[PATCH] tracing: Add comments explaining cpu online filter for trace events

[Shreyas B. Prabhu]

trace_mm_page_pcpu_drain, trace_kmem_cache_free, trace_mm_page_free
and trace_tlb_flush can be potentially called from an offlined cpu.
Since trace points use RCU and RCU should not be used from offlined
cpus, we have checks to filter out such calls. Add comments to explain
this.

Signed-off-by: Shreyas B. Prabhu <shreyas@xxxxxxxxxxxxxxxxxx>
---
This applies on top of patches posted here:
https://lkml.org/lkml/2015/5/8/527

 include/trace/events/kmem.h | 15 +++++++++++++++
 include/trace/events/tlb.h  |  5 +++++
 2 files changed, 20 insertions(+)

diff --git a/include/trace/events/kmem.h b/include/trace/events/kmem.h
index 6cd975f..9883f2f 100644
--- a/include/trace/events/kmem.h
+++ b/include/trace/events/kmem.h
@@ -146,6 +146,11 @@ DEFINE_EVENT_CONDITION(kmem_free, kmem_cache_free,
 
 	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.
+	 */
 	TP_CONDITION(cpu_online(smp_processor_id()))
 );
 
@@ -155,6 +160,11 @@ TRACE_EVENT_CONDITION(mm_page_free,
 
 	TP_ARGS(page, order),
 
+	/*
+	 * 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.
+	 */
 	TP_CONDITION(cpu_online(smp_processor_id())),
 
 	TP_STRUCT__entry(
@@ -263,6 +273,11 @@ TRACE_EVENT_CONDITION(mm_page_pcpu_drain,
 
 	TP_ARGS(page, order, migratetype),
 
+	/*
+	 * 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.
+	 */
 	TP_CONDITION(cpu_online(smp_processor_id())),
 
 	TP_STRUCT__entry(
diff --git a/include/trace/events/tlb.h b/include/trace/events/tlb.h
index 4250f36..2afc3ed 100644
--- a/include/trace/events/tlb.h
+++ b/include/trace/events/tlb.h
@@ -38,6 +38,11 @@ TRACE_EVENT_CONDITION(tlb_flush,
 	TP_PROTO(int reason, unsigned long pages),
 	TP_ARGS(reason, pages),
 
+	/*
+	 * 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.
+	 */
 	TP_CONDITION(cpu_online(smp_processor_id())),
 
 	TP_STRUCT__entry(
-- 
1.9.3

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