[PATCH v3] sched: Restart fair hrtick after same-task repicks
From: Shubhang Kaushik (Ampere)
Date: Tue Sep 15 2026 - 21:29:10 EST
Fair hrticks are one-shot timers. A same-task repick after hrtick expiry
skips set_next_task_fair() and leaves no hrtick armed for the next fair
preemption point:
hrtick()
-> task_tick_fair(..., queued=1)
-> entity_tick()
-> resched_curr()
-> schedule()
-> pick_task_fair() selects current
-> put_prev_set_next_task(rq, prev, next)
-> next == prev
-> return
Introduce SNT_REPICK for the next == prev path:
put_prev_set_next_task(rq, prev, next)
-> next == prev
-> next->sched_class->set_next_task(rq, next, SNT_REPICK)
-> set_next_task_fair()
-> hrtick_start_fair()
set_next_task_fair() skips task-transition work for SNT_REPICK and
restarts the fair hrtick. hrtick_start() records the delay during
schedule(), and hrtick_schedule_exit() rearms the timer.
Do not restart the DL hrtick for SNT_REPICK. put_prev_task_dl() is
skipped for next == prev, so dl_se->runtime may be stale.
Signed-off-by: Shubhang Kaushik (Ampere) <sh@xxxxxxxxxx>
---
Testing:
- HRTICK and DELAY_DEQUEUE enabled, base_slice_ns=3000000:
p90: 3.998ms baseline -> 3.053ms v3
p99: 5.144ms baseline -> 4.533ms v3
>4ms: 7.89% baseline -> 2.26% v3
>6ms: 0.309% baseline -> 0.013% v3
- HRTICK_DL stress-ng smoke test: baseline and v3 completed 2691
bogo ops with no new dmesg warnings.
- Also built with CONFIG_HIGH_RES_TIMERS=n, which disables
CONFIG_SCHED_HRTICK and CONFIG_HRTIMER_REARM_DEFERRED. The kernel
booted successfully; a 30-second stress-ng --cpu 2 run completed
without new dmesg warnings.
---
Changes in v3:
- Drop the SCHED_DEADLINE SNT_REPICK hrtick rearm from v2.
Link to v2: https://lore.kernel.org/r/20260911-sched-fair-hrtick-restart-v2-1-0d34db26ecd1@xxxxxxxxxx
Changes in v2:
- Replace the fair-specific hrtick rearm state with SNT_REPICK.
- Restart hrticks for both fair and SCHED_DEADLINE same-task repicks.
- Remove the delayed-dequeue runnable-count condition.
Link to v1: https://lore.kernel.org/r/20260813-sched-fair-hrtick-restart-v1-1-4230d1e18fbb@xxxxxxxxxx
---
kernel/sched/core.c | 2 +-
kernel/sched/deadline.c | 7 +++++--
kernel/sched/ext/ext.c | 5 ++++-
kernel/sched/fair.c | 15 ++++++++++-----
kernel/sched/idle.c | 5 ++++-
kernel/sched/rt.c | 7 +++++--
kernel/sched/sched.h | 16 ++++++++++++----
kernel/sched/stop_task.c | 5 ++++-
8 files changed, 45 insertions(+), 17 deletions(-)
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 7885ff76e69f28e1c81d72d87e5ceef0562ccea9..4f7d2139f29d5e6a9241e0bb87369bc20359955c 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -7222,7 +7222,7 @@ static void __sched notrace __schedule(int sched_mode)
* on_cpu.
*/
donor->sched_class->put_prev_task(rq, donor, donor);
- donor->sched_class->set_next_task(rq, donor, true);
+ donor->sched_class->set_next_task(rq, donor, SNT_PICK);
}
} else {
rq_set_donor(rq, next);
diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 0663c00c41c04213fda2ebaf16a0bdf34e0849a8..1502a45d76aaf44079d618d8ae193c9b213fc401 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -2773,11 +2773,14 @@ static void start_hrtick_dl(struct rq *rq, struct sched_dl_entity *dl_se)
* DL keeps current in tree, because ->deadline is not typically changed while
* a task is runnable.
