An rq clock becomes valid when it is updated using update_rq_clock()
and invalidated when the rq is unlocked using rq_unpin_lock(). Also,
after long-running operations -- ops.running() and ops.update_idle() --
in a BPF scheduler, the sched_ext core invalidates the rq clock.

Let's suppose the following timeline in the scheduler core:

   T1. rq_lock(rq)
   T2. update_rq_clock(rq)
   T3. a sched_ext BPF operation
   T4. rq_unlock(rq)
   T5. a sched_ext BPF operation
   T6. rq_lock(rq)
   T7. update_rq_clock(rq)

For [T2, T4), we consider that rq clock is valid (SCX_RQ_CLK_VALID is
set), so scx_bpf_clock_get_ns() calls during [T2, T4) (including T3)
will return the rq clock updated at T2. For duration [T4, T7),
when a BPF scheduler can still call scx_bpf_clock_get_ns() (T5), we
consider the rq clock is invalid (SCX_RQ_CLK_VALID is unset at T4). So
when calling scx_bpf_clock_get_ns() at T5, we will return a fresh clock
value by calling sched_clock() internally.

One example of calling scx_bpf_clock_get_ns(), when the rq clock is
invalid (like T5), is in scx_central [1]. The scx_central scheduler uses
a BPF timer for preemptive scheduling. In every msec, the timer callback
checks if the currently running tasks exceed their timeslice. At the
beginning of the BPF timer callback (central_timerfn in scx_central.bpf.c),
scx_central gets the current time. When the BPF timer callback runs, the rq
clock could be invalid, the same as T5. In this case, scx_bpf_clock_get_ns()
returns a fresh clock value rather than returning the old one (T2).

[1] https://github.com/sched-ext/scx/blob/main/scheds/c/scx_central.bpf.c

Signed-off-by: Changwoo Min <changwoo@igalia.com>
---
 kernel/sched/core.c  | 6 +++++-
 kernel/sched/ext.c   | 3 +++
 kernel/sched/sched.h | 2 +-
 3 files changed, 9 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index 95e40895a519..ab8015c8cab4 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -789,6 +789,7 @@ static void update_rq_clock_task(struct rq *rq, s64 delta)
 void update_rq_clock(struct rq *rq)
 {
 	s64 delta;
+	u64 clock;
 
 	lockdep_assert_rq_held(rq);
 
@@ -800,11 +801,14 @@ void update_rq_clock(struct rq *rq)
 		SCHED_WARN_ON(rq->clock_update_flags & RQCF_UPDATED);
 	rq->clock_update_flags |= RQCF_UPDATED;
 #endif
+	clock = sched_clock_cpu(cpu_of(rq));
+	scx_rq_clock_update(rq, clock);
 
-	delta = sched_clock_cpu(cpu_of(rq)) - rq->clock;
+	delta = clock - rq->clock;
 	if (delta < 0)
 		return;
 	rq->clock += delta;
+
 	update_rq_clock_task(rq, delta);
 }
 
diff --git a/kernel/sched/ext.c b/kernel/sched/ext.c
index 7fff1d045477..ac279a657d50 100644
--- a/kernel/sched/ext.c
+++ b/kernel/sched/ext.c
@@ -2928,6 +2928,8 @@ static void set_next_task_scx(struct rq *rq, struct task_struct *p, bool first)
 	if (SCX_HAS_OP(running) && (p->scx.flags & SCX_TASK_QUEUED))
 		SCX_CALL_OP_TASK(SCX_KF_REST, running, p);
 
+	scx_rq_clock_stale(rq);
+
 	clr_task_runnable(p, true);
 
 	/*
@@ -3590,6 +3592,7 @@ void __scx_update_idle(struct rq *rq, bool idle)
 {
 	int cpu = cpu_of(rq);
 
+	scx_rq_clock_stale(rq);
 	if (SCX_HAS_OP(update_idle) && !scx_rq_bypassing(rq)) {
 		SCX_CALL_OP(SCX_KF_REST, update_idle, cpu_of(rq), idle);
 		if (!static_branch_unlikely(&scx_builtin_idle_enabled))
diff --git a/kernel/sched/sched.h b/kernel/sched/sched.h
index 09d5a64f78ce..1c1345274b02 100644
--- a/kernel/sched/sched.h
+++ b/kernel/sched/sched.h
@@ -1766,7 +1766,7 @@ static inline void rq_unpin_lock(struct rq *rq, struct rq_flags *rf)
 	if (rq->clock_update_flags > RQCF_ACT_SKIP)
 		rf->clock_update_flags = RQCF_UPDATED;
 #endif
-
+	scx_rq_clock_stale(rq);
 	lockdep_unpin_lock(__rq_lockp(rq), rf->cookie);
 }
 
-- 
2.47.1

On Mon, Dec 02, 2024 at 01:38:46PM +0900, Changwoo Min wrote:
> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
> index 95e40895a519..ab8015c8cab4 100644
> --- a/kernel/sched/core.c
> +++ b/kernel/sched/core.c
> @@ -789,6 +789,7 @@ static void update_rq_clock_task(struct rq *rq, s64 delta)
>  void update_rq_clock(struct rq *rq)
>  {
>  	s64 delta;
> +	u64 clock;
>  
>  	lockdep_assert_rq_held(rq);
>  
> @@ -800,11 +801,14 @@ void update_rq_clock(struct rq *rq)
>  		SCHED_WARN_ON(rq->clock_update_flags & RQCF_UPDATED);
>  	rq->clock_update_flags |= RQCF_UPDATED;
>  #endif
> +	clock = sched_clock_cpu(cpu_of(rq));
> +	scx_rq_clock_update(rq, clock);

This adds a write to a second cacheline for *everyone* always.

Please don't do that.

Hello Peter,

On 24. 12. 2. 18:58, Peter Zijlstra wrote:
> On Mon, Dec 02, 2024 at 01:38:46PM +0900, Changwoo Min wrote:
>> diff --git a/kernel/sched/core.c b/kernel/sched/core.c
>> index 95e40895a519..ab8015c8cab4 100644
>> --- a/kernel/sched/core.c
>> +++ b/kernel/sched/core.c
>> @@ -789,6 +789,7 @@ static void update_rq_clock_task(struct rq *rq, s64 delta)
>>   void update_rq_clock(struct rq *rq)
>>   {
>>   	s64 delta;
>> +	u64 clock;
>>   
>>   	lockdep_assert_rq_held(rq);
>>   
>> @@ -800,11 +801,14 @@ void update_rq_clock(struct rq *rq)
>>   		SCHED_WARN_ON(rq->clock_update_flags & RQCF_UPDATED);
>>   	rq->clock_update_flags |= RQCF_UPDATED;
>>   #endif
>> +	clock = sched_clock_cpu(cpu_of(rq));
>> +	scx_rq_clock_update(rq, clock);
> 
> This adds a write to a second cacheline for *everyone* always.
> 
> Please don't do that.
> 

Thank you for the suggestion! I will update the flags only when
the scx is enabled by checking it inside the
scx_rq_clock_update() and scx_rq_clock_stale() functions.

Regards,
Changwoo Min