[PATCH 02/12] sched/deadline: Do not divide by zero active CPUs of a root domain

[Mathieu Desnoyers <mathieu.desnoyers@efficios.com>]

__dl_sub() and __dl_add() spread a change of a root domain's total_bw
over the extra_bw of its active CPUs, dividing it by their number, which
their callers take from dl_bw_cpus(). That number is 0 for a root domain
whose CPUs all went offline, such as the default root domain, which
holds the offline CPUs: the division then traps.

A debugfs write of a fair server's runtime or period reaches it while
its CPU goes offline or comes online. From the rebuild of the root
domains in sched_cpu_deactivate() until the CPU is offline, and from the
CPU online until sched_cpu_activate(), the CPU sits in the default root
domain, with no active CPU. A write that leaves the server's bandwidth
as it is passes admission there, __dl_overflow() comparing that root
domain's capacity, 0, with total_bw less the old bandwidth plus the new,
0 as well, and dl_server_apply_params() divides. Any other value is
refused with EBUSY, higher directly, lower through the unsigned
subtraction. Once the CPU is offline the write is refused, since
commit 4043f5498416 ("sched/deadline: Reject debugfs dl_server writes
for offline CPUs").

Writing a fair server's own runtime back in a loop while its CPU goes
offline and online oopses on the first round, in 8-CPU VMs of v7.4-rc1
with the four deadline fixes of sched/urgent:

  Oops: divide error: 0000 [#1] SMP NOPTI
  CPU: 6 UID: 0 PID: 110 Comm: dl_server_div0
  RIP: 0010:dl_server_apply_params+0x335/0x380
  Call Trace:
   <TASK>
   sched_server_write_common.isra.0+0x1c0/0x260
   full_proxy_write+0x58/0x90
   vfs_write+0xcb/0x550
   ksys_write+0x6c/0xe0
   do_syscall_64+0x109/0x5d0
   entry_SYSCALL_64_after_hwframe+0x77/0x7f

inactive_task_timer() can reach it too, found by reading: it releases
the bandwidth of a task that left SCHED_DEADLINE, or died, before its
0-lag time from the root domain of the CPU the task is on, and that CPU
may go offline before the timer fires. With admission control off, so
may sched_setattr() to SCHED_DEADLINE of a task asleep on an offline
CPU.

With no active CPU there is no extra_bw to update: change total_bw
alone. 20 rounds of the reproducer leave no warning then.

Fixes: daec57983670 ("sched/deadline: Reclaim bandwidth not used by dl tasks")
Signed-off-by: Mathieu Desnoyers <mathieu.desnoyers@efficios.com>
---
 kernel/sched/deadline.c | 11 +++++++++--
 1 file changed, 9 insertions(+), 2 deletions(-)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 963fda6ddb7f..dadda1a528ec 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -189,18 +189,25 @@ void __dl_update(struct dl_bw *dl_b, s64 bw)
 	}
 }
 
+/*
+ * @cpus is the number of active CPUs of the root domain, whose extra_bw
+ * shares the change: none once they all went offline, and total_bw alone
+ * changes then.
+ */
 static inline
 void __dl_sub(struct dl_bw *dl_b, u64 tsk_bw, int cpus)
 {
 	dl_b->total_bw -= tsk_bw;
-	__dl_update(dl_b, (s32)tsk_bw / cpus);
+	if (cpus)
+		__dl_update(dl_b, (s32)tsk_bw / cpus);
 }
 
 static inline
 void __dl_add(struct dl_bw *dl_b, u64 tsk_bw, int cpus)
 {
 	dl_b->total_bw += tsk_bw;
-	__dl_update(dl_b, -((s32)tsk_bw / cpus));
+	if (cpus)
+		__dl_update(dl_b, -((s32)tsk_bw / cpus));
 }
 
 static inline bool
-- 
2.47.3