*/
-static void set_next_task_dl(struct rq *rq, struct task_struct *p, bool first)
+static void set_next_task_dl(struct rq *rq, struct task_struct *p, enum snt_e type)
{
struct sched_dl_entity *dl_se = &p->dl;
struct dl_rq *dl_rq = &rq->dl;
+ if (type == SNT_REPICK)
+ return;
+
p->se.exec_start = rq_clock_task(rq);
if (on_dl_rq(&p->dl))
update_stats_wait_end_dl(dl_rq, dl_se);
@@ -2788,7 +2791,7 @@ static void set_next_task_dl(struct rq *rq, struct task_struct *p, bool first)
WARN_ON_ONCE(dl_rq->curr);
dl_rq->curr = dl_se;
- if (!first)
+ if (type != SNT_PICK)
return;
if (rq->donor->sched_class != &dl_sched_class)
diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
index 51de1d8b72a1ede6d16c07b58defb1284285b8a6..31a300f2d3b202d588ca666d6b8f6d4705ee6c79 100644
--- a/kernel/sched/ext/ext.c
+++ b/kernel/sched/ext/ext.c
@@ -3003,10 +3003,13 @@ static enum scx_dsp_verdict dispatch_one(struct rq *rq, struct task_struct *prev
return verdict;
}
-static void set_next_task_scx(struct rq *rq, struct task_struct *p, bool first)
+static void set_next_task_scx(struct rq *rq, struct task_struct *p, enum snt_e type)
{
struct scx_sched *sch = scx_task_sched(p);
+ if (type == SNT_REPICK)
+ return;
+
if (p->scx.flags & SCX_TASK_QUEUED) {
/*
* Core-sched might decide to execute @p before it is
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 7455a83a6a9907b4fd18c3a99df707438818709b..b46b180d363cf8a0e8fd1ad330c1cd36b71b313b 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -15253,14 +15253,18 @@ static void switched_to_fair(struct rq *rq, struct task_struct *p)
}
}
-static void set_next_task_fair(struct rq *rq, struct task_struct *p, bool first)
+static void set_next_task_fair(struct rq *rq, struct task_struct *p, enum snt_e type)
{
struct sched_entity *se = &p->se;
- bool throttled = false;
struct cfs_rq *cfs_rq = &rq->cfs;
unsigned long weight = NICE_0_LOAD;
+ bool first = type == SNT_PICK;
+ bool throttled = false;
bool on_rq = se->on_rq;
+ if (type == SNT_REPICK)
+ goto repick;
+
clear_buddies(cfs_rq, se);
if (on_rq)
@@ -15304,11 +15308,12 @@ static void set_next_task_fair(struct rq *rq, struct task_struct *p, bool first)
WARN_ON_ONCE(se->sched_delayed);
- if (hrtick_enabled_fair(rq))
- hrtick_start_fair(rq, p);
-
update_misfit_status(p, rq);
sched_fair_update_stop_tick(rq, p);
+
+repick:
+ if (hrtick_enabled_fair(rq))
+ hrtick_start_fair(rq, p);
}
void init_cfs_rq(struct cfs_rq *cfs_rq)
diff --git a/kernel/sched/idle.c b/kernel/sched/idle.c
index eb73b65ce6c4714db1e07066c86a8cf39adbc328..76f3c84ca6846afc30609fdc512553c6797e9557 100644
--- a/kernel/sched/idle.c
+++ b/kernel/sched/idle.c
@@ -487,8 +487,11 @@ static void put_prev_task_idle(struct rq *rq, struct task_struct *prev, struct t
update_rq_avg_idle(rq);
}
-static void set_next_task_idle(struct rq *rq, struct task_struct *next, bool first)
+static void set_next_task_idle(struct rq *rq, struct task_struct *next, enum snt_e type)
{
+ if (type == SNT_REPICK)
+ return;
+
update_idle_core(rq);
scx_update_idle(rq, true, true);
schedstat_inc(rq->sched_goidle);
diff --git a/kernel/sched/rt.c b/kernel/sched/rt.c
