[PATCH v3 1/2] sched_ext: document the rolling-cursor requirement for dsq_vtime

[Tao Cui <cui.tao@linux.dev>]

From: Tao Cui <cuitao@kylinos.cn>

scx_bpf_dsq_insert_vtime() orders tasks by time_before64(), which is
only meaningful when the values within a given DSQ stay less than 2^63
apart. This is implicit in how a vtime scheduler works but not spelled
out anywhere. Document it so BPF scheduler authors know the
constraint.

Signed-off-by: Tao Cui <cuitao@kylinos.cn>
---
 kernel/sched/ext/ext.c | 4 +++-
 1 file changed, 3 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
index 8041c87a3562..81a506a4c8ab 100644
--- a/kernel/sched/ext/ext.c
+++ b/kernel/sched/ext/ext.c
@@ -8903,7 +8903,9 @@ struct scx_bpf_dsq_insert_vtime_args {
  *
  * @args->vtime ordering is according to time_before64() which considers
  * wrapping. A numerically larger vtime may indicate an earlier position in the
- * ordering and vice-versa.
+ * ordering and vice-versa. vtime is a rolling cursor and values used for
+ * ordering within a given DSQ should stay less than 2^63 apart for
+ * time_before64() ordering to remain well-defined.
  *
  * A DSQ can only be used as a FIFO or priority queue at any given time and this
  * function must not be called on a DSQ which already has one or more FIFO tasks
-- 
2.43.0