (!sched_energy_enabled())
+ return false;
+
+ rq_util_min = uclamp_rq_get(cpu_rq(cpu), UCLAMP_MIN);
+ rq_util_max = uclamp_rq_get(cpu_rq(cpu), UCLAMP_MAX);

/* Return true only if the utilization doesn't fit CPU's capacity */
return !util_fits_cpu(cpu_util_cfs(cpu), rq_util_min, rq_util_max, cpu);
}

-static inline void update_overutilized_status(struct rq *rq)
+static inline void set_rd_overutilized_status(struct root_domain *rd,
+ unsigned int status)
{
- if (!READ_ONCE(rq->rd->overutilized) && cpu_overutilized(rq->cpu)) {
- WRITE_ONCE(rq->rd->overutilized, SG_OVERUTILIZED);
- trace_sched_overutilized_tp(rq->rd, SG_OVERUTILIZED);
- }
+ if (!sched_energy_enabled())
+ return;
+
+ WRITE_ONCE(rd->overutilized, status);
+ trace_sched_overutilized_tp(rd, !!status);
+}
+
+static inline void check_update_overutilized_status(struct rq *rq)
+{
+ /*
+ * overutilized field is used for load balancing decisions only
+ * if energy aware scheduler is being used
+ */
+ if (!sched_energy_enabled())
+ return;
+
+ if (!READ_ONCE(rq->rd->overutilized) && cpu_overutilized(rq->cpu))
+ set_rd_overutilized_status(rq->rd, SG_OVERUTILIZED);
}
#else
-static inline void update_overutilized_status(struct rq *rq) { }
+static inline void check_update_overutilized_status(struct rq *rq) { }
#endif

/* Runqueue only has SCHED_IDLE tasks enqueued */
@@ -6147,7 +6167,7 @@ enqueue_task_fair(struct rq *rq, struct task_struct *p, int flags)
* and the following generally works well enough in practice.
*/
if (!task_new)
- update_overutilized_status(rq);
+ check_update_overutilized_status(rq);

enqueue_throttle:
assert_list_leaf_cfs_rq(rq);
@@ -9923,19 +9943,14 @@ static inline void update_sd_lb_stats(struct lb_env *env, struct sd_lb_stats *sd
env->fbq_type = fbq_classify_group(&sds->busiest_stat);

if (!env->sd->parent) {
- struct root_domain *rd = env->dst_rq->rd;
-
/* update overload indicator if we are at root domain */
- WRITE_ONCE(rd->overload, sg_status & SG_OVERLOAD);
+ WRITE_ONCE(env->dst_rq->rd->overload, sg_status & SG_OVERLOAD);

/* Update over-utilization (tipping point, U >= 0) indicator */
- WRITE_ONCE(rd->overutilized, sg_status & SG_OVERUTILIZED);
- trace_sched_overutilized_tp(rd, sg_status & SG_OVERUTILIZED);
+ set_rd_overutilized_status(env->dst_rq->rd,
+ sg_status & SG_OVERUTILIZED);
} else if (sg_status & SG_OVERUTILIZED) {
- struct root_domain *rd = env->dst_rq->rd;
-
- WRITE_ONCE(rd->overutilized, SG_OVERUTILIZED);
- trace_sched_overutilized_tp(rd, SG_OVERUTILIZED);
+ set_rd_overutilized_status(env->dst_rq->rd, SG_OVERUTILIZED);
}

update_idle_cpu_scan(env, sum_util);
@@ -11849,7 +11864,7 @@ static void task_tick_fair(struct rq *rq, struct task_struct *curr, int queued)
task_tick_numa(rq, curr);

update_misfit_status(curr, rq);
- update_overutilized_status(task_rq(curr));
+ check_update_overutilized_status(task_rq(curr));

task_tick_core(rq, curr);
}
--
2.43.0