sched, sched_ext: Replace scx_next_task_picked() with sched_class->switch_class()

scx_next_task_picked() is used by sched_ext to notify the BPF scheduler when
a CPU is taken away by a task dispatched from a higher priority sched_class
so that the BPF scheduler can, e.g., punt the task[s] which was running or
were waiting for the CPU to other CPUs.

Replace the sched_ext specific hook scx_next_task_picked() with a new
sched_class operation switch_class().

The changes are straightforward and the code looks better afterwards.
However, when !CONFIG_SCHED_CLASS_EXT, this ends up adding an unused hook
which is unlikely to be useful to other sched_classes. For further
discussion on this subject, please refer to the following:

  http://lkml.kernel.org/r/CAHk-=wjFPLqo7AXu8maAGEGnOy6reUg-F4zzFhVB0Kyu22h7pw@mail.gmail.com

Signed-off-by: Tejun Heo <tj@kernel.org>
Suggested-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 5eec6639773bab67442a20316ee7412315d9a38f..1092955a7d6e8533b57a10832ffe9c4c774a00c6 100644 (file)
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -5907,7 +5907,10 @@ restart:
        for_each_active_class(class) {
                p = class->pick_next_task(rq);
                if (p) {
-                       scx_next_task_picked(rq, p, class);
+                       const struct sched_class *prev_class = prev->sched_class;
+
+                       if (class != prev_class && prev_class->switch_class)
+                               prev_class->switch_class(rq, p);
                        return p;
                }
        }

diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index f814e84ceeb397d53d9fbc5c669bbcc026f8b80c..390623a4a376b144c53ade08a447c3fefc66f897 100644 (file)
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -2749,10 +2749,9 @@ preempt_reason_from_class(const struct sched_class *class)
        return SCX_CPU_PREEMPT_UNKNOWN;
 }
 
-void scx_next_task_picked(struct rq *rq, struct task_struct *p,
-                         const struct sched_class *active)
+static void switch_class_scx(struct rq *rq, struct task_struct *next)
 {
-       lockdep_assert_rq_held(rq);
+       const struct sched_class *next_class = next->sched_class;
 
        if (!scx_enabled())
                return;
@@ -2769,12 +2768,11 @@ void scx_next_task_picked(struct rq *rq, struct task_struct *p,
 
        /*
         * The callback is conceptually meant to convey that the CPU is no
-        * longer under the control of SCX. Therefore, don't invoke the
-        * callback if the CPU is is staying on SCX, or going idle (in which
-        * case the SCX scheduler has actively decided not to schedule any
-        * tasks on the CPU).
+        * longer under the control of SCX. Therefore, don't invoke the callback
+        * if the next class is below SCX (in which case the BPF scheduler has
+        * actively decided not to schedule any tasks on the CPU).
         */
-       if (likely(active >= &ext_sched_class))
+       if (sched_class_above(&ext_sched_class, next_class))
                return;
 
        /*
@@ -2789,8 +2787,8 @@ void scx_next_task_picked(struct rq *rq, struct task_struct *p,
        if (!rq->scx.cpu_released) {
                if (SCX_HAS_OP(cpu_release)) {
                        struct scx_cpu_release_args args = {
-                               .reason = preempt_reason_from_class(active),
-                               .task = p,
+                               .reason = preempt_reason_from_class(next_class),
+                               .task = next,
                        };
 
                        SCX_CALL_OP(SCX_KF_CPU_RELEASE,
@@ -3496,6 +3494,8 @@ DEFINE_SCHED_CLASS(ext) = {
        .put_prev_task          = put_prev_task_scx,
        .set_next_task          = set_next_task_scx,
 
+       .switch_class           = switch_class_scx,
+
 #ifdef CONFIG_SMP
        .balance                = balance_scx,
        .select_task_rq         = select_task_rq_scx,

diff --git a/kernel/sched/ext.h b/kernel/sched/ext.h
index c41d742b5d62c176eac776365ccf03022c59a799..bf6f2cfa49d54634dde1ad9349d1a0bd2febbdaf 100644 (file)
--- a/kernel/sched/ext.h
+++ b/kernel/sched/ext.h
@@ -33,8 +33,6 @@ static inline bool task_on_scx(const struct task_struct *p)
        return scx_enabled() && p->sched_class == &ext_sched_class;
 }
 
-void scx_next_task_picked(struct rq *rq, struct task_struct *p,
-                         const struct sched_class *active);
 void scx_tick(struct rq *rq);
 void init_scx_entity(struct sched_ext_entity *scx);
 void scx_pre_fork(struct task_struct *p);
@@ -82,8 +80,6 @@ bool scx_prio_less(const struct task_struct *a, const struct task_struct *b,
 #define scx_enabled()          false
 #define scx_switched_all()     false
 
-static inline void scx_next_task_picked(struct rq *rq, struct task_struct *p,
-                                       const struct sched_class *active) {}
 static inline void scx_tick(struct rq *rq) {}
 static inline void scx_pre_fork(struct task_struct *p) {}
 static inline int scx_fork(struct task_struct *p) { return 0; }

diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index c0d6e42c99ccc694edcdb92f67e8be582f8e68ed..3989bf8f2a1b204c40fc06462d2cbe9f98d4d2e5 100644 (file)
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -2355,6 +2355,8 @@ struct sched_class {
        void (*put_prev_task)(struct rq *rq, struct task_struct *p);
        void (*set_next_task)(struct rq *rq, struct task_struct *p, bool first);
 
+       void (*switch_class)(struct rq *rq, struct task_struct *next);
+
 #ifdef CONFIG_SMP
        int (*balance)(struct rq *rq, struct task_struct *prev, struct rq_flags *rf);
        int  (*select_task_rq)(struct task_struct *p, int task_cpu, int flags);