sched/fair: Fix NO_RUN_TO_PARITY case

EEVDF expects the scheduler to allocate a time quantum to the selected
entity and then pick a new entity for next quantum.
Although this notion of time quantum is not strictly doable in our case,
we can ensure a minimum runtime for each task most of the time and pick a
new entity after a minimum time has elapsed.
Reuse the slice protection of run to parity to ensure such runtime
quantum.

Signed-off-by: Vincent Guittot <vincent.guittot@linaro.org>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20250708165630.1948751-3-vincent.guittot@linaro.org

diff --git a/include/linux/sched.h b/include/linux/sched.h
index 4802fcf738cded1a10eb3693f4d252772ef44d99..55921385927d8ccb632b285cf29e6f353205d1e0 100644 (file)
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -583,7 +583,15 @@ struct sched_entity {
        u64                             sum_exec_runtime;
        u64                             prev_sum_exec_runtime;
        u64                             vruntime;
-       s64                             vlag;
+       union {
+               /*
+                * When !@on_rq this field is vlag.
+                * When cfs_rq->curr == se (which implies @on_rq)
+                * this field is vprot. See protect_slice().
+                */
+               s64                     vlag;
+               u64                     vprot;
+       };
        u64                             slice;
 
        u64                             nr_migrations;

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 43fe5c831dd59787fdafac19b967bde50b9da706..8d288df79a6bd750e2e79cf1c412d5482bd2521b 100644 (file)
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -882,23 +882,35 @@ struct sched_entity *__pick_first_entity(struct cfs_rq *cfs_rq)
 }
 
 /*
- * HACK, stash a copy of deadline at the point of pick in vlag,
- * which isn't used until dequeue.
+ * Set the vruntime up to which an entity can run before looking
+ * for another entity to pick.
+ * In case of run to parity, we protect the entity up to its deadline.
+ * When run to parity is disabled, we give a minimum quantum to the running
+ * entity to ensure progress.
  */
 static inline void set_protect_slice(struct sched_entity *se)
 {
-       se->vlag = se->deadline;
+       u64 slice = se->slice;
+       u64 vprot = se->deadline;
+
+       if (!sched_feat(RUN_TO_PARITY))
+               slice = min(slice, normalized_sysctl_sched_base_slice);
+
+       if (slice != se->slice)
+               vprot = min_vruntime(vprot, se->vruntime + calc_delta_fair(slice, se));
+
+       se->vprot = vprot;
 }
 
 static inline bool protect_slice(struct sched_entity *se)
 {
-       return se->vlag == se->deadline;
+       return ((s64)(se->vprot - se->vruntime) > 0);
 }
 
 static inline void cancel_protect_slice(struct sched_entity *se)
 {
        if (protect_slice(se))
-               se->vlag = se->deadline + 1;
+               se->vprot = se->vruntime;
 }
 
 /*
@@ -937,7 +949,7 @@ static struct sched_entity *pick_eevdf(struct cfs_rq *cfs_rq)
        if (curr && (!curr->on_rq || !entity_eligible(cfs_rq, curr)))
                curr = NULL;
 
-       if (sched_feat(RUN_TO_PARITY) && curr && protect_slice(curr))
+       if (curr && protect_slice(curr))
                return curr;
 
        /* Pick the leftmost entity if it's eligible */
@@ -1156,11 +1168,8 @@ static inline void update_curr_task(struct task_struct *p, s64 delta_exec)
        cgroup_account_cputime(p, delta_exec);
 }
 
-static inline bool did_preempt_short(struct cfs_rq *cfs_rq, struct sched_entity *curr)
+static inline bool resched_next_slice(struct cfs_rq *cfs_rq, struct sched_entity *curr)
 {
-       if (!sched_feat(PREEMPT_SHORT))
-               return false;
-
        if (protect_slice(curr))
                return false;
 
@@ -1248,7 +1257,7 @@ static void update_curr(struct cfs_rq *cfs_rq)
        if (cfs_rq->nr_queued == 1)
                return;
 
-       if (resched || did_preempt_short(cfs_rq, curr)) {
+       if (resched || resched_next_slice(cfs_rq, curr)) {
                resched_curr_lazy(rq);
                clear_buddies(cfs_rq, curr);
        }