[PATCH 9/9] sched,fair: Alternative sched_slice()

From: Peter Zijlstra
Date: Fri Mar 26 2021 - 06:44:52 EST


The current sched_slice() seems to have issues; there's two possible
things that could be improved:

- the 'nr_running' used for __sched_period() is daft when cgroups are
considered. Using the RQ wide h_nr_running seems like a much more
consistent number.

- (esp) cgroups can slice it real fine, which makes for easy
over-scheduling, ensure min_gran is what the name says.

Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
---
kernel/sched/fair.c | 15 ++++++++++++++-
kernel/sched/features.h | 3 +++
2 files changed, 17 insertions(+), 1 deletion(-)

--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -680,7 +680,16 @@ static u64 __sched_period(unsigned long
*/
static u64 sched_slice(struct cfs_rq *cfs_rq, struct sched_entity *se)
{
- u64 slice = __sched_period(cfs_rq->nr_running + !se->on_rq);
+ unsigned int nr_running = cfs_rq->nr_running;
+ u64 slice;
+
+ if (sched_feat(ALT_PERIOD))
+ nr_running = rq_of(cfs_rq)->cfs.h_nr_running;
+
+ slice = __sched_period(nr_running + !se->on_rq);
+
+ if (sched_feat(BASE_SLICE))
+ slice -= sysctl_sched_min_granularity;

for_each_sched_entity(se) {
struct load_weight *load;
@@ -697,6 +706,10 @@ static u64 sched_slice(struct cfs_rq *cf
}
slice = __calc_delta(slice, se->load.weight, load);
}
+
+ if (sched_feat(BASE_SLICE))
+ slice += sysctl_sched_min_granularity;
+
return slice;
}

--- a/kernel/sched/features.h
+++ b/kernel/sched/features.h
@@ -90,3 +90,6 @@ SCHED_FEAT(WA_BIAS, true)
*/
SCHED_FEAT(UTIL_EST, true)
SCHED_FEAT(UTIL_EST_FASTUP, true)
+
+SCHED_FEAT(ALT_PERIOD, true)
+SCHED_FEAT(BASE_SLICE, true)