[PATCH 10/12] sched_ext: Move event_stats_cpu into scx_sched

Tejun Heo
2025-04-25T21:58:55Z

The event counters are going to become per scheduler instance. Move
event_stats_cpu into scx_sched.

- [__]scx_add_event() are updated to take @sch. While at it, add missing
parentheses around @cnt expansions.

- scx_read_events() is updated to take @sch.

- scx_bpf_events() accesses scx_root under RCU read lock.

v2: - Replace stale scx_bpf_get_event_stat() reference in a comment with
scx_bpf_events().

- Trivial goto label rename.

Signed-off-by: Tejun Heo <tj@kernel.org>
---
kernel/sched/ext.c | 100 ++++++++++++++++++++++++++-------------------
1 file changed, 58 insertions(+), 42 deletions(-)

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 885cd71a847e..154c354856fc 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -838,6 +838,13 @@ struct scx_sched {
struct rhashtable dsq_hash;
struct scx_dispatch_q **global_dsqs;

+ /*
+ * The event counters are in a per-CPU variable to minimize the
+ * accounting overhead. A system-wide view on the event counter is
+ * constructed when requested by scx_bpf_events().
+ */
+ struct scx_event_stats __percpu *event_stats_cpu;
+
bool warned_zero_slice;

atomic_t exit_kind;
@@ -1599,34 +1606,29 @@ static struct task_struct *scx_task_iter_next_locked(struct scx_task_iter *iter)
return p;
}

-/*
- * The event counter is organized by a per-CPU variable to minimize the
- * accounting overhead without synchronization. A system-wide view on the
- * event counter is constructed when requested by scx_bpf_get_event_stat().
- */
-static DEFINE_PER_CPU(struct scx_event_stats, event_stats_cpu);
-
/**
* scx_add_event - Increase an event counter for 'name' by 'cnt'
+ * @sch: scx_sched to account events for
* @name: an event name defined in struct scx_event_stats
* @cnt: the number of the event occured
*
* This can be used when preemption is not disabled.
*/
-#define scx_add_event(name, cnt) do { \
- this_cpu_add(event_stats_cpu.name, cnt); \
- trace_sched_ext_event(#name, cnt); \
+#define scx_add_event(sch, name, cnt) do { \
+ this_cpu_add((sch)->event_stats_cpu->name, (cnt)); \
+ trace_sched_ext_event(#name, (cnt)); \
} while(0)

/**
* __scx_add_event - Increase an event counter for 'name' by 'cnt'
+ * @sch: scx_sched to account events for
* @name: an event name defined in struct scx_event_stats
* @cnt: the number of the event occured
*
* This should be used only when preemption is disabled.
*/
-#define __scx_add_event(name, cnt) do { \
- __this_cpu_add(event_stats_cpu.name, cnt); \
+#define __scx_add_event(sch, name, cnt) do { \
+ __this_cpu_add((sch)->event_stats_cpu->name, (cnt)); \
trace_sched_ext_event(#name, cnt); \
} while(0)

@@ -1651,7 +1653,8 @@ static DEFINE_PER_CPU(struct scx_event_stats, event_stats_cpu);
} while (0)

-static void scx_read_events(struct scx_event_stats *events);
+static void scx_read_events(struct scx_sched *sch,
+ struct scx_event_stats *events);

static enum scx_enable_state scx_enable_state(void)
{
@@ -1877,7 +1880,7 @@ static void dsq_mod_nr(struct scx_dispatch_q *dsq, s32 delta)
static void refill_task_slice_dfl(struct task_struct *p)
{
p->scx.slice = SCX_SLICE_DFL;
- __scx_add_event(SCX_EV_REFILL_SLICE_DFL, 1);
+ __scx_add_event(scx_root, SCX_EV_REFILL_SLICE_DFL, 1);
}

static void dispatch_enqueue(struct scx_dispatch_q *dsq, struct task_struct *p,
@@ -2206,7 +2209,7 @@ static void do_enqueue_task(struct rq *rq, struct task_struct *p, u64 enq_flags,
goto local;

if (scx_rq_bypassing(rq)) {
- __scx_add_event(SCX_EV_BYPASS_DISPATCH, 1);
+ __scx_add_event(sch, SCX_EV_BYPASS_DISPATCH, 1);
goto global;
}

@@ -2216,14 +2219,14 @@ static void do_enqueue_task(struct rq *rq, struct task_struct *p, u64 enq_flags,
/* see %SCX_OPS_ENQ_EXITING */
if (!(scx_root->ops.flags & SCX_OPS_ENQ_EXITING) &&
unlikely(p->flags & PF_EXITING)) {
- __scx_add_event(SCX_EV_ENQ_SKIP_EXITING, 1);
+ __scx_add_event(sch, SCX_EV_ENQ_SKIP_EXITING, 1);
goto local;
}

