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Subject[PATCH 35/37] perf callchain: Save eh/debug frame offset for dwarf unwind
Date
When libunwind tries to resolve callchains it needs to know the offset
of .eh_frame_hdr or .debug_frame to access the dso. Since it calls
dso__data_fd(), it'll try to grab dso->lock everytime for same
information. So save it to dso_data struct and reuse it.

Note that there's a window between dso__data_fd() and actual use of
the fd. The fd could be closed by other threads to deal with the open
file limit in dso cache code. But I think it's ok since in that case
elf_section_offset() will return 0 so it'll be tried in next acess.

Signed-off-by: Namhyung Kim <namhyung@kernel.org>
---
tools/perf/util/dso.h | 1 +
tools/perf/util/unwind-libunwind.c | 31 ++++++++++++++++++++-----------
2 files changed, 21 insertions(+), 11 deletions(-)

diff --git a/tools/perf/util/dso.h b/tools/perf/util/dso.h
index 28e8bb320495..5cf8dfe04eac 100644
--- a/tools/perf/util/dso.h
+++ b/tools/perf/util/dso.h
@@ -143,6 +143,7 @@ struct dso {
u32 status_seen;
size_t file_size;
struct list_head open_entry;
+ u64 frame_offset;
} data;

union { /* Tool specific area */
diff --git a/tools/perf/util/unwind-libunwind.c b/tools/perf/util/unwind-libunwind.c
index 94929ec491f3..3cb78c36faad 100644
--- a/tools/perf/util/unwind-libunwind.c
+++ b/tools/perf/util/unwind-libunwind.c
@@ -244,14 +244,17 @@ static int read_unwind_spec_eh_frame(struct dso *dso, struct machine *machine,
u64 *fde_count)
{
int ret = -EINVAL, fd;
- u64 offset;
+ u64 offset = dso->data.frame_offset;

- fd = dso__data_fd(dso, machine);
- if (fd < 0)
- return -EINVAL;
+ if (offset == 0) {
+ fd = dso__data_fd(dso, machine);
+ if (fd < 0)
+ return -EINVAL;

- /* Check the .eh_frame section for unwinding info */
- offset = elf_section_offset(fd, ".eh_frame_hdr");
+ /* Check the .eh_frame section for unwinding info */
+ offset = elf_section_offset(fd, ".eh_frame_hdr");
+ dso->data.frame_offset = offset;
+ }

if (offset)
ret = unwind_spec_ehframe(dso, machine, offset,
@@ -265,14 +268,20 @@ static int read_unwind_spec_eh_frame(struct dso *dso, struct machine *machine,
static int read_unwind_spec_debug_frame(struct dso *dso,
struct machine *machine, u64 *offset)
{
- int fd = dso__data_fd(dso, machine);
+ int fd;
+ u64 ofs = dso->data.frame_offset;

- if (fd < 0)
- return -EINVAL;
+ if (ofs == 0) {
+ fd = dso__data_fd(dso, machine);
+ if (fd < 0)
+ return -EINVAL;

- /* Check the .debug_frame section for unwinding info */
- *offset = elf_section_offset(fd, ".debug_frame");
+ /* Check the .debug_frame section for unwinding info */
+ ofs = elf_section_offset(fd, ".debug_frame");
+ dso->data.frame_offset = ofs;
+ }

+ *offset = ofs;
if (*offset)
return 0;

--
2.1.3


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