Re: [PATCH 2/5] sched_ext: Manage the validity of scx_rq_clock

[Changwoo Min]

Hello,

On 24. 11. 17. 04:32, Peter Zijlstra wrote:
> It is not at all clear why you think you need to keep a second copy of
> that value. You like cache misses?

Of course not. :-) I always try to avoid cache misses whenever
possible.

The main reason to keep the second copy (rq->scx.clock) is that
a BPF scheduler can call scx_bpf_clock_get_ns() at almost any
time in any context, including any of sched_ext operations, BPF
timer callbacks, BPF syscalls, kprobes, and so on.

However, the rq->clock is supposed to be updated while holding
the rq lock (lockdep_assert_rq_held), which is not always the
case in a BPF scheduler. Also, rq->clock is also used in other
places (e.g., PELT), so updating rq->clock in arbitrary points by
a BPF scheduler will create unnecessary dependencies.

Another approach would be to extend struct sched_clock_data (in
kernel/sched/clock.c) to store the update flag
(SCX_RQ_CLK_UPDATED). This would be the best regarding the number
of cache line accesses. However, that would be an overkill since
now sched_clock_data stores the sched_ext-specific data.
I thought it would be better to keep sched_ext specific data in
one place, struct scx_rq, for managibility.

Regards,
Changwoo Min