[PATCH 30/34] sched_ext: Implement core-sched support

From: Tejun Heo
Date: Mon Jul 10 2023 - 21:17:27 EST

The core-sched support is composed of the following parts:

* task_struct->scx.core_sched_at is added. This is a timestamp which can be
used to order tasks. Depending on whether the BPF scheduler implements
custom ordering, it tracks either global FIFO ordering of all tasks or
local-DSQ ordering within the dispatched tasks on a CPU.

* prio_less() is updated to call scx_prio_less() when comparing SCX tasks.
scx_prio_less() calls ops.core_sched_before() if available or uses the
core_sched_at timestamp. For global FIFO ordering, the BPF scheduler
doesn't need to do anything. Otherwise, it should implement
ops.core_sched_before() which reflects the ordering.

* When core-sched is enabled, balance_scx() balances all SMT siblings so
that they all have tasks dispatched if necessary before pick_task_scx() is
called. pick_task_scx() picks between the current task and the first
dispatched task on the local DSQ based on availability and the
core_sched_at timestamps. Note that FIFO ordering is expected among the
already dispatched tasks whether running or on the local DSQ, so this path
always compares core_sched_at instead of calling into
ops.core_sched_before().

qmap_core_sched_before() is added to scx_qmap. It scales the
distances from the heads of the queues to compare the tasks across different
priority queues and seems to behave as expected.

v3: * Fixed build error when !CONFIG_SCHED_SMT reported by Andrea Righi.

v2: * Sched core added the const qualifiers to prio_less task arguments.
Explicitly drop them for ops.core_sched_before() task arguments. BPF
enforces access control through the verifier, so the qualifier isn't
actually operative and only gets in the way when interacting with
various helpers.

Signed-off-by: Tejun Heo <tj@xxxxxxxxxx>
Reviewed-by: David Vernet <dvernet@xxxxxxxx>
Reviewed-by: Josh Don <joshdon@xxxxxxxxxx>
Cc: Andrea Righi <andrea.righi@xxxxxxxxxxxxx>
---
include/linux/sched/ext.h | 21 ++++
kernel/Kconfig.preempt | 2 +-
kernel/sched/core.c | 10 +-
kernel/sched/ext.c | 220 +++++++++++++++++++++++++++++++--
kernel/sched/ext.h | 13 ++
tools/sched_ext/scx_qmap.bpf.c | 87 ++++++++++++-
tools/sched_ext/scx_qmap.c | 5 +-
7 files changed, 341 insertions(+), 17 deletions(-)

diff --git a/include/linux/sched/ext.h b/include/linux/sched/ext.h
index c17957bd75df..9ef4c3a90199 100644
--- a/include/linux/sched/ext.h
+++ b/include/linux/sched/ext.h
@@ -316,6 +316,24 @@ struct sched_ext_ops {
*/
bool (*yield)(struct task_struct *from, struct task_struct *to);

+ /**
+ * core_sched_before - Task ordering for core-sched
+ * @a: task A
+ * @b: task B
+ *
+ * Used by core-sched to determine the ordering between two tasks. See
+ * Documentation/admin-guide/hw-vuln/core-scheduling.rst for details on
+ * core-sched.
+ *
+ * Both @a and @b are runnable and may or may not currently be queued on
+ * the BPF scheduler. Should return %true if @a should run before @b.
+ * %false if there's no required ordering or @b should run before @a.
+ *
+ * If not specified, the default is ordering them according to when they
+ * became runnable.
+ */
+ bool (*core_sched_before)(struct task_struct *a,struct task_struct *b);
+
/**
* set_weight - Set task weight
* @p: task to set weight for
@@ -627,6 +645,9 @@ struct sched_ext_entity {
struct task_struct *kf_tasks[2]; /* see SCX_CALL_OP_TASK() */
atomic64_t ops_state;
unsigned long runnable_at;
+#ifdef CONFIG_SCHED_CORE
+ u64 core_sched_at; /* see scx_prio_less() */
+#endif

