sched/deadline: Reset extra_bw to max_bw when clearing root domains

dl_clear_root_domain() doesn't take into account the fact that per-rq
extra_bw variables retain values computed before root domain changes,
resulting in broken accounting.

Fix it by resetting extra_bw to max_bw before restoring back dl-servers
contributions.

Fixes: 2ff899e ("sched/deadline: Rebuild root domain accounting after every update")
Reported-by: Marcel Ziswiler <marcel.ziswiler@codethink.co.uk>
Signed-off-by: Juri Lelli <juri.lelli@redhat.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Tested-by: Marcel Ziswiler <marcel.ziswiler@codethink.co.uk> # nuc & rock5b
Link: https://lore.kernel.org/r/20250627115118.438797-3-juri.lelli@redhat.com

Author: jlelli

kernel/sched/deadline.c

@@ -2924,7 +2924,14 @@ void dl_clear_root_domain(struct root_domain *rd)
 	int i;
 
 	guard(raw_spinlock_irqsave)(&rd->dl_bw.lock);
+
+	/*
+	 * Reset total_bw to zero and extra_bw to max_bw so that next
+	 * loop will add dl-servers contributions back properly,
+	 */
 	rd->dl_bw.total_bw = 0;
+	for_each_cpu(i, rd->span)
+		cpu_rq(i)->dl.extra_bw = cpu_rq(i)->dl.max_bw;
 
 	/*
 	 * dl_servers are not tasks. Since dl_add_task_root_domain ignores