index 85303add726df4c2da846c427aab2d627fca9589..1535046a23ffa0705d90dce301fa4c5b015703a1 100644
--- a/kernel/sched/rt.c
+++ b/kernel/sched/rt.c
@@ -1654,11 +1654,14 @@ static void wakeup_preempt_rt(struct rq *rq, struct task_struct *p, int flags)
check_preempt_equal_prio(rq, p);
}
-static inline void set_next_task_rt(struct rq *rq, struct task_struct *p, bool first)
+static inline void set_next_task_rt(struct rq *rq, struct task_struct *p, enum snt_e type)
{
struct sched_rt_entity *rt_se = &p->rt;
struct rt_rq *rt_rq = &rq->rt;
+ if (type == SNT_REPICK)
+ return;
+
p->se.exec_start = rq_clock_task(rq);
if (on_rt_rq(&p->rt))
update_stats_wait_end_rt(rt_rq, rt_se);
@@ -1666,7 +1669,7 @@ static inline void set_next_task_rt(struct rq *rq, struct task_struct *p, bool f
/* The running task is never eligible for pushing */
dequeue_pushable_task(rq, p);
- if (!first)
+ if (type != SNT_PICK)
return;
/*
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index e656c7059bf864d1ed91d4ec3d4624850aded7e0..defcf7fd023fa5bb3c34c3c29f99afe3c42a87de 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2621,6 +2621,12 @@ struct affinity_context {
extern s64 update_curr_common(struct rq *rq);
+enum snt_e {
+ SNT_NORMAL,
+ SNT_PICK,
+ SNT_REPICK,
+};
+
struct sched_class {
#ifdef CONFIG_UCLAMP_TASK
@@ -2678,7 +2684,7 @@ struct sched_class {
* __schedule: rq->lock
*/
void (*put_prev_task)(struct rq *rq, struct task_struct *p, struct task_struct *next);
- void (*set_next_task)(struct rq *rq, struct task_struct *p, bool first);
+ void (*set_next_task)(struct rq *rq, struct task_struct *p, enum snt_e type);
/*
* select_task_rq: p->pi_lock
@@ -2781,7 +2787,7 @@ static inline void put_prev_task(struct rq *rq, struct task_struct *prev)
static inline void set_next_task(struct rq *rq, struct task_struct *next)
{
- next->sched_class->set_next_task(rq, next, false);
+ next->sched_class->set_next_task(rq, next, SNT_NORMAL);
}
static inline void
@@ -2802,11 +2808,13 @@ static inline void put_prev_set_next_task(struct rq *rq,
__put_prev_set_next_dl_server(rq, prev, next);
- if (next == prev)
+ if (next == prev) {
+ next->sched_class->set_next_task(rq, next, SNT_REPICK);
return;
+ }
prev->sched_class->put_prev_task(rq, prev, next);
- next->sched_class->set_next_task(rq, next, true);
+ next->sched_class->set_next_task(rq, next, SNT_PICK);
}
/*
diff --git a/kernel/sched/stop_task.c b/kernel/sched/stop_task.c
index c909ca0d8c87c1055ad5ffe58ae9f044194ee358..1e0109ec36b3f9b84f6f7290e584e38257db701c 100644
--- a/kernel/sched/stop_task.c
+++ b/kernel/sched/stop_task.c
@@ -27,8 +27,11 @@ wakeup_preempt_stop(struct rq *rq, struct task_struct *p, int flags)
/* we're never preempted */
}
-static void set_next_task_stop(struct rq *rq, struct task_struct *stop, bool first)
+static void set_next_task_stop(struct rq *rq, struct task_struct *stop, enum snt_e type)
{
+ if (type == SNT_REPICK)
+ return;
+
stop->se.exec_start = rq_clock_task(rq);
}
---
base-commit: f6e7b42bf05b2427fb8a7a1d1c387a86638bb413
change-id: 20260813-sched-fair-hrtick-restart-ab9d3d47ef78
Best regards,
--
Shubhang Kaushik (Ampere) <sh@xxxxxxxxxx>