[PATCH 1/2] sched_ext: Drop unlocked scx_rq_clock_invalidate() from scx_root_disable()

[Tejun Heo <tj@kernel.org>]

scx_root_disable() invalidates each rq's clock before taking the rq lock.
scx_rq_clock_invalidate() is a plain read-modify-write of rq->scx.flags and
every other writer of the word runs under the rq lock, so the unlocked
update can race a concurrent flags update and lose one side's bits.

The invalidation is also redundant. The dl_server rebalancing right below
cycles each rq's lock and rq_unpin_lock() clears SCX_RQ_CLK_VALID on every
unlock, while __scx_enabled is turned off only later in the function, so the
clocks end up invalidated either way. Drop the call and note the
invalidation in the comment.

Fixes: 3a9910b5904d ("sched_ext: Implement scx_bpf_now()")
Signed-off-by: Tejun Heo <tj@kernel.org>
Cc: Changwoo Min <changwoo@igalia.com>
---
 kernel/sched/ext/ext.c | 13 ++++++-------
 1 file changed, 6 insertions(+), 7 deletions(-)

diff --git a/kernel/sched/ext/ext.c b/kernel/sched/ext/ext.c
index 2adf2bde0cb1..d301fe757193 100644
--- a/kernel/sched/ext/ext.c
+++ b/kernel/sched/ext/ext.c
@@ -6476,12 +6476,13 @@ static void scx_root_disable(struct scx_sched *sch)
 	percpu_up_write(&scx_fork_rwsem);
 
 	/*
-	 * Invalidate all the rq clocks to prevent getting outdated
-	 * rq clocks from a previous scx scheduler.
+	 * Re-balance the dl_server bandwidth reservations: detach ext_server
+	 * (no more sched_ext tasks) and reinstate fair_server if it was
+	 * previously detached because we were running in full mode.
 	 *
-	 * Also re-balance the dl_server bandwidth reservations: detach
-	 * ext_server (no more sched_ext tasks) and reinstate fair_server if it
-	 * was previously detached because we were running in full mode.
+	 * The rq lock cycle also invalidates each rq's clock (rq_unpin_lock()
+	 * clears SCX_RQ_CLK_VALID on unlock), preventing the next enable from
+	 * seeing outdated rq clocks from this scheduler.
 	 *
 	 * Unlike the enable path, this runs on a recovery path that cannot
 	 * fail, so we use dl_server_swap_bw() to atomically free ext_server's
@@ -6494,8 +6495,6 @@ static void scx_root_disable(struct scx_sched *sch)
 	for_each_possible_cpu(cpu) {
 		struct rq *rq = cpu_rq(cpu);
 
-		scx_rq_clock_invalidate(rq);
-
 		scoped_guard(rq_lock_irqsave, rq) {
 			update_rq_clock(rq);
 			if (was_switched_all) {
-- 
2.55.0