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SubjectRe: [PATCH V2] cgroup: Trace event cgroup id fields should be u64
On Wed,  1 Dec 2021 09:07:46 -0700
William Kucharski <william.kucharski@oracle.com> wrote:

> Various trace event fields that store cgroup IDs were declared as
> ints, but cgroup_id(() returns a u64 and the structures and associated
> TP_printk() calls were not updated to reflect this.
>
> Fixes: 743210386c03 ("cgroup: use cgrp->kn->id as the cgroup ID")
> Signed-off-by: William Kucharski <william.kucharski@oracle.com>
> ---
> V2: Do not remove spaces from macro arguments
>
> include/trace/events/cgroup.h | 12 ++++++------
> 1 file changed, 6 insertions(+), 6 deletions(-)
>
> diff --git a/include/trace/events/cgroup.h b/include/trace/events/cgroup.h
> index 7f42a3de59e6..1b68c842ac46 100644
> --- a/include/trace/events/cgroup.h
> +++ b/include/trace/events/cgroup.h
> @@ -59,7 +59,7 @@ DECLARE_EVENT_CLASS(cgroup,
>
> TP_STRUCT__entry(
> __field( int, root )
> - __field( int, id )
> + __field( u64, id )
> __field( int, level )

It's best to move them around to prevent holes. This is not a packed
structure, and the above will create a structure on the ring buffer that
looks like:

int root;
u64 id;
int level;
short path_offset;
short path_size;

(the string() macro is a 4 byte word where the half is the offset of the
actual string and the other half is the size of the string).

Having a 8 byte word between two 4 byte words, will add a 4 byte padding
after the first 4 byte word (before the 8 byte word), and waste space on
the ring buffer.

Better to move it around to be:

TP_STRUCT__entry(
__field( int, root )
__field( int, level )
__field( u64, id )

That way the two 4 byte words will be together followed directly by the 8
byte word.

> __string( path, path )
> ),
> @@ -71,7 +71,7 @@ DECLARE_EVENT_CLASS(cgroup,
> __assign_str(path, path);
> ),
>
> - TP_printk("root=%d id=%d level=%d path=%s",
> + TP_printk("root=%d id=%llu level=%d path=%s",
> __entry->root, __entry->id, __entry->level, __get_str(path))
> );
>
> @@ -126,7 +126,7 @@ DECLARE_EVENT_CLASS(cgroup_migrate,
>
> TP_STRUCT__entry(
> __field( int, dst_root )
> - __field( int, dst_id )
> + __field( u64, dst_id )

Same here. Just move dst_level above dst_id.

> __field( int, dst_level )
> __field( int, pid )
> __string( dst_path, path )
> @@ -142,7 +142,7 @@ DECLARE_EVENT_CLASS(cgroup_migrate,
> __assign_str(comm, task->comm);
> ),
>
> - TP_printk("dst_root=%d dst_id=%d dst_level=%d dst_path=%s pid=%d comm=%s",
> + TP_printk("dst_root=%d dst_id=%llu dst_level=%d dst_path=%s pid=%d comm=%s",
> __entry->dst_root, __entry->dst_id, __entry->dst_level,
> __get_str(dst_path), __entry->pid, __get_str(comm))
> );
> @@ -171,7 +171,7 @@ DECLARE_EVENT_CLASS(cgroup_event,
>
> TP_STRUCT__entry(
> __field( int, root )
> - __field( int, id )
> + __field( u64, id )

And here.

> __field( int, level )
> __string( path, path )
> __field( int, val )
> @@ -185,7 +185,7 @@ DECLARE_EVENT_CLASS(cgroup_event,
> __entry->val = val;
> ),
>
> - TP_printk("root=%d id=%d level=%d path=%s val=%d",
> + TP_printk("root=%d id=%llu level=%d path=%s val=%d",
> __entry->root, __entry->id, __entry->level, __get_str(path),
> __entry->val)
> );



-- Steve

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