[RFC][RT][PATCH] mm: Do not use stop_machine() for__zone_pcp_udpate() for CONFIG_PREEMPT_RT_FULL

[Aaditya Kumar]

The code path of __zone_pcp_update() has following locks, which in
CONFIG_PREEMPT_RT_FULL=y are rt-mutex.
  - pa_lock locked by cpu_lock_irqsave()
  - zone->lock locked by free_pcppages_bulk()

Since __zone_pcp_update() is called from stop_machine(), so with
CONFIG_PREEMPT_RT_FULL=y
we get following backtrace when __zone_pcp_update() is called during
memory hot plugging while
doing heavy file I/O.

I think stop_machine() may not be required for calling __zone_pcp_update()
in case of CONFIG_PREEMPT_RT_FULL=y as acquiring pa_lock in __zone_pcp_update()
should be sufficient to isolate pcp pages and to setup per cpu pagesets.

Can someone please let me know if am missing anything here?


The backtrace that this patch fixes:
 BUG: scheduling while atomic: migration/0/7/0x00000002
 Modules linked in: v2p
 Backtrace:
 [<800111a0>] (dump_backtrace+0x0/0x10c) from [<802d7b7c>]
(dump_stack+0x18/0x1c)
  r6:80c8fc28 r5:80c8f9a0 r4:00000000 r3:60000013
 [<802d7b64>] (dump_stack+0x0/0x1c) from [<8001e81c>] (__schedule_bug+0x64/0x74)
 [<8001e7b8>] (__schedule_bug+0x0/0x74) from [<802d7fa0>]
(__schedule+0x68/0x604)
  r4:8051bf00 r3:00000000
 [<802d7f38>] (__schedule+0x0/0x604) from [<802d8a78>] (schedule+0x98/0xbc)
 [<802d89e0>] (schedule+0x0/0xbc) from [<802d9e14>]
(rt_spin_lock_slowlock+0x168/0x240)
  r4:805228f4 r3:00000000
 [<802d9cac>] (rt_spin_lock_slowlock+0x0/0x240) from [<802da234>]
(rt_spin_lock+0x10/0x14)
 [<802da224>] (rt_spin_lock+0x0/0x14) from [<8008694c>]
(__zone_pcp_update+0x58/0xd8)
 [<800868f4>] (__zone_pcp_update+0x0/0xd8) from [<800603ec>]
(stop_machine_cpu_stop+0xb0/0x104)
 [<8006033c>] (stop_machine_cpu_stop+0x0/0x104) from [<80060200>]
(cpu_stopper_thread+0xd4/0x188)


Signed-off-by: Aaditya Kumar <aaditya.kumar@xxxxxxxxxxx>

---
 mm/page_alloc.c |    4 	4 +	0 -	0 !
 1 file changed, 4 insertions(+)

Index: b/mm/page_alloc.c
===================================================================
--- a/mm/page_alloc.c
+++ b/mm/page_alloc.c
@@ -3868,7 +3868,11 @@ static int __zone_pcp_update(void *data)

 void zone_pcp_update(struct zone *zone)
 {
+#ifndef CONFIG_PREEMPT_RT_FULL
 	stop_machine(__zone_pcp_update, zone, NULL);
+#else
+	__zone_pcp_update(zone);
+#endif
 }

 static __meminit void zone_pcp_init(struct zone *zone)
--
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