[PATCH 6.12.y 1/3] sched/deadline: Less agressive dl_server handling
From: Lukas Beckmann <hidden>
Date: 2026-05-22 21:31:34
Also in:
stable
Subsystem:
scheduler, the rest · Maintainers:
Ingo Molnar, Peter Zijlstra, Juri Lelli, Vincent Guittot, Linus Torvalds
From: Peter Zijlstra <peterz@infradead.org>
commit cccb45d7c4295bbfeba616582d0249f2d21e6df5 upstream.
Chris reported that commit 5f6bd380c7bd ("sched/rt: Remove default
bandwidth control") caused a significant dip in his favourite
benchmark of the day. Simply disabling dl_server cured things.
His workload hammers the 0->1, 1->0 transitions, and the
dl_server_{start,stop}() overhead kills it -- fairly obviously a bad
idea in hind sight and all that.
Change things around to only disable the dl_server when there has not
been a fair task around for a whole period. Since the default period
is 1 second, this ensures the benchmark never trips this, overhead
gone.
Fixes: 557a6bfc662c ("sched/fair: Add trivial fair server")
Reported-by: Chris Mason <redacted>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Juri Lelli <juri.lelli@redhat.com>
Acked-by: Juri Lelli <juri.lelli@redhat.com>
Link: https://lkml.kernel.org/r/20250702121158.465086194@infradead.org
[ adjust context for renamed/removed variable names ]
Signed-off-by: Lukas Beckmann <redacted>
---
include/linux/sched.h | 1 +
kernel/sched/deadline.c | 25 ++++++++++++++++++++++---
kernel/sched/fair.c | 9 ---------
3 files changed, 23 insertions(+), 12 deletions(-)
diff --git a/include/linux/sched.h b/include/linux/sched.h
index 2e4c437c7c90..299a65a92d2e 100644
--- a/include/linux/sched.h
+++ b/include/linux/sched.h@@ -674,6 +674,7 @@ struct sched_dl_entity { unsigned int dl_defer : 1; unsigned int dl_defer_armed : 1; unsigned int dl_defer_running : 1; + unsigned int dl_server_idle : 1; /* * Bandwidth enforcement timer. Each -deadline task has its
diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 1ef891f8e3f2..9c5fa95b345a 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c@@ -1201,6 +1201,8 @@ static void __push_dl_task(struct rq *rq, struct rq_flags *rf) /* a defer timer will not be reset if the runtime consumed was < dl_server_min_res */ static const u64 dl_server_min_res = 1 * NSEC_PER_MSEC; +static bool dl_server_stopped(struct sched_dl_entity *dl_se); + static enum hrtimer_restart dl_server_timer(struct hrtimer *timer, struct sched_dl_entity *dl_se) { struct rq *rq = rq_of_dl_se(dl_se);
@@ -1220,6 +1222,7 @@ static enum hrtimer_restart dl_server_timer(struct hrtimer *timer, struct sched_ if (!dl_se->server_has_tasks(dl_se)) { replenish_dl_entity(dl_se); + dl_server_stopped(dl_se); return HRTIMER_NORESTART; }
@@ -1626,8 +1629,10 @@ void dl_server_update_idle_time(struct rq *rq, struct task_struct *p) void dl_server_update(struct sched_dl_entity *dl_se, s64 delta_exec) { /* 0 runtime = fair server disabled */ - if (dl_se->dl_runtime) + if (dl_se->dl_runtime) { + dl_se->dl_server_idle = 0; update_curr_dl_se(dl_se->rq, dl_se, delta_exec); + } } /*
@@ -1850,7 +1855,7 @@ void dl_server_start(struct sched_dl_entity *dl_se) setup_new_dl_entity(dl_se); } - if (!dl_se->dl_runtime) + if (!dl_se->dl_runtime || dl_se->dl_server_active) return; dl_se->dl_server_active = 1;
@@ -1871,6 +1876,20 @@ void dl_server_stop(struct sched_dl_entity *dl_se) dl_se->dl_server_active = 0; } +static bool dl_server_stopped(struct sched_dl_entity *dl_se) +{ + if (!dl_se->dl_server_active) + return false; + + if (dl_se->dl_server_idle) { + dl_server_stop(dl_se); + return true; + } + + dl_se->dl_server_idle = 1; + return false; +} + void dl_server_init(struct sched_dl_entity *dl_se, struct rq *rq, dl_server_has_tasks_f has_tasks, dl_server_pick_f pick_task)
@@ -2628,7 +2647,7 @@ static struct task_struct *__pick_task_dl(struct rq *rq) if (dl_server(dl_se)) { p = dl_se->server_pick_task(dl_se); if (!p) { - if (dl_server_active(dl_se)) { + if (!dl_server_stopped(dl_se)) { dl_se->dl_yielded = 1; update_curr_dl_se(rq, dl_se, 0); }
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index a0a47e50b71c..d26e078d0623 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c@@ -5972,7 +5972,6 @@ static bool throttle_cfs_rq(struct cfs_rq *cfs_rq) struct cfs_bandwidth *cfs_b = tg_cfs_bandwidth(cfs_rq->tg); struct sched_entity *se; long queued_delta, runnable_delta, idle_task_delta, delayed_delta, dequeue = 1; - long rq_h_nr_queued = rq->cfs.h_nr_queued; raw_spin_lock(&cfs_b->lock); /* This will start the period timer if necessary */
@@ -6059,10 +6058,6 @@ static bool throttle_cfs_rq(struct cfs_rq *cfs_rq) /* At this point se is NULL and we are at root level*/ sub_nr_running(rq, queued_delta); - - /* Stop the fair server if throttling resulted in no runnable tasks */ - if (rq_h_nr_queued && !rq->cfs.h_nr_queued) - dl_server_stop(&rq->fair_server); done: /* * Note: distribution will already see us throttled via the
@@ -7162,7 +7157,6 @@ static void set_next_buddy(struct sched_entity *se); static int dequeue_entities(struct rq *rq, struct sched_entity *se, int flags) { bool was_sched_idle = sched_idle_rq(rq); - int rq_h_nr_queued = rq->cfs.h_nr_queued; bool task_sleep = flags & DEQUEUE_SLEEP; bool task_delayed = flags & DEQUEUE_DELAYED; struct task_struct *p = NULL;
@@ -7251,9 +7245,6 @@ static int dequeue_entities(struct rq *rq, struct sched_entity *se, int flags) sub_nr_running(rq, h_nr_queued); - if (rq_h_nr_queued && !rq->cfs.h_nr_queued) - dl_server_stop(&rq->fair_server); - /* balance early to pull high priority tasks */ if (unlikely(!was_sched_idle && sched_idle_rq(rq))) rq->next_balance = jiffies;
--
2.54.0