/* BPF scheduler modifiable fields */

diff --git a/kernel/Kconfig.preempt b/kernel/Kconfig.preempt
index 0afcda19bc50..e12a057ead7b 100644
--- a/kernel/Kconfig.preempt
+++ b/kernel/Kconfig.preempt
@@ -135,7 +135,7 @@ config SCHED_CORE

config SCHED_CLASS_EXT
bool "Extensible Scheduling Class"
- depends on BPF_SYSCALL && BPF_JIT && !SCHED_CORE
+ depends on BPF_SYSCALL && BPF_JIT
help
This option enables a new scheduler class sched_ext (SCX), which
allows scheduling policies to be implemented as BPF programs to
diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 878e84694a6e..972996c05263 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -167,7 +167,10 @@ static inline int __task_prio(const struct task_struct *p)
if (p->sched_class == &idle_sched_class)
return MAX_RT_PRIO + NICE_WIDTH; /* 140 */

- return MAX_RT_PRIO + MAX_NICE; /* 120, squash fair */
+ if (task_on_scx(p))
+ return MAX_RT_PRIO + MAX_NICE + 1; /* 120, squash ext */
+
+ return MAX_RT_PRIO + MAX_NICE; /* 119, squash fair */
}

/*
@@ -196,6 +199,11 @@ static inline bool prio_less(const struct task_struct *a,
if (pa == MAX_RT_PRIO + MAX_NICE) /* fair */
return cfs_prio_less(a, b, in_fi);

+#ifdef CONFIG_SCHED_CLASS_EXT
+ if (pa == MAX_RT_PRIO + MAX_NICE + 1) /* ext */
+ return scx_prio_less(a, b, in_fi);
+#endif
+
return false;
}

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index e54d8c7d19a9..115d4d76cbc6 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -529,6 +529,49 @@ static int ops_sanitize_err(const char *ops_name, s32 err)
return -EPROTO;
}

+/**
+ * touch_core_sched - Update timestamp used for core-sched task ordering
+ * @rq: rq to read clock from, must be locked
+ * @p: task to update the timestamp for
+ *
+ * Update @p->scx.core_sched_at timestamp. This is used by scx_prio_less() to
+ * implement global or local-DSQ FIFO ordering for core-sched. Should be called
+ * when a task becomes runnable and its turn on the CPU ends (e.g. slice
+ * exhaustion).
+ */
+static void touch_core_sched(struct rq *rq, struct task_struct *p)
+{
+#ifdef CONFIG_SCHED_CORE
+ /*
+ * It's okay to update the timestamp spuriously. Use
+ * sched_core_disabled() which is cheaper than enabled().
+ */
+ if (!sched_core_disabled())
+ p->scx.core_sched_at = rq_clock_task(rq);
+#endif
+}
+
+/**
+ * touch_core_sched_dispatch - Update core-sched timestamp on dispatch
+ * @rq: rq to read clock from, must be locked
+ * @p: task being dispatched
+ *
+ * If the BPF scheduler implements custom core-sched ordering via
+ * ops.core_sched_before(), @p->scx.core_sched_at is used to implement FIFO
+ * ordering within each local DSQ. This function is called from dispatch paths
+ * and updates @p->scx.core_sched_at if custom core-sched ordering is in effect.
+ */
+static void touch_core_sched_dispatch(struct rq *rq, struct task_struct *p)
+{
+ lockdep_assert_rq_held(rq);
+ assert_clock_updated(rq);
+
+#ifdef CONFIG_SCHED_CORE
+ if (SCX_HAS_OP(core_sched_before))
+ touch_core_sched(rq, p);
+#endif
+}
+
static void update_curr_scx(struct rq *rq)
{
struct task_struct *curr = rq->curr;
@@ -544,8 +587,11 @@ static void update_curr_scx(struct rq *rq)
account_group_exec_runtime(curr, delta_exec);
cgroup_account_cputime(curr, delta_exec);

- if (curr->scx.slice != SCX_SLICE_INF)
+ if (curr->scx.slice != SCX_SLICE_INF) {
curr->scx.slice -= min(curr->scx.slice, delta_exec);
+ if (!curr->scx.slice)
+ touch_core_sched(rq, curr);
+ }
}

static void dispatch_enqueue(struct scx_dispatch_q *dsq, struct task_struct *p,
@@ -701,6 +747,8 @@ static void direct_dispatch(struct task_struct *ddsp_task, struct task_struct *p
return;
}

