Messages in this thread Patch in this message |  | | | From | David Sharp <> | | Subject | [PATCH 04/15] ftrace: pack event structures. | | Date | Fri, 3 Dec 2010 16:13:18 -0800 |
| |
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
|  |