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Subject[PATCH 04/15] ftrace: pack event structures.
Date
Ftrace event structures have a 12-byte struct trace_entry at the beginning.
If the structure is aligned, this means that if the first field is 64-bits,
there will be 4 bytes of padding. Ironically, due to the 4-byte ringbuffer
header, this will make 64-bit writes unaligned, if the ring buffer position
is currently 64-bit aligned:
4(rb)+12(ftrace)+4(pad) = 20; 20%8 = 4

Adding __attribute__((packed)) to the event structures removes the extra
space from the trace events, and actually improves alignment of trace
events with a first field that is 64-bits.

About 65 tracepoints have a 4-byte pad at offset 12:
# find events -name format | xargs -n1 awk '
$1=="name:" {name=$2}
$1=="format:"{FS="\t"}
$3=="offset:12;" && $4=="size:4;"{okay=1}
$3=="offset:16;" && !okay {print name}' | wc -l
65

With all 'syscalls' and 'timer' events enabled, this results in a 5%
improvement in a simple 512MB read benchmark with warm caches.

Tested:

setup:
# echo 1 >events/syscalls/enable
# echo 1 >events/timer/enable
# echo 0 > tracing_enabled
off:
# for n in $(seq 10) ; do \
time dd if=/dev/hda3 of=/dev/null bs=1K count=512K ; \
done
on:
# for n in $(seq 10) ; do \
echo 1 >tracing_enabled; \
time dd if=/dev/hda3 of=/dev/null bs=1K count=512K ; \
echo 0 >tracing_enabled; \
echo > trace; \
done

real time mean/median/stdev
w/o patch off: 1.1679/1.164/0.0169
w/o patch on : 1.9432/1.936/0.0274
w/ patch off: 1.1715/1.159/0.0431
w/ patch on : 1.8425/1.836/0.0138
"on" delta: -0.1007 --> -5.2%

Google-Bug-Id: 2895627

Signed-off-by: David Sharp <dhsharp@google.com>
---
include/trace/ftrace.h | 5 +++--
kernel/trace/trace.h | 2 +-
2 files changed, 4 insertions(+), 3 deletions(-)

diff --git a/include/trace/ftrace.h b/include/trace/ftrace.h
index a9377c0..51d1f52 100644
--- a/include/trace/ftrace.h
+++ b/include/trace/ftrace.h
@@ -48,7 +48,8 @@
#define __array(type, item, len) type item[len];

#undef __dynamic_array
-#define __dynamic_array(type, item, len) u32 __data_loc_##item;
+#define __dynamic_array(type, item, len) \
+ u32 __data_loc_##item __attribute__((aligned(4)));

#undef __string
#define __string(item, src) __dynamic_array(char, item, -1)
@@ -62,7 +63,7 @@
struct trace_entry ent; \
tstruct \
char __data[0]; \
- }; \
+ } __attribute__((packed)); \
\
static struct ftrace_event_class event_class_##name;

diff --git a/kernel/trace/trace.h b/kernel/trace/trace.h
index 9021f8c..2e80433 100644
--- a/kernel/trace/trace.h
+++ b/kernel/trace/trace.h
@@ -60,7 +60,7 @@ enum trace_type {
struct struct_name { \
struct trace_entry ent; \
tstruct \
- }
+ } __attribute__((packed))

#undef TP_ARGS
#define TP_ARGS(args...) args
--
1.7.3.1


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