[PATCH v3] sched/deadline: fix the hang in dl_task_of

[Huang Shijie <shijie@os.amperecomputing.com>]

When we enable the schedstats, we will meet an OS hang like this:
  --------------------------------------------------------
	[  134.104253] kernel BUG at kernel/sched/deadline.c:63!
	[  134.132013] ------------[ cut here ]------------
	[  134.133441]  x27: 0000000000000001
	[  134.138048] kernel BUG at kernel/sched/deadline.c:63!
	[  134.146478] x26: 0000000000000001 x25: 0000000000000000 x24: 0000000000000001
	[  134.153607] x23: 0000000000000001 x22: 0000000000000000 x21: 0000000000000001
	[  134.160734] x20: ffff007dbf1b6d00 x19: ffff007dbf1b7610 x18: 0000000000000014
	[  134.162027] ------------[ cut here ]------------
	[  134.167861] x17: 000000009deab6cd x16: 00000000527c9a1c x15: 00000000000000dc
	[  134.172473] kernel BUG at kernel/sched/deadline.c:63!
	[  134.179595] x14: 0000000001200011 x13: 0000000040001000 x12: 0000ffffb6df05bc
	[  134.191760] x11: ffff007dbf1b6d00 x10: ffff0001062dd2e8 x9 : ffff8000801215ac
	[  134.192036] ------------[ cut here ]------------
	[  134.198888] x8 : 0000000000000000 x7 : 0000000000000021 x6 : ffff0001764ed280
	[  134.203498] kernel BUG at kernel/sched/deadline.c:63!
	[  134.210622] x5 : 0000000000000000 x4 : 0000000000000001 x3 : ffff807d3dd24000
	[  134.222787] x2 : 000000028b77a140 x1 : 0000003400000000 x0 : ffff007dbf1b6c80
	[  134.229915] Call trace:
	[  134.232353]  dl_task_of.part.0+0x0/0x10
	[  134.236182]  dl_server_start+0x54/0x158
	[  134.240013]  enqueue_task_fair+0x138/0x420
	[  134.244100]  enqueue_task+0x44/0xb0
	[  134.247584]  wake_up_new_task+0x1c0/0x3a0
	[  134.251584]  kernel_clone+0xe8/0x3e8
	[  134.252022] ------------[ cut here ]------------
	[  134.255156]  __do_sys_clone+0x70/0xa8
	[  134.259764] kernel BUG at kernel/sched/deadline.c:63!
	[  134.263412]  __arm64_sys_clone+0x28/0x40
	[  134.272360]  invoke_syscall+0x50/0x120
	[  134.276101]  el0_svc_common+0x44/0xf8
	[  134.279753]  do_el0_svc+0x28/0x40
	[  134.283058]  el0_svc+0x40/0x150
	[  134.286195]  el0t_64_sync_handler+0x100/0x130
	[  134.290546]  el0t_64_sync+0x1a4/0x1a8
	[  134.294200] Code: 35ffffa2 17ffffe3 d4210000 17ffffb4 (d4210000)
	[  134.300283] ---[ end trace 0000000000000000 ]---
	[  134.304890] Kernel panic - not syncing: Oops - BUG: Fatal exception
	[  134.311147] SMP: stopping secondary CPUs
	[  135.365096] SMP: failed to stop secondary CPUs 8-9,16,30,43,86,88,121,149
	[  135.371884] Kernel Offset: disabled
	[  135.375361] CPU features: 0x00,00100003,80153d29,d75ffea7
	[  135.380749] Memory Limit: none
	[  135.383793] ---[ end Kernel panic - not syncing: Oops - BUG: Fatal exception ]
  --------------------------------------------------------

In dl_server_start(), we set the dl_se->dl_server with 1. When schedstats
is enabled, in the following:
   dl_server_start() --> enqueue_dl_entity() --> update_stats_enqueue_dl()
	__schedstats_from_dl_se() -->dl_task_of()

we will meet the BUG_ON.

Since the fair task has already had its own schedstats, there is no need
to track anything for the associated dl_server.

So add check in:
            update_stats_wait_start_dl()
	    update_stats_wait_end_dl()
	    update_stats_enqueue_dl()
	    update_stats_dequeue_dl()

return early for a dl_server dl_se.

Tested this patch with memcached in Altra.

Fixes: 5f6bd380c7bd ("sched/rt: Remove default bandwidth control")
Signed-off-by: Huang Shijie <shijie@os.amperecomputing.com>
---
v2 --> v3:
   Return early in:
            update_stats_wait_start_dl()
	    update_stats_wait_end_dl()
	    update_stats_enqueue_dl()
	    update_stats_dequeue_dl()
   
   The v2 link:
    https://lore.kernel.org/lkml/770da6f7-89f6-49b7-b8db-a7318abbd828@arm.com/T/

v1 --> v2:
   Skip the update_stats_enqueue_dl() for a dl_server dl_se
   in enqueue_dl_entity().

   Btw, the update_stats_{wait_start,wait_end}_dl has already had
   the dl_server() check.

   The v1 link: https://lore.kernel.org/lkml/20240826021115.9284-1-shijie@os.amperecomputing.com/T/

---
 kernel/sched/deadline.c | 12 ++++++++++++
 1 file changed, 12 insertions(+)

diff --git a/kernel/sched/deadline.c b/kernel/sched/deadline.c
index 0f2df67f710b..b1ccc1b6e7b9 100644
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1910,6 +1910,9 @@ update_stats_wait_start_dl(struct dl_rq *dl_rq, struct sched_dl_entity *dl_se)
 	if (!schedstat_enabled())
 		return;
 
+	if (dl_server(dl_se))
+		return;
+
 	stats = __schedstats_from_dl_se(dl_se);
 	__update_stats_wait_start(rq_of_dl_rq(dl_rq), dl_task_of(dl_se), stats);
 }
@@ -1922,6 +1925,9 @@ update_stats_wait_end_dl(struct dl_rq *dl_rq, struct sched_dl_entity *dl_se)
 	if (!schedstat_enabled())
 		return;
 
+	if (dl_server(dl_se))
+		return;
+
 	stats = __schedstats_from_dl_se(dl_se);
 	__update_stats_wait_end(rq_of_dl_rq(dl_rq), dl_task_of(dl_se), stats);
 }
@@ -1945,6 +1951,9 @@ update_stats_enqueue_dl(struct dl_rq *dl_rq, struct sched_dl_entity *dl_se,
 	if (!schedstat_enabled())
 		return;
 
+	if (dl_server(dl_se))
+		return;
+
 	if (flags & ENQUEUE_WAKEUP)
 		update_stats_enqueue_sleeper_dl(dl_rq, dl_se);
 }
@@ -1958,6 +1967,9 @@ update_stats_dequeue_dl(struct dl_rq *dl_rq, struct sched_dl_entity *dl_se,
 	if (!schedstat_enabled())
 		return;
 
+	if (dl_server(dl_se))
+		return;
+
 	if ((flags & DEQUEUE_SLEEP)) {
 		unsigned int state;
 
-- 
2.40.1