Consider a 4-CPU big.LITTLE system with CPUs 0-1 as LITTLEs and CPUs 2-3 as
bigs. The resulting sched_domain hierarchy is:
DIE [ ]
MC [ ][ ]
0 1 2 3
When running a multithreaded CPU-bound workload (i.e. 1 hog per CPU), the
expected behaviour is to have the about-to-idle big CPUs pull a hog from
the LITTLEs, since bigs will complete their work sooner than LITTLEs.
Further Consider a scenario where:
- CPU0 is idle (e.g. its hog got migrated to one of the big CPUs)
- CPU1 is currently executing a per-CPU kworker, preempting the CPU hog
- CPU2 and CPU3 are executing CPU-hogs
CPU0 goes through load_balance() at MC level, and tries to pick stuff from
CPU1, but:
- the hog can't be pulled, because it's task_hot()
- the kworker can't be pulled, because it's pinned to CPU1, which sets
LBF_SOME_PINNED
This load balance attempts ends with no load pulled, LBF_SOME_PINNED set,
and as a consequence we set the imbalance flag of DIE's [0, 1]
sched_group_capacity.
Shortly after, CPU2 completes its work and is about to go idle. It goes
through the newidle_balance(), and we would really like it to active
balance the hog running on CPU1 (which is a misfit task). However,
sgc->imbalance is set for the LITTLE group at DIE level, so the group gets
classified as group_imbalanced rather than group_misfit_task.
Unlike group_misfit_task (via migrate_misfit), the active balance logic
doesn't have any specific case for group_imbalanced, so CPU2 ends up going
idle. We'll have to wait for a load balance on CPU0 or CPU1 to happen and
clear the imbalance flag, and then for another DIE-level load-balance on
CPU2 to happen to pull the task off of CPU1. That's several precious
milliseconds wasted down the drain.
Giving group_misfit_task a higher group_classify() priority than
group_imbalance doesn't seem like the right thing to do. Instead, make
need_active_balance() return true for any migration_type when the
destination CPU is idle and the source CPU has a misfit task.
While at it, add an sd_has_asym_cpucapacity() guard in
need_active_balance().
Signed-off-by: Valentin Schneider <valentin.schnei...@arm.com>
---
kernel/sched/fair.c | 13 +++++++++++++
1 file changed, 13 insertions(+)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 0ac2f876b86f..cba9f97d9beb 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -9557,9 +9557,22 @@ static int need_active_balance(struct lb_env *env)
return 1;
}
+ if (!sd_has_asym_cpucapacity(sd))
+ return 0;
+
if (env->migration_type == migrate_misfit)
return 1;
+ /*
+ * If we failed to pull anything and the src_rq has a misfit task, but
+ * the busiest group_type was higher than group_misfit_task, try to
+ * go for a misfit active balance anyway.
+ */
+ if ((env->idle != CPU_NOT_IDLE) &&
+ env->src_rq->misfit_task_load &&
+ cpu_capacity_greater(env->dst_cpu, env->src_cpu))
+ return 1;
+
return 0;
}
--
2.27.0