+ touch_core_sched_dispatch(task_rq(p), p);
+
dsq = find_dsq_for_dispatch(task_rq(p), dsq_id, p);
dispatch_enqueue(dsq, p, enq_flags | SCX_ENQ_CLEAR_OPSS);

@@ -784,12 +832,19 @@ static void do_enqueue_task(struct rq *rq, struct task_struct *p, u64 enq_flags,
return;

local:
+ /*
+ * For task-ordering, slice refill must be treated as implying the end
+ * of the current slice. Otherwise, the longer @p stays on the CPU, the
+ * higher priority it becomes from scx_prio_less()'s POV.
+ */
+ touch_core_sched(rq, p);
p->scx.slice = SCX_SLICE_DFL;
local_norefill:
dispatch_enqueue(&rq->scx.local_dsq, p, enq_flags);
return;

global:
+ touch_core_sched(rq, p); /* see the comment in local: */
p->scx.slice = SCX_SLICE_DFL;
dispatch_enqueue(&scx_dsq_global, p, enq_flags);
}
@@ -846,6 +901,9 @@ static void enqueue_task_scx(struct rq *rq, struct task_struct *p, int enq_flags
if (SCX_HAS_OP(runnable))
SCX_CALL_OP_TASK(SCX_KF_REST, runnable, p, enq_flags);

+ if (enq_flags & SCX_ENQ_WAKEUP)
+ touch_core_sched(rq, p);
+
do_enqueue_task(rq, p, enq_flags, sticky_cpu);
}

@@ -1296,6 +1354,7 @@ static void finish_dispatch(struct rq *rq, struct rq_flags *rf,
struct scx_dispatch_q *dsq;
u64 opss;

+ touch_core_sched_dispatch(rq, p);
retry:
/*
* No need for _acquire here. @p is accessed only after a successful
@@ -1373,8 +1432,8 @@ static void flush_dispatch_buf(struct rq *rq, struct rq_flags *rf)
dspc->buf_cursor = 0;
}

-static int balance_scx(struct rq *rq, struct task_struct *prev,
- struct rq_flags *rf)
+static int balance_one(struct rq *rq, struct task_struct *prev,
+ struct rq_flags *rf, bool local)
{
struct scx_rq *scx_rq = &rq->scx;
struct scx_dsp_ctx *dspc = this_cpu_ptr(&scx_dsp_ctx);
@@ -1398,7 +1457,7 @@ static int balance_scx(struct rq *rq, struct task_struct *prev,
}

if (prev_on_scx) {
- WARN_ON_ONCE(prev->scx.flags & SCX_TASK_BAL_KEEP);
+ WARN_ON_ONCE(local && (prev->scx.flags & SCX_TASK_BAL_KEEP));
update_curr_scx(rq);

/*
@@ -1410,10 +1469,16 @@ static int balance_scx(struct rq *rq, struct task_struct *prev,
*
* See scx_ops_disable_workfn() for the explanation on the
* disabling() test.
+ *
+ * When balancing a remote CPU for core-sched, there won't be a
+ * following put_prev_task_scx() call and we don't own
+ * %SCX_TASK_BAL_KEEP. Instead, pick_task_scx() will test the
+ * same conditions later and pick @rq->curr accordingly.
*/
if ((prev->scx.flags & SCX_TASK_QUEUED) &&
prev->scx.slice && !scx_ops_disabling()) {
- prev->scx.flags |= SCX_TASK_BAL_KEEP;
+ if (local)
+ prev->scx.flags |= SCX_TASK_BAL_KEEP;
return 1;
}
}
@@ -1469,10 +1534,56 @@ static int balance_scx(struct rq *rq, struct task_struct *prev,
return 0;
}

