Re: [RFC v3 net-next 1/7] net: add rx_sk to trace_kfree_skb

[Yan Zhai]

On Mon, Jun 10, 2024 at 11:54 AM Steven Rostedt <rostedt@xxxxxxxxxxx> wrote:
>
> On Thu, 6 Jun 2024 10:37:46 -0500
> Yan Zhai <yan@xxxxxxxxxxxxxx> wrote:
>
> > > name: kfree_skb
> > > ID: 1799
> > > format:
> > >         field:unsigned short common_type;       offset:0;       size:2; signed:0;
> > >         field:unsigned char common_flags;       offset:2;       size:1; signed:0;
> > >         field:unsigned char common_preempt_count;       offset:3;       size:1; signed:0;
> > >         field:int common_pid;   offset:4;       size:4; signed:1;
> > >
> > >         field:void * skbaddr;   offset:8;       size:8; signed:0;
> > >         field:void * location;  offset:16;      size:8; signed:0;
> > >         field:unsigned short protocol;  offset:24;      size:2; signed:0;
> > >         field:enum skb_drop_reason reason;      offset:28;      size:4; signed:0;
> > >
> > > Notice that "protocol" is 2 bytes in size at offset 24, but "reason" starts
> > > at offset 28. This means at offset 26, there's a 2 byte hole.
> > >
> > The reason I added the pointer as the last argument is trying to
> > minimize the surprise to existing TP users, because for common ABIs
> > it's fine to omit later arguments when defining a function, but it
> > needs change and recompilation if the order of arguments changed.
>
> Nothing should be hard coding the offsets of the fields. This is
> exported to user space so that tools can see where the fields are.
> That's the purpose of libtraceevent. The fields should be movable and
> not affect anything. There should be no need to recompile.
>
Oh I misunderstood previously. I was also thinking about the argument
order in TP_PROTO, but what you mentioned is just the order in
TP_STRUCT__entry, correct? I'd prefer to not change the argument order
but the struct field order can definitely be aligned better here.

Yan

> >
> > Looking at the actual format after the change, it does not add a new
> > hole since protocol and reason are already packed into the same 8-byte
> > block, so rx_skaddr starts at 8-byte aligned offset:
> >
> > # cat /sys/kernel/debug/tracing/events/skb/kfree_skb/format
> > name: kfree_skb
> > ID: 2260
> > format:
> >         field:unsigned short common_type;       offset:0;
> > size:2; signed:0;
> >         field:unsigned char common_flags;       offset:2;
> > size:1; signed:0;
> >         field:unsigned char common_preempt_count;       offset:3;
> >  size:1; signed:0;
> >         field:int common_pid;   offset:4;       size:4; signed:1;
> >
> >         field:void * skbaddr;   offset:8;       size:8; signed:0;
> >         field:void * location;  offset:16;      size:8; signed:0;
> >         field:unsigned short protocol;  offset:24;      size:2; signed:0;
> >         field:enum skb_drop_reason reason;      offset:28;
> > size:4; signed:0;
> >         field:void * rx_skaddr; offset:32;      size:8; signed:0;
> >
> > Do you think we still need to change the order?
>
> Up to you, just wanted to point it out.
>
> -- Steve
>