[PATCH] libtraceevent: A new API for trace page size

[PATCH] libtraceevent: A new API for trace page size

[Tzvetomir Stoyanov (VMware)]

Currently, the size of a trace buffer page is equal to the system memory
page size. This may change in the future, so this assumption may not be
valid. The proper way to determine the trace buffer page size is by
using the information from "events/header_page" ftrace file:
 ...
 field: char data;	offset:16;	size:4080;	signed:1;
The trace buffer page size is the size of the "data" filed + its offset.

A new libtraceevent API is introduced, for getting the trace buffer page
size using that formula:
 tep_get_trace_page_size()

Note, that some old kernels may not have "events/header_page" file. For
those, the API returns the system memory page size.

Signed-off-by: Tzvetomir Stoyanov (VMware) <tz.stoyanov@gmail.com>
---
 src/event-parse-api.c | 16 ++++++++++++++++
 src/event-parse.h     |  1 +
 2 files changed, 17 insertions(+)

diff --git a/src/event-parse-api.c b/src/event-parse-api.c
index f8361e4..b5be8d3 100644
--- a/src/event-parse-api.c
+++ b/src/event-parse-api.c
@@ -4,6 +4,7 @@
  *
  */
 
+#include <unistd.h>
 #include "event-parse.h"
 #include "event-parse-local.h"
 #include "event-utils.h"
@@ -248,6 +249,21 @@ void tep_set_page_size(struct tep_handle *tep, int _page_size)
 		tep->page_size = _page_size;
 }
 
+/**
+ * tep_get_trace_page_size - get the size of a trace page on the traced machine
+ * @tep: a handle to the tep_handle
+ *
+ * This returns the size of a trace page on the traced machine
+ * If @tep is NULL or the kernel is old, user space page size is returned.
+ */
+int tep_get_trace_page_size(struct tep_handle *tep)
+{
+	if (tep && !tep->old_format)
+		return tep->header_page_data_size + tep->header_page_data_offset;
+
+	return getpagesize();
+}
+
 /**
  * tep_is_file_bigendian - return the endian of the file
  * @tep: a handle to the tep_handle
diff --git a/src/event-parse.h b/src/event-parse.h
index 0833893..ea8df35 100644
--- a/src/event-parse.h
+++ b/src/event-parse.h
@@ -572,6 +572,7 @@ void tep_set_cpus(struct tep_handle *tep, int cpus);
 int tep_get_long_size(struct tep_handle *tep);
 void tep_set_long_size(struct tep_handle *tep, int long_size);
 int tep_get_page_size(struct tep_handle *tep);
+int tep_get_trace_page_size(struct tep_handle *tep);
 void tep_set_page_size(struct tep_handle *tep, int _page_size);
 bool tep_is_file_bigendian(struct tep_handle *tep);
 void tep_set_file_bigendian(struct tep_handle *tep, enum tep_endian endian);
-- 
2.31.1

Re: [PATCH] libtraceevent: A new API for trace page size

[Steven Rostedt]

On Fri,  1 Oct 2021 09:23:38 +0300
"Tzvetomir Stoyanov (VMware)" <tz.stoyanov@gmail.com> wrote:

> Currently, the size of a trace buffer page is equal to the system memory
> page size. This may change in the future, so this assumption may not be
> valid. The proper way to determine the trace buffer page size is by
> using the information from "events/header_page" ftrace file:
>  ...
>  field: char data;	offset:16;	size:4080;	signed:1;
> The trace buffer page size is the size of the "data" filed + its offset.
> 
> A new libtraceevent API is introduced, for getting the trace buffer page
> size using that formula:
>  tep_get_trace_page_size()
> 
> Note, that some old kernels may not have "events/header_page" file. For
> those, the API returns the system memory page size.
> 
> Signed-off-by: Tzvetomir Stoyanov (VMware) <tz.stoyanov@gmail.com>
> ---
>  src/event-parse-api.c | 16 ++++++++++++++++
>  src/event-parse.h     |  1 +
>  2 files changed, 17 insertions(+)
> 
> diff --git a/src/event-parse-api.c b/src/event-parse-api.c
> index f8361e4..b5be8d3 100644
> --- a/src/event-parse-api.c
> +++ b/src/event-parse-api.c
> @@ -4,6 +4,7 @@
>   *
>   */
>  
> +#include <unistd.h>
>  #include "event-parse.h"
>  #include "event-parse-local.h"
>  #include "event-utils.h"
> @@ -248,6 +249,21 @@ void tep_set_page_size(struct tep_handle *tep, int _page_size)
>  		tep->page_size = _page_size;
>  }
>  
> +/**
> + * tep_get_trace_page_size - get the size of a trace page on the traced machine
> + * @tep: a handle to the tep_handle
> + *
> + * This returns the size of a trace page on the traced machine
> + * If @tep is NULL or the kernel is old, user space page size is returned.
> + */
> +int tep_get_trace_page_size(struct tep_handle *tep)

Let's call it:

  tep_get_sub_buffer_size()

as it has nothing to do with pages. (the header_page is really a misnomer).

> +{
> +	if (tep && !tep->old_format)

The old format should be updated to produce the same, and not have the
check here. And if tep is NULL, it should return -1;

> +		return tep->header_page_data_size + tep->header_page_data_offset;
> +
> +	return getpagesize();
> +}

That is, in tep_parse_header_page() we should have:

	if (!size) {
		/*
		 * Old kernels did not have header page info.
		 * Sorry but we just use what we find here in user space.
		 */
		tep->header_page_ts_size = sizeof(long long);
		tep->header_page_size_size = long_size;
		tep->header_page_data_offset = sizeof(long long) + long_size;
+		tep->header_page_data_size = getpagesize() - tep->header_page_data_offset;
		tep->old_format = 1;
		return -1;
	}

-- Steve


> +
>  /**
>   * tep_is_file_bigendian - return the endian of the file
>   * @tep: a handle to the tep_handle
> diff --git a/src/event-parse.h b/src/event-parse.h
> index 0833893..ea8df35 100644
> --- a/src/event-parse.h
> +++ b/src/event-parse.h
> @@ -572,6 +572,7 @@ void tep_set_cpus(struct tep_handle *tep, int cpus);
>  int tep_get_long_size(struct tep_handle *tep);
>  void tep_set_long_size(struct tep_handle *tep, int long_size);
>  int tep_get_page_size(struct tep_handle *tep);
> +int tep_get_trace_page_size(struct tep_handle *tep);
>  void tep_set_page_size(struct tep_handle *tep, int _page_size);
>  bool tep_is_file_bigendian(struct tep_handle *tep);
>  void tep_set_file_bigendian(struct tep_handle *tep, enum tep_endian endian);