+static int balance_scx(struct rq *rq, struct task_struct *prev,
+ struct rq_flags *rf)
+{
+ int ret;
+
+ ret = balance_one(rq, prev, rf, true);
+
+#ifdef CONFIG_SCHED_SMT
+ /*
+ * When core-sched is enabled, this ops.balance() call will be followed
+ * by put_prev_scx() and pick_task_scx() on this CPU and pick_task_scx()
+ * on the SMT siblings. Balance the siblings too.
+ */
+ if (sched_core_enabled(rq)) {
+ const struct cpumask *smt_mask = cpu_smt_mask(cpu_of(rq));
+ int scpu;
+
+ for_each_cpu_andnot(scpu, smt_mask, cpumask_of(cpu_of(rq))) {
+ struct rq *srq = cpu_rq(scpu);
+ struct rq_flags srf;
+ struct task_struct *sprev = srq->curr;
+
+ /*
+ * While core-scheduling, rq lock is shared among
+ * siblings but the debug annotations and rq clock
+ * aren't. Do pinning dance to transfer the ownership.
+ */
+ WARN_ON_ONCE(__rq_lockp(rq) != __rq_lockp(srq));
+ rq_unpin_lock(rq, rf);
+ rq_pin_lock(srq, &srf);
+
+ update_rq_clock(srq);
+ balance_one(srq, sprev, &srf, false);
+
+ rq_unpin_lock(srq, &srf);
+ rq_repin_lock(rq, rf);
+ }
+ }
+#endif
+ return ret;
+}
+
static void set_next_task_scx(struct rq *rq, struct task_struct *p, bool first)
{
if (p->scx.flags & SCX_TASK_QUEUED) {
- WARN_ON_ONCE(atomic64_read(&p->scx.ops_state) != SCX_OPSS_NONE);
+ /*
+ * Core-sched might decide to execute @p before it is
+ * dispatched. Call ops_dequeue() to notify the BPF scheduler.
+ */
+ ops_dequeue(p, SCX_DEQ_CORE_SCHED_EXEC);
dispatch_dequeue(&rq->scx, p);
}

@@ -1555,7 +1666,8 @@ static void put_prev_task_scx(struct rq *rq, struct task_struct *p)
/*
* If @p has slice left and balance_scx() didn't tag it for
* keeping, @p is getting preempted by a higher priority
- * scheduler class. Leave it at the head of the local DSQ.
+ * scheduler class or core-sched forcing a different task. Leave
+ * it at the head of the local DSQ.
*/
if (p->scx.slice && !scx_ops_disabling()) {
dispatch_enqueue(&rq->scx.local_dsq, p, SCX_ENQ_HEAD);
@@ -1612,6 +1724,84 @@ static struct task_struct *pick_next_task_scx(struct rq *rq)
return p;
}

