[PATCH 1/2] tracing/mm: don't trace kfree on offline CPUs

[Denis Kirjanov <kda@linux-powerpc.org>]

Tracepoints use RCU for protection and they must not be called on
offline CPUS. So make this tracepoint conditional.

INFO: suspicious RCU usage. ]
[   99.749382] 4.4.0-00006-g0fe53e8-dirty #34 Tainted: G S
[   99.749383] -------------------------------
[   99.749385] include/trace/events/kmem.h:141 suspicious rcu_dereference_check() usage!
[   99.749385]
other info that might help us debug this:

[   99.749387]
RCU used illegally from offline CPU!
rcu_scheduler_active = 1, debug_locks = 1
[   99.749388] no locks held by swapper/8/0.
[   99.749388]
stack backtrace:
[   99.749390] CPU: 8 PID: 0 Comm: swapper/8 Tainted: G S              4.4.0-00006-g0fe53e8-dirty #34
[   99.749391] Call Trace:
[   99.749399] [c0000005b76c78d0] [c0000000008b9540] .dump_stack+0x98/0xd4 (unreliable)
[   99.749402] [c0000005b76c7950] [c00000000010c898] .lockdep_rcu_suspicious+0x108/0x170
[   99.749405] [c0000005b76c79e0] [c00000000029adc0] .kfree+0x390/0x440
[   99.749409] [c0000005b76c7a80] [c000000000055f74] .destroy_context+0x44/0x100
[   99.749412] [c0000005b76c7b00] [c0000000000934a0] .__mmdrop+0x60/0x150
[   99.749415] [c0000005b76c7b90] [c0000000000e3ff0] .idle_task_exit+0x130/0x140
[   99.749417] [c0000005b76c7c20] [c000000000075804] .pseries_mach_cpu_die+0x64/0x310
[   99.749420] [c0000005b76c7cd0] [c000000000043e7c] .cpu_die+0x3c/0x60
[   99.749423] [c0000005b76c7d40] [c0000000000188d8] .arch_cpu_idle_dead+0x28/0x40
[   99.749426] [c0000005b76c7db0] [c000000000101e6c] .cpu_startup_entry+0x50c/0x560
[   99.749428] [c0000005b76c7ed0] [c000000000043bd8] .start_secondary+0x328/0x360
[   99.749431] [c0000005b76c7f90] [c000000000008a6c] start_secondary_prolog+0x10/0x14

Signed-off-by: Denis Kirjanov <kda@linux-powerpc.org>
---
 include/trace/events/kmem.h | 15 +++++++++++++--
 1 file changed, 13 insertions(+), 2 deletions(-)

diff --git a/include/trace/events/kmem.h b/include/trace/events/kmem.h
index f7554fd..9473c2d 100644
--- a/include/trace/events/kmem.h
+++ b/include/trace/events/kmem.h
@@ -133,11 +133,22 @@ DECLARE_EVENT_CLASS(kmem_free,
 	TP_printk("call_site=%lx ptr=%p", __entry->call_site, __entry->ptr)
 );
 
-DEFINE_EVENT(kmem_free, kfree,
+DEFINE_EVENT_CONDITION(kmem_free, kfree,
 
 	TP_PROTO(unsigned long call_site, const void *ptr),
 
-	TP_ARGS(call_site, ptr)
+	TP_ARGS(call_site, ptr),
+
+	/*
+	 * This trace can be potentially called from an offlined cpu.
+	 * Since trace points use RCU and RCU should not be used from
+	 * offline cpus, filter such calls out.
+	 * While this trace can be called from a preemptable section,
+	 * it has no impact on the condition since tasks can migrate
+	 * only from online cpus to other online cpus. Thus its safe
+	 * to use raw_smp_processor_id.
+	 */
+	TP_CONDITION(cpu_online(raw_smp_processor_id()))
 );
 
 DEFINE_EVENT_CONDITION(kmem_free, kmem_cache_free,
-- 
2.4.0