/* see %SCX_OPS_ENQ_MIGRATION_DISABLED */
if (!(scx_root->ops.flags & SCX_OPS_ENQ_MIGRATION_DISABLED) &&
is_migration_disabled(p)) {
- __scx_add_event(SCX_EV_ENQ_SKIP_MIGRATION_DISABLED, 1);
+ __scx_add_event(sch, SCX_EV_ENQ_SKIP_MIGRATION_DISABLED, 1);
goto local;
}

@@ -2346,7 +2349,7 @@ static void enqueue_task_scx(struct rq *rq, struct task_struct *p, int enq_flags

if ((enq_flags & SCX_ENQ_CPU_SELECTED) &&
unlikely(cpu_of(rq) != p->scx.selected_cpu))
- __scx_add_event(SCX_EV_SELECT_CPU_FALLBACK, 1);
+ __scx_add_event(scx_root, SCX_EV_SELECT_CPU_FALLBACK, 1);
}

static void ops_dequeue(struct rq *rq, struct task_struct *p, u64 deq_flags)
@@ -2574,7 +2577,8 @@ static bool task_can_run_on_remote_rq(struct task_struct *p, struct rq *rq,

if (!scx_rq_online(rq)) {
if (enforce)
- __scx_add_event(SCX_EV_DISPATCH_LOCAL_DSQ_OFFLINE, 1);
+ __scx_add_event(scx_root,
+ SCX_EV_DISPATCH_LOCAL_DSQ_OFFLINE, 1);
return false;
}

@@ -3096,7 +3100,7 @@ static int balance_one(struct rq *rq, struct task_struct *prev)
if (prev_on_rq &&
(!(scx_root->ops.flags & SCX_OPS_ENQ_LAST) || scx_rq_bypassing(rq))) {
rq->scx.flags |= SCX_RQ_BAL_KEEP;
- __scx_add_event(SCX_EV_DISPATCH_KEEP_LAST, 1);
+ __scx_add_event(sch, SCX_EV_DISPATCH_KEEP_LAST, 1);
goto has_tasks;
}
rq->scx.flags &= ~SCX_RQ_IN_BALANCE;
@@ -3427,6 +3431,7 @@ bool scx_prio_less(const struct task_struct *a, const struct task_struct *b,

static int select_task_rq_scx(struct task_struct *p, int prev_cpu, int wake_flags)
{
+ struct scx_sched *sch = scx_root;
bool rq_bypass;

/*
@@ -3443,7 +3448,7 @@ static int select_task_rq_scx(struct task_struct *p, int prev_cpu, int wake_flag
return prev_cpu;

rq_bypass = scx_rq_bypassing(task_rq(p));
- if (likely(SCX_HAS_OP(scx_root, select_cpu)) && !rq_bypass) {
+ if (likely(SCX_HAS_OP(sch, select_cpu)) && !rq_bypass) {
s32 cpu;
struct task_struct **ddsp_taskp;

@@ -3472,7 +3477,7 @@ static int select_task_rq_scx(struct task_struct *p, int prev_cpu, int wake_flag
p->scx.selected_cpu = cpu;

if (rq_bypass)
- __scx_add_event(SCX_EV_BYPASS_DISPATCH, 1);
+ __scx_add_event(sch, SCX_EV_BYPASS_DISPATCH, 1);
return cpu;
}
}
@@ -4413,6 +4418,8 @@ static void scx_sched_free_rcu_work(struct work_struct *work)
struct scx_dispatch_q *dsq;
int node;

+ free_percpu(sch->event_stats_cpu);
+
for_each_node_state(node, N_POSSIBLE)
kfree(sch->global_dsqs[node]);
kfree(sch->global_dsqs);
@@ -4455,10 +4462,11 @@ SCX_ATTR(ops);
static ssize_t scx_attr_events_show(struct kobject *kobj,
struct kobj_attribute *ka, char *buf)
{
+ struct scx_sched *sch = container_of(kobj, struct scx_sched, kobj);
struct scx_event_stats events;
int at = 0;

- scx_read_events(&events);
+ scx_read_events(sch, &events);
at += scx_attr_event_show(buf, at, &events, SCX_EV_SELECT_CPU_FALLBACK);
at += scx_attr_event_show(buf, at, &events, SCX_EV_DISPATCH_LOCAL_DSQ_OFFLINE);
at += scx_attr_event_show(buf, at, &events, SCX_EV_DISPATCH_KEEP_LAST);
@@ -4591,25 +4599,29 @@ static void scx_bypass(bool bypass)
{
static DEFINE_RAW_SPINLOCK(bypass_lock);
static unsigned long bypass_timestamp;
-
- int cpu;
+ struct scx_sched *sch;
unsigned long flags;
+ int cpu;

