On Tue, Nov 19, 2024 at 10:19:44AM +0900, Changwoo Min wrote:
Let's suppose the following timeline:
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_UPDATED is set), so scx_bpf_clock_get_ns calls during
[T2, T4) (including T3) will return the rq clock updated at T2.
Let's think about what we should do for the duration [T4, T7)
when a BPF scheduler can still call scx_bpf_clock_get_ns (T5).
During that duration, we consider the rq clock is invalid
(SCX_RQ_CLK_UPDATED is unset). So when calling
scx_bpf_clock_get_ns at T5, we call sched_clock() to get the
fresh clock.
So the question then becomes, what is T5 doing and is it 'right' for it
to get a fresh clock value.
Please give an example of T5 -- I really don't know this BPF crap much
-- and reason about how the clock should behave.