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sched/deadline: Less agressive dl_server handling

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Chris reported that commit 5f6bd380c7 ("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: 557a6bfc66 ("sched/fair: Add trivial fair server")
Reported-by: Chris Mason <clm@meta.com>
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
This commit is contained in:
Peter Zijlstra 2025-05-20 11:19:30 +02:00
parent 570c8efd5e
commit cccb45d7c4
3 changed files with 23 additions and 12 deletions
repo.diff.show_diff_stats Download patch file Download diff file Expand all files Collapse all files

1
include/linux/sched.h
View file

@ -698,6 +698,7 @@ 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

25
kernel/sched/deadline.c
View file

@ -1150,6 +1150,8 @@ 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);
@ -1169,6 +1171,7 @@ 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;
}
@ -1572,8 +1575,10 @@ 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);
}
}
void dl_server_start(struct sched_dl_entity *dl_se)
@ -1596,7 +1601,7 @@ 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;
@ -1617,6 +1622,20 @@ 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)
@ -2354,7 +2373,7 @@ 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);
}

9
kernel/sched/fair.c
View file

@ -5802,7 +5802,6 @@ 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_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 */
@ -5886,10 +5885,6 @@ 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
@ -6966,7 +6961,6 @@ 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;
@ -7050,9 +7044,6 @@ 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;