sched/deadline: Reject debugfs dl_server writes for offline CPUs

Writing runtime or period via the per-CPU dl_server debugfs files
(/sys/kernel/debug/sched/{fair,ext}_server/cpu*/{runtime,period}) on an
offline CPU can trigger two distinct kernel issues:

1) Divide-by-zero in dl_server_apply_params():

  Oops: divide error: 0000 [#1] SMP NOPTI
  RIP: 0010:dl_server_apply_params+0x239/0x3a0
  Call Trace:
   sched_server_write_common.isra.0+0x21a/0x3c0
   full_proxy_write+0x78/0xd0
   vfs_write+0xe7/0x6e0

  Both __dl_sub() and __dl_add() divide by cpus internally, which can be
  0 once the CPU has been removed from any active root-domain span (this
  has been latent since the debugfs interface was introduced).

2) WARN_ON_ONCE in dl_server_start():

  WARNING: kernel/sched/deadline.c:1805 at dl_server_start+0x232/0x270

  Commit ee6e44dfe6 ("sched/deadline: Stop dl_server before CPU goes
  offline") added this check to catch enqueueing the server on an
  offline rq.

There's no meaningful semantics for re-configuring the per-CPU dl_server
bandwidth while the CPU is offline, so simply reject the write with
-EBUSY so userspace gets a clear error.

Closes: https://lore.kernel.org/all/20260526092228.3B6891F00A3A@smtp.kernel.org/
Fixes: d741f297bc ("sched/fair: Fair server interface")
Reported-by: Sashiko <sashiko-bot@kernel.org>
Signed-off-by: Andrea Righi <arighi@nvidia.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Reviewed-by: Juri Lelli <juri.lelli@redhat.com>
Tested-by: abaci-kreproducer <abaci@linux.alibaba.com>
Link: https://patch.msgid.link/20260526100502.575774-1-arighi@nvidia.com

kernel/sched/debug.c

@@ -416,6 +416,9 @@ static ssize_t sched_server_write_common(struct file *filp, const char __user *u
 			return  -EINVAL;
 		}
 
+		if (!cpu_online(cpu_of(rq)))
+			return -EBUSY;
+
 		update_rq_clock(rq);
 		dl_server_stop(dl_se);
 		retval = dl_server_apply_params(dl_se, runtime, period, 0);