+#ifdef CONFIG_SCHED_CORE
+/**
+ * scx_prio_less - Task ordering for core-sched
+ * @a: task A
+ * @b: task B
+ *
+ * Core-sched is implemented as an additional scheduling layer on top of the
+ * usual sched_class'es and needs to find out the expected task ordering. For
+ * SCX, core-sched calls this function to interrogate the task ordering.
+ *
+ * Unless overridden by ops.core_sched_before(), @p->scx.core_sched_at is used
+ * to implement the default task ordering. The older the timestamp, the higher
+ * prority the task - the global FIFO ordering matching the default scheduling
+ * behavior.
+ *
+ * When ops.core_sched_before() is enabled, @p->scx.core_sched_at is used to
+ * implement FIFO ordering within each local DSQ. See pick_task_scx().
+ */
+bool scx_prio_less(const struct task_struct *a, const struct task_struct *b,
+ bool in_fi)
+{
+ /*
+ * The const qualifiers are dropped from task_struct pointers when
+ * calling ops.core_sched_before(). Accesses are controlled by the
+ * verifier.
+ */
+ if (SCX_HAS_OP(core_sched_before) && !scx_ops_disabling())
+ return SCX_CALL_OP_2TASKS_RET(SCX_KF_REST, core_sched_before,
+ (struct task_struct *)a,
+ (struct task_struct *)b);
+ else
+ return time_after64(a->scx.core_sched_at, b->scx.core_sched_at);
+}
+
+/**
+ * pick_task_scx - Pick a candidate task for core-sched
+ * @rq: rq to pick the candidate task from
+ *
+ * Core-sched calls this function on each SMT sibling to determine the next
+ * tasks to run on the SMT siblings. balance_one() has been called on all
+ * siblings and put_prev_task_scx() has been called only for the current CPU.
+ *
+ * As put_prev_task_scx() hasn't been called on remote CPUs, we can't just look
+ * at the first task in the local dsq. @rq->curr has to be considered explicitly
+ * to mimic %SCX_TASK_BAL_KEEP.
+ */
+static struct task_struct *pick_task_scx(struct rq *rq)
+{
+ struct task_struct *curr = rq->curr;
+ struct task_struct *first = first_local_task(rq);
+
+ if (curr->scx.flags & SCX_TASK_QUEUED) {
+ /* is curr the only runnable task? */
+ if (!first)
+ return curr;
+
+ /*
+ * Does curr trump first? We can always go by core_sched_at for
+ * this comparison as it represents global FIFO ordering when
+ * the default core-sched ordering is used and local-DSQ FIFO
+ * ordering otherwise.
+ *
+ * We can have a task with an earlier timestamp on the DSQ. For
+ * example, when a current task is preempted by a sibling
+ * picking a different cookie, the task would be requeued at the
+ * head of the local DSQ with an earlier timestamp than the
+ * core-sched picked next task. Besides, the BPF scheduler may
+ * dispatch any tasks to the local DSQ anytime.
+ */
+ if (curr->scx.slice && time_before64(curr->scx.core_sched_at,
+ first->scx.core_sched_at))
+ return curr;
+ }
+
+ return first; /* this may be %NULL */
+}
+#endif /* CONFIG_SCHED_CORE */
+
static enum scx_cpu_preempt_reason
preempt_reason_from_class(const struct sched_class *class)
{
@@ -1921,11 +2111,13 @@ static void task_tick_scx(struct rq *rq, struct task_struct *curr, int queued)
update_curr_scx(rq);

/*
- * While disabling, always resched as we can't trust the slice
- * management.
+ * While disabling, always resched and refresh core-sched timestamp as
+ * we can't trust the slice management or ops.core_sched_before().
*/
- if (scx_ops_disabling())
+ if (scx_ops_disabling()) {
curr->scx.slice = 0;
+ touch_core_sched(rq, curr);
+ }

if (!curr->scx.slice)
resched_curr(rq);
@@ -2388,6 +2580,10 @@ DEFINE_SCHED_CLASS(ext) = {
.rq_offline = rq_offline_scx,
#endif

+#ifdef CONFIG_SCHED_CORE
+ .pick_task = pick_task_scx,
+#endif
+
.task_tick = task_tick_scx,

.switching_to = switching_to_scx,
@@ -2716,9 +2912,11 @@ static void scx_ops_disable_workfn(struct kthread_work *work)
*
* b. balance_scx() never sets %SCX_TASK_BAL_KEEP as the slice value
* can't be trusted. Whenever a tick triggers, the running task is
- * rotated to the tail of the queue.
+ * rotated to the tail of the queue with core_sched_at touched.
*
* c. pick_next_task() suppresses zero slice warning.
+ *
+ * d. scx_prio_less() reverts to the default core_sched_at order.
*/
scx_ops.enqueue = scx_ops_fallback_enqueue;
scx_ops.dispatch = scx_ops_fallback_dispatch;
diff --git a/kernel/sched/ext.h b/kernel/sched/ext.h
index c71bf641f7a5..cd14970c239d 100644
--- a/kernel/sched/ext.h
+++ b/kernel/sched/ext.h
@@ -68,6 +68,14 @@ enum scx_enq_flags {
enum scx_deq_flags {
/* expose select DEQUEUE_* flags as enums */
SCX_DEQ_SLEEP = DEQUEUE_SLEEP,
+
+ /* high 32bits are SCX specific */
+
+ /*
+ * The generic core-sched layer decided to execute the task even though
+ * it hasn't been dispatched yet. Dequeue from the BPF side.
+ */
+ SCX_DEQ_CORE_SCHED_EXEC = 1LLU << 32,
};

enum scx_pick_idle_cpu_flags {
@@ -193,6 +201,11 @@ static inline const struct sched_class *next_active_class(const struct sched_cla
for_active_class_range(class, (prev_class) > &ext_sched_class ? \
&ext_sched_class : (prev_class), (end_class))

