On Fri, 12 Jan 2024 10:06:41 -0500
Steven Rostedt <rost...@goodmis.org> wrote:
> I'm thinking both may be good, as the number of dropped events are not
> added if there's no room to put it at the end of the ring buffer. When
> there's no room, it just sets a flag that there was missed events but
> doesn't give how many events were missed.
>
> If anything, it should be in both locations. In the sub-buffer header, to
> be consistent with the read/splice version, and in the meta page were if
> there's no room to add the count, it can be accessed in the meta page.
I think the meta data page should look like this:
struct trace_buffer_meta {
__u32 meta_page_size;
__u32 meta_struct_len;
__u32 subbuf_size;
__u32 nr_subbufs;
struct {
__u64 lost_events;
__u32 id;
__u32 read;
} reader;
__u64 entries;
__u64 overrun;
__u64 read;
};
1) meta_page_size
The size of this meta page. It's the first thing the application
needs to know how much to mmap.
2) meta_struct_len
The actual length of the structure. It's the second thing the
application will look at to know what version it is dealing with.
3) subbuf_size
4) nr_subbufs
The next two is the next thing the application should do. That is to
memory map in all the sub-buffers. With 1) and this, it knows how to
do that.
The first four are needed for setup, and never changes once mapped. The
rest gets updated during the trace.
5) reader
5a) lost_events
5b) id
5c) read
This is actively needed during tracing. It is what is used to know
where and how to read the reader sub-buffer.
6) entries
7) overrun
8) read
These are useful statistics, but probably seldom really looked at.
They should be at the end.
Also notice that I converted all "unsigned long" to __u64. This is because
it is possible to have a 32 bit userspace running this on top of a 64 bit
kernel. If we were to use "long" it would be inconsistent and buggy.
Now if you need subbufs_touched and subbufs_lost. When that gets opened, it
would update the meta_struct_len accordingly, and the structure would look
like:
struct trace_buffer_meta {
__u32 meta_page_size;
__u32 meta_struct_len;
__u32 subbuf_size;
__u32 nr_subbufs;
struct {
__u64 lost_events;
__u32 id;
__u32 read;
} reader;
__u64 entries;
__u64 overrun;
__u64 read;
__u64 subbufs_touched;
__u64 subbufs_lost;
};
Make sense?
-- Steve
