linux-kernel - Re: [PATCH V2] cgroup: Trace event cgroup id fields should be u64

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Date:   Wed, 1 Dec 2021 16:27:54 +0000
From:   William Kucharski <william.kucharski@...cle.com>
To:     Steven Rostedt <rostedt@...dmis.org>
CC:     Ingo Molnar <mingo@...hat.com>, Tejun Heo <tj@...nel.org>,
        Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        LKML <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH V2] cgroup: Trace event cgroup id fields should be u64

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.

> On Dec 1, 2021, at 9:14 AM, Steven Rostedt <rostedt@...dmis.org> wrote:
> 
> On Wed,  1 Dec 2021 09:07:46 -0700
> William Kucharski <william.kucharski@...cle.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@...cle.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