linux-kernel - Re: [RFC PATCH bpf-next 0/6] bpf, printk: add BTF-based type printing

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Date:   Mon, 20 Apr 2020 17:54:24 -0300
From:   Arnaldo Carvalho de Melo <arnaldo.melo@...il.com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc:     Alan Maguire <alan.maguire@...cle.com>, ast@...nel.org,
        daniel@...earbox.net, yhs@...com, kafai@...com,
        songliubraving@...com, andriin@...com, john.fastabend@...il.com,
        kpsingh@...omium.org, linux-kernel@...r.kernel.org,
        netdev@...r.kernel.org, bpf@...r.kernel.org
Subject: Re: [RFC PATCH bpf-next 0/6] bpf, printk: add BTF-based type printing

Em Sat, Apr 18, 2020 at 09:05:36AM -0700, Alexei Starovoitov escreveu:
> On Fri, Apr 17, 2020 at 11:42:34AM +0100, Alan Maguire wrote:
> > ...gives us:
> > 
> > {{{.next=00000000c7916e9c,.prev=00000000c7916e9c,{.dev=00000000c7916e9c|.dev_scratch=0}}|.rbnode={.__rb_parent_color=0,
 
> This is unreadable.
> I like the choice of C style output, but please format it similar to drgn. Like:
> *(struct task_struct *)0xffff889ff8a08000 = {
> 	.thread_info = (struct thread_info){
> 		.flags = (unsigned long)0,
> 		.status = (u32)0,
> 	},
> 	.state = (volatile long)1,
> 	.stack = (void *)0xffffc9000c4dc000,
> 	.usage = (refcount_t){
> 		.refs = (atomic_t){
> 			.counter = (int)2,
> 		},
> 	},
> 	.flags = (unsigned int)4194560,
> 	.ptrace = (unsigned int)0,
 
> I like Arnaldo's idea as well, but I prefer zeros to be dropped by default.
> Just like %d doesn't print leading zeros by default.
> "%p0<struct sk_buff>" would print them.

I was thinking about another way to compress the output of a given data
structure someone is tracking, having to print it from time to time,
which is to store a copy of the struct as you print it and then, when
printing it again, print just its pointer, i.e. that:

*(struct task_struct *)0xffff889ff8a08000 = {

Line, then just printing the fields that changed, say just that refcount
was bumped, so it first print:

*(struct task_struct *)0xffff889ff8a08000 = {
      .thread_info = (struct thread_info){
              .flags = (unsigned long)0,
              .status = (u32)0,
      },
      .state = (volatile long)1,
      .stack = (void *)0xffffc9000c4dc000,
      .usage = (refcount_t){
              .refs = (atomic_t){
                      .counter = (int)2,
              },
      },
      .flags = (unsigned int)4194560,
      .ptrace = (unsigned int)0,

Then, the next time it would print:

*(struct task_struct *)0xffff889ff8a08000 = {
      .usage = (refcount_t){
              .refs = (atomic_t){
                      .counter = (int)3,
              },
      },
},

- Arnaldo