+#ifdef CONFIG_SCHED_CORE
+bool scx_prio_less(const struct task_struct *a, const struct task_struct *b,
+ bool in_fi);
+#endif
+
#else /* CONFIG_SCHED_CLASS_EXT */

#define scx_enabled() false
diff --git a/tools/sched_ext/scx_qmap.bpf.c b/tools/sched_ext/scx_qmap.bpf.c
index bfffbfd3368b..b6365df0fb64 100644
--- a/tools/sched_ext/scx_qmap.bpf.c
+++ b/tools/sched_ext/scx_qmap.bpf.c
@@ -13,6 +13,7 @@
* - Sleepable per-task storage allocation using ops.prep_enable().
* - Using ops.cpu_release() to handle a higher priority scheduling class taking
* the CPU away.
+ * - Core-sched support.
*
* This scheduler is primarily for demonstration and testing of sched_ext
* features and unlikely to be useful for actual workloads.
@@ -62,9 +63,21 @@ struct {
},
};

+/*
+ * Per-queue sequence numbers to implement core-sched ordering.
+ *
+ * Tail seq is assigned to each queued task and incremented. Head seq tracks the
+ * sequence number of the latest dispatched task. The distance between the a
+ * task's seq and the associated queue's head seq is called the queue distance
+ * and used when comparing two tasks for ordering. See qmap_core_sched_before().
+ */
+static u64 core_sched_head_seqs[5];
+static u64 core_sched_tail_seqs[5];
+
/* Per-task scheduling context */
struct task_ctx {
bool force_local; /* Dispatch directly to local_dsq */
+ u64 core_sched_seq;
};

struct {
@@ -84,6 +97,7 @@ struct {

/* Statistics */
unsigned long nr_enqueued, nr_dispatched, nr_reenqueued, nr_dequeued;
+unsigned long nr_core_sched_execed;

s32 BPF_STRUCT_OPS(qmap_select_cpu, struct task_struct *p,
s32 prev_cpu, u64 wake_flags)
@@ -150,8 +164,18 @@ void BPF_STRUCT_OPS(qmap_enqueue, struct task_struct *p, u64 enq_flags)
return;
}

- /* Is select_cpu() is telling us to enqueue locally? */
- if (tctx->force_local) {
+ /*
+ * All enqueued tasks must have their core_sched_seq updated for correct
+ * core-sched ordering, which is why %SCX_OPS_ENQ_LAST is specified in
+ * qmap_ops.flags.
+ */
+ tctx->core_sched_seq = core_sched_tail_seqs[idx]++;
+
+ /*
+ * If qmap_select_cpu() is telling us to or this is the last runnable
+ * task on the CPU, enqueue locally.
+ */
+ if (tctx->force_local || (enq_flags & SCX_ENQ_LAST)) {
tctx->force_local = false;
scx_bpf_dispatch(p, SCX_DSQ_LOCAL, slice_ns, enq_flags);
return;
@@ -195,6 +219,19 @@ void BPF_STRUCT_OPS(qmap_enqueue, struct task_struct *p, u64 enq_flags)
void BPF_STRUCT_OPS(qmap_dequeue, struct task_struct *p, u64 deq_flags)
{
__sync_fetch_and_add(&nr_dequeued, 1);
+ if (deq_flags & SCX_DEQ_CORE_SCHED_EXEC)
+ __sync_fetch_and_add(&nr_core_sched_execed, 1);
+}
+
+static void update_core_sched_head_seq(struct task_struct *p)
+{
+ struct task_ctx *tctx = bpf_task_storage_get(&task_ctx_stor, p, 0, 0);
+ int idx = weight_to_idx(p->scx.weight);
+
+ if (tctx)
+ core_sched_head_seqs[idx] = tctx->core_sched_seq;
+ else
+ scx_bpf_error("task_ctx lookup failed");
}

