[PATCH 2/5] sched/core: Keep a sibling's DL server link in the core-wide pick

[Fernand Sieber <sieberf@amazon.com>]

The core-wide selection records in rq_i->core_dl_server the DL server
each sibling's pick came through, then clears it for every sibling in
the cookie matching loop, including when the pick is kept. The sibling
takes the fast path, which sets rq->dl_server from rq->core_dl_server,
and runs the task its server picked with p->dl_server NULL. Only the
selecting CPU keeps the link, as its pick goes through rq->dl_server.

A running task is ranked for core scheduling by p->dl_server. While
such a task runs on a sibling, each following core-wide selection
ranks it as a plain fair or ext task rather than by its server's
deadline, so a higher class task on another sibling takes the core
back before the server's runtime is used. The RV nomiss monitor, which
follows the server through next->dl_server at sched_switch, does not
see the server run either.

Keep the server that pick_task() left in rq_i->dl_server. With the
previous patch it is NULL when the class pick was replaced. Drop the
assignment in the first loop, which the matching loop always
overwrites.

Fixes: bd9bbc96e835 ("sched: Rework dl_server")
Signed-off-by: Fernand Sieber <sieberf@amazon.com>
---
 kernel/sched/core.c | 3 +--
 1 file changed, 1 insertion(+), 2 deletions(-)

diff --git a/kernel/sched/core.c b/kernel/sched/core.c
index f7ac13186ee59..61d4e6e990bba 100644
--- a/kernel/sched/core.c
+++ b/kernel/sched/core.c
@@ -6431,7 +6431,6 @@ pick_next_task(struct rq *rq, struct rq_flags *rf)
 		}
 
 		rq_i->core_pick = p;
-		rq_i->core_dl_server = rq_i->dl_server;
 
 		if (!max || prio_less(max, p, fi_before))
 			max = p;
@@ -6462,7 +6461,7 @@ pick_next_task(struct rq *rq, struct rq_flags *rf)
 		}
 
 		rq_i->core_pick = p;
-		rq_i->core_dl_server = NULL;
+		rq_i->core_dl_server = rq_i->dl_server;
 
 		if (p == rq_i->idle) {
 			if (rq_i->nr_running) {
-- 
2.47.3




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