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Message-ID: <20250702121158.465086194@infradead.org>
Date: Wed, 02 Jul 2025 13:49:26 +0200
From: Peter Zijlstra <peterz@...radead.org>
To: mingo@...hat.com,
juri.lelli@...hat.com,
vincent.guittot@...aro.org,
dietmar.eggemann@....com,
rostedt@...dmis.org,
bsegall@...gle.com,
mgorman@...e.de,
vschneid@...hat.com,
clm@...a.com
Cc: linux-kernel@...r.kernel.org,
peterz@...radead.org
Subject: [PATCH v2 02/12] sched/deadline: Less agressive dl_server handling
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 <clm@...a.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
Link: https://lkml.kernel.org/r/20250520101727.507378961@infradead.org
---
include/linux/sched.h | 1 +
kernel/sched/deadline.c | 25 ++++++++++++++++++++++---
kernel/sched/fair.c | 9 ---------
3 files changed, 23 insertions(+), 12 deletions(-)
--- a/include/linux/sched.h
+++ b/include/linux/sched.h
@@ -701,6 +701,7 @@ 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
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1215,6 +1215,8 @@ static void __push_dl_task(struct rq *rq
/* 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);
@@ -1234,6 +1236,7 @@ static enum hrtimer_restart dl_server_ti
if (!dl_se->server_has_tasks(dl_se)) {
replenish_dl_entity(dl_se);
+ dl_server_stopped(dl_se);
return HRTIMER_NORESTART;
}
@@ -1639,8 +1642,10 @@ void dl_server_update_idle_time(struct r
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);
+ }
}
void dl_server_start(struct sched_dl_entity *dl_se)
@@ -1663,7 +1668,7 @@ void dl_server_start(struct sched_dl_ent
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;
@@ -1684,6 +1689,20 @@ void dl_server_stop(struct sched_dl_enti
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)
@@ -2435,7 +2454,7 @@ static struct task_struct *__pick_task_d
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);
}
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -5879,7 +5879,6 @@ static bool throttle_cfs_rq(struct cfs_r
struct cfs_bandwidth *cfs_b = tg_cfs_bandwidth(cfs_rq->tg);
struct sched_entity *se;
long queued_delta, runnable_delta, idle_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 */
@@ -5963,10 +5962,6 @@ static bool throttle_cfs_rq(struct cfs_r
/* 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
@@ -7060,7 +7055,6 @@ static void set_next_buddy(struct sched_
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;
@@ -7144,9 +7138,6 @@ static int dequeue_entities(struct rq *r
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;
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