Re: perf probe and bitfields

[David Ahern <dsahern@gmail.com>]

On 1/26/18 10:04 PM, Masami Hiramatsu wrote:
> On Fri, 26 Jan 2018 09:28:49 -0700
> David Ahern <dsahern@gmail.com> wrote:
> 
>> On 1/25/18 11:56 PM, Namhyung Kim wrote:
>>> Hi David,
>>>
>>> On Fri, Jan 19, 2018 at 08:43:50AM -0800, David Ahern wrote:
>>>> Hi Arnaldo / Masami:
>>>>
>>>> Are there known issues / limitations with perf probe and bitfields?
>>>>
>>>> Here's an example: pkt_type is a bitfield in sk_buff. When I put a probe
>>>> in arp_rcv to examine the pkt_type value:
>>>>
>>>> $ perf probe 'arp_rcv ptype=skb->pkt_type dev=dev->name:string'
>>>
>>> Bitfield type takes 3 parameters according to the Doc/perf-probe.txt.
>>> Maybe it's like 'ptype=skb->pkt_type:b3@0/8' ?
>>
>> missed that. Thanks, Namyhung.
> 
> 
> 
> Hmm, if you have debuginfo, it must be automatically translated.
> E.g.
> 
> $ sudo perf probe -D "arp_rcv ptype=skb->pkt_type dev=dev->name:string"
> p:probe/arp_rcv _text+8135904 ptype=+144(%di):b3@5/8 dev=+0(%si):string
> 
> So as you can see the ptype automatically has ":b3@5/8" types.

I was expecting it to automatically figure that out. The kernels in
question were custom built and the vmlinux must not have been available.

Thanks the tip.