raw_spin_lock_irqsave(&bypass_lock, flags);
+ sch = rcu_dereference_bh(scx_root);
+
if (bypass) {
scx_bypass_depth++;
WARN_ON_ONCE(scx_bypass_depth <= 0);
if (scx_bypass_depth != 1)
goto unlock;
bypass_timestamp = ktime_get_ns();
- scx_add_event(SCX_EV_BYPASS_ACTIVATE, 1);
+ if (sch)
+ scx_add_event(sch, SCX_EV_BYPASS_ACTIVATE, 1);
} else {
scx_bypass_depth--;
WARN_ON_ONCE(scx_bypass_depth < 0);
if (scx_bypass_depth != 0)
goto unlock;
- scx_add_event(SCX_EV_BYPASS_DURATION,
- ktime_get_ns() - bypass_timestamp);
+ if (sch)
+ scx_add_event(sch, SCX_EV_BYPASS_DURATION,
+ ktime_get_ns() - bypass_timestamp);
}

atomic_inc(&scx_breather_depth);
@@ -5182,7 +5194,7 @@ static void scx_dump_state(struct scx_exit_info *ei, size_t dump_len)
dump_line(&s, "Event counters");
dump_line(&s, "--------------");

- scx_read_events(&events);
+ scx_read_events(scx_root, &events);
scx_dump_event(s, &events, SCX_EV_SELECT_CPU_FALLBACK);
scx_dump_event(s, &events, SCX_EV_DISPATCH_LOCAL_DSQ_OFFLINE);
scx_dump_event(s, &events, SCX_EV_DISPATCH_KEEP_LAST);
@@ -5290,6 +5302,10 @@ static struct scx_sched *scx_alloc_and_add_sched(struct sched_ext_ops *ops)
sch->global_dsqs[node] = dsq;
}

+ sch->event_stats_cpu = alloc_percpu(struct scx_event_stats);
+ if (!sch->event_stats_cpu)
+ goto err_free_gdsqs;
+
atomic_set(&sch->exit_kind, SCX_EXIT_NONE);
sch->ops = *ops;
ops->priv = sch;
@@ -5297,10 +5313,12 @@ static struct scx_sched *scx_alloc_and_add_sched(struct sched_ext_ops *ops)
sch->kobj.kset = scx_kset;
ret = kobject_init_and_add(&sch->kobj, &scx_ktype, NULL, "root");
if (ret < 0)
- goto err_free_gdsqs;
+ goto err_free_event_stats;

return sch;

+err_free_event_stats:
+ free_percpu(sch->event_stats_cpu);
err_free_gdsqs:
for_each_node_state(node, N_POSSIBLE)
kfree(sch->global_dsqs[node]);
@@ -5376,15 +5394,6 @@ static int scx_enable(struct sched_ext_ops *ops, struct bpf_link *link)

mutex_lock(&scx_enable_mutex);

- /*
- * Clear event counters so a new scx scheduler gets
- * fresh event counter values.
- */
- for_each_possible_cpu(cpu) {
- struct scx_event_stats *e = per_cpu_ptr(&event_stats_cpu, cpu);
- memset(e, 0, sizeof(*e));
- }
-
if (!scx_helper) {
WRITE_ONCE(scx_helper, scx_create_rt_helper("sched_ext_helper"));
if (!scx_helper) {
@@ -7405,7 +7414,7 @@ __bpf_kfunc u64 scx_bpf_now(void)
return clock;
}

-static void scx_read_events(struct scx_event_stats *events)
+static void scx_read_events(struct scx_sched *sch, struct scx_event_stats *events)
{
struct scx_event_stats *e_cpu;
int cpu;
@@ -7413,7 +7422,7 @@ static void scx_read_events(struct scx_event_stats *events)
/* Aggregate per-CPU event counters into @events. */
memset(events, 0, sizeof(*events));
for_each_possible_cpu(cpu) {
- e_cpu = per_cpu_ptr(&event_stats_cpu, cpu);
+ e_cpu = per_cpu_ptr(sch->event_stats_cpu, cpu);
scx_agg_event(events, e_cpu, SCX_EV_SELECT_CPU_FALLBACK);
scx_agg_event(events, e_cpu, SCX_EV_DISPATCH_LOCAL_DSQ_OFFLINE);
scx_agg_event(events, e_cpu, SCX_EV_DISPATCH_KEEP_LAST);
@@ -7434,9 +7443,16 @@ static void scx_read_events(struct scx_event_stats *events)
__bpf_kfunc void scx_bpf_events(struct scx_event_stats *events,
size_t events__sz)
{
+ struct scx_sched *sch;
struct scx_event_stats e_sys;

- scx_read_events(&e_sys);
+ rcu_read_lock();
+ sch = rcu_dereference(scx_root);
+ if (sch)
+ scx_read_events(sch, &e_sys);
+ else
+ memset(&e_sys, 0, sizeof(e_sys));
+ rcu_read_unlock();

/*
* We cannot entirely trust a BPF-provided size since a BPF program
--
2.49.0