void BPF_STRUCT_OPS(qmap_dispatch, s32 cpu, struct task_struct *prev)
@@ -247,6 +284,7 @@ void BPF_STRUCT_OPS(qmap_dispatch, s32 cpu, struct task_struct *prev)

p = bpf_task_from_pid(pid);
if (p) {
+ update_core_sched_head_seq(p);
__sync_fetch_and_add(&nr_dispatched, 1);
scx_bpf_dispatch(p, SCX_DSQ_GLOBAL, slice_ns, 0);
bpf_task_release(p);
@@ -258,6 +296,49 @@ void BPF_STRUCT_OPS(qmap_dispatch, s32 cpu, struct task_struct *prev)
}
}

+/*
+ * The distance from the head of the queue scaled by the weight of the queue.
+ * The lower the number, the older the task and the higher the priority.
+ */
+static s64 task_qdist(struct task_struct *p)
+{
+ int idx = weight_to_idx(p->scx.weight);
+ struct task_ctx *tctx;
+ s64 qdist;
+
+ tctx = bpf_task_storage_get(&task_ctx_stor, p, 0, 0);
+ if (!tctx) {
+ scx_bpf_error("task_ctx lookup failed");
+ return 0;
+ }
+
+ qdist = tctx->core_sched_seq - core_sched_head_seqs[idx];
+
+ /*
+ * As queue index increments, the priority doubles. The queue w/ index 3
+ * is dispatched twice more frequently than 2. Reflect the difference by
+ * scaling qdists accordingly. Note that the shift amount needs to be
+ * flipped depending on the sign to avoid flipping priority direction.
+ */
+ if (qdist >= 0)
+ return qdist << (4 - idx);
+ else
+ return qdist << idx;
+}
+
+/*
+ * This is called to determine the task ordering when core-sched is picking
+ * tasks to execute on SMT siblings and should encode about the same ordering as
+ * the regular scheduling path. Use the priority-scaled distances from the head
+ * of the queues to compare the two tasks which should be consistent with the
+ * dispatch path behavior.
+ */
+bool BPF_STRUCT_OPS(qmap_core_sched_before,
+ struct task_struct *a, struct task_struct *b)
+{
+ return task_qdist(a) > task_qdist(b);
+}
+
void BPF_STRUCT_OPS(qmap_cpu_release, s32 cpu, struct scx_cpu_release_args *args)
{
u32 cnt;
@@ -309,10 +390,12 @@ struct sched_ext_ops qmap_ops = {
.enqueue = (void *)qmap_enqueue,
.dequeue = (void *)qmap_dequeue,
.dispatch = (void *)qmap_dispatch,
+ .core_sched_before = (void *)qmap_core_sched_before,
.cpu_release = (void *)qmap_cpu_release,
.prep_enable = (void *)qmap_prep_enable,
.init = (void *)qmap_init,
.exit = (void *)qmap_exit,
+ .flags = SCX_OPS_ENQ_LAST,
.timeout_ms = 5000U,
.name = "qmap",
};
diff --git a/tools/sched_ext/scx_qmap.c b/tools/sched_ext/scx_qmap.c
index b47be72ee66e..0a02aa166b47 100644
--- a/tools/sched_ext/scx_qmap.c
+++ b/tools/sched_ext/scx_qmap.c
@@ -92,9 +92,10 @@ int main(int argc, char **argv)
long nr_enqueued = skel->bss->nr_enqueued;
long nr_dispatched = skel->bss->nr_dispatched;

- printf("enq=%lu, dsp=%lu, delta=%ld, reenq=%lu, deq=%lu\n",
+ printf("enq=%lu, dsp=%lu, delta=%ld, reenq=%lu, deq=%lu, core=%lu\n",
nr_enqueued, nr_dispatched, nr_enqueued - nr_dispatched,
- skel->bss->nr_reenqueued, skel->bss->nr_dequeued);
+ skel->bss->nr_reenqueued, skel->bss->nr_dequeued,
+ skel->bss->nr_core_sched_execed);
fflush(stdout);
sleep(1);
}
--
2.41.0