Re: [PATCH] perf session: Try to read pipe data from file

[Jiri Olsa <jolsa@redhat.com>]

On Fri, May 01, 2020 at 01:34:47PM +0200, Jiri Olsa wrote:
> From: Jiri Olsa <jolsa@redhat.com>
> 
> Ian came with the idea of having support to read the pipe
> data also from file [1]. Currently pipe mode files fails
> like:
> 
>   $ perf record -o - sleep 1 > /tmp/perf.pipe.data
>   $ perf report -i /tmp/perf.pipe.data
>   incompatible file format (rerun with -v to learn more)
> 
> This patch adds the support to do that by trying the pipe
> header first, and if its successfully detected, switching
> the perf data to pipe mode.
> 
> [1] https://lore.kernel.org/lkml/20200409185744.255881-1-irogers@google.com/
> Original-patch-by: Ian Rogers <irogers@google.com>
> Signed-off-by: Jiri Olsa <jolsa@kernel.org>

actualy.. I found another issue while trying this on tracepoints:

  # ./perf record -g  -e 'raw_syscalls:sys_enter' -o -  true > data
  [ perf record: Woken up 1 times to write data ]
  [ perf record: Captured and wrote 0.000 MB - ]
  # ./perf script -i ./data
  perf_event__process_tracing_data: tracing data size mismatch0x1034 [0xc]: failed to process type: 66

it's because some of the pipe synthesize code calls lseek, which
fails on pipe, but succeeds on normal file (with pipe data)

patch below fixes that for me, but I wonder there are other leftovers
like this.. I'll check on post it all together

jirka


---
diff --git a/tools/perf/util/header.c b/tools/perf/util/header.c
index 8ca709f938b8..33e299674121 100644
--- a/tools/perf/util/header.c
+++ b/tools/perf/util/header.c
@@ -3955,13 +3955,8 @@ int perf_event__process_tracing_data(struct perf_session *session,
 {
 	ssize_t size_read, padding, size = event->tracing_data.size;
 	int fd = perf_data__fd(session->data);
-	off_t offset = lseek(fd, 0, SEEK_CUR);
 	char buf[BUFSIZ];
 
-	/* setup for reading amidst mmap */
-	lseek(fd, offset + sizeof(struct perf_record_header_tracing_data),
-	      SEEK_SET);
-
 	size_read = trace_report(fd, &session->tevent,
 				 session->repipe);
 	padding = PERF_ALIGN(size_read, sizeof(u64)) - size_read;
diff --git a/tools/perf/util/session.c b/tools/perf/util/session.c
index c11d89e0ee55..b75df19feaf1 100644
--- a/tools/perf/util/session.c
+++ b/tools/perf/util/session.c
@@ -1543,7 +1543,8 @@ static s64 perf_session__process_user_event(struct perf_session *session,
 		return 0;
 	case PERF_RECORD_HEADER_TRACING_DATA:
 		/* setup for reading amidst mmap */
-		lseek(fd, file_offset, SEEK_SET);
+		if (!perf_data__is_pipe(session->data))
+			lseek(fd, file_offset, SEEK_SET);
 		return tool->tracing_data(session, event);
 	case PERF_RECORD_HEADER_BUILD_ID:
 		return tool->build_id(session, event);