From: Petr Mladek <pmladek@suse.com>
To: Alan Maguire <alan.maguire@oracle.com>
Cc: rostedt@goodmis.org, sergey.senozhatsky@gmail.com,
Linus Torvalds <torvalds@linux-foundation.org>,
ast@kernel.org, daniel@iogearbox.net, yhs@fb.com, andