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SubjectRe: [PATCH V2] cgroup: Trace event cgroup id fields should be u64
On Wed, 1 Dec 2021 16:27:54 +0000
William Kucharski <william.kucharski@oracle.com> wrote:

> I had pondered that as a consequence for two of the three uses but wasn't sure
> if it was worth reordering things; I can easily do so.
>
> What do you suggest for cgroup_migrate as that will have a hole either way
> as it's:
>
> TP_STRUCT__entry(
> __field( int, dst_root )
> __field( u64, dst_id )
> __field( int, dst_level )
> __field( int, pid )
> __string( dst_path, path )
> __string( comm, task->comm )
> ),
>
> if I put dst_level above dst_id, the int for pid field will leave a hole
> anyway because the string pointer for dst_path will want to be 64-bit
> aligned.
>
> Thanks in advance.
>

It's not actually a string pointer, both string() macros are really 4 byte
words. The above would be best compressed as:

TP_STRUCT__entry(
__field( int, dst_root )
__field( int, dst_level )
__field( u64, dst_id )
__field( int, pid )
__string( dst_path, path )
__string( comm, task->comm )
),

What that would turn into is:

struct __entry {
int dst_root;
int dst_level;
u64 dst_id;
int pid;
int dst_path_offset_size;
int comm_offset_size;
char strings[];
}

Remember, this is used for storing data onto the ring buffer. I'll make up
two strings to show an example.

dst_path = "/path/to/dst_path";
comm = "bash";

The string(dst_path) and string(comm) would be where the data to find the
strings are. The strings will be stored in the strings[] field of the
structure, and the dst_path and comm fields will be used to find those
strings.

dst_root = 1;
dst_level = 2;
dst_id = 3;
pid = 4;

dst_path = (28 << 16) | 18;
comm = ((28 + 18) << 16 | 5;

strings = "/path/to/dst_path\0bash\0";


The dst_path holds how to find the string it represents. It starts at
offset 28[*] (sizeof(int) * 5 + sizeof(u64))
And has size of 18 (strlen("/path/to/dst_path") + 1)

The comm string starts at offset 46 (28 + size of dst_path), and has he
size of 5 (strlen("bash") + 1).

So consider "string()" macros to be only 4 bytes in size. And leave the
"holes" at the end, especially if there's strings, because the will start
the actual strings nicely aligned.

-- Steve

[*] It really starts at a later offset, because there's common fields added
before the first field (common_type, common_flags, common_preempt_count,
common_pid) that is also included in that offset count.

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