NVIDIA: SAUCE: sched/deadline: Skip DL server initialization on nohz_full CPUs

The DL scheduler server (fair_server) is initialized on every online CPU
with a default runtime of 50ms/1000ms. On nohz_full isolated CPUs, the
DL server's dl_task_timer hrtimer fires periodically, which triggers
hrtimer_interrupt(). This re-activates the tick's sched_timer (via
tick_nohz_handler returning HRTIMER_RESTART) before the IRQ-exit path
can suppress it through tick_nohz_full_update_tick(), causing the
scheduler tick to run at full HZ rate on isolated CPUs.

This is a regression from v6.8 where the DL server did not exist and
nohz_full CPUs had no spurious hrtimer activity.

Measured with osnoise tracer on ARM64 (nohz_full=4-64):
  Before: IRQ=~1000/s SIRQ=~30/s on isolated CPUs
  After:  IRQ=0       SIRQ=0     on isolated CPUs (100.00000%)

Verified across 6 reboot cycles with the osnoise method (CPUs 0-60,
10s runtime) - all isolated CPUs consistently at 100.00000% CPU
availability.

Skip DL server initialization for nohz_full CPUs since they are
isolated and do not need CFS bandwidth protection.

Fixes: bd9bbc9 ("sched: Rework dl_server")
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Daniel Bristot de Oliveira <bristot@kernel.org>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Frederic Weisbecker <frederic@kernel.org>
Signed-off-by: Koba Ko <kobak@nvidia.com>

Author: kobak2026

kernel/sched/deadline.c

@@ -1620,6 +1620,9 @@ void sched_init_dl_servers(void)
 		u64 runtime =  50 * NSEC_PER_MSEC;
 		u64 period = 1000 * NSEC_PER_MSEC;
 
+		if (tick_nohz_full_cpu(cpu))
+			continue;
+
 		rq = cpu_rq(cpu);
 
 		guard(rq_lock_irq)(rq);