By default FAIR tasks start without clamps, i.e. neither boosted nor capped, and they run at the best frequency matching their utilization demand. This default behavior does not fit RT tasks which instead are expected to run at the maximum available frequency, if not otherwise required by explicitly capping them.
Enforce the correct behavior for RT tasks by setting util_min to max whenever: 1. the task is switched to the RT class and it does not already have a user-defined clamp value assigned. 2. an RT task is forked from a parent with RESET_ON_FORK set. NOTE: utilization clamp values are cross scheduling class attributes and thus they are never changed/reset once a value has been explicitly defined from user-space. Signed-off-by: Patrick Bellasi <patrick.bell...@arm.com> Cc: Ingo Molnar <mi...@redhat.com> Cc: Peter Zijlstra <pet...@infradead.org> --- kernel/sched/core.c | 26 ++++++++++++++++++++++++++ 1 file changed, 26 insertions(+) diff --git a/kernel/sched/core.c b/kernel/sched/core.c index bdebdabe9bc4..71c9dd6487b1 100644 --- a/kernel/sched/core.c +++ b/kernel/sched/core.c @@ -1042,6 +1042,28 @@ static int uclamp_validate(struct task_struct *p, static void __setscheduler_uclamp(struct task_struct *p, const struct sched_attr *attr) { + unsigned int clamp_id; + + /* + * On scheduling class change, reset to default clamps for tasks + * without a task-specific value. + */ + for (clamp_id = 0; clamp_id < UCLAMP_CNT; ++clamp_id) { + struct uclamp_se *uc_se = &p->uclamp_req[clamp_id]; + unsigned int clamp_value = uclamp_none(clamp_id); + + /* Keep using defined clamps across class changes */ + if (uc_se->user_defined) + continue; + + /* By default, RT tasks always get 100% boost */ + if (unlikely(rt_task(p) && clamp_id == UCLAMP_MIN)) + clamp_value = uclamp_none(UCLAMP_MAX); + + uc_se->bucket_id = uclamp_bucket_id(clamp_value); + uc_se->value = clamp_value; + } + if (likely(!(attr->sched_flags & SCHED_FLAG_UTIL_CLAMP))) return; @@ -1077,6 +1099,10 @@ static void uclamp_fork(struct task_struct *p) for (clamp_id = 0; clamp_id < UCLAMP_CNT; ++clamp_id) { unsigned int clamp_value = uclamp_none(clamp_id); + /* By default, RT tasks always get 100% boost */ + if (unlikely(rt_task(p) && clamp_id == UCLAMP_MIN)) + clamp_value = uclamp_none(UCLAMP_MAX); + p->uclamp_req[clamp_id].user_defined = false; p->uclamp_req[clamp_id].value = clamp_value; p->uclamp_req[clamp_id].bucket_id = uclamp_bucket_id(clamp_value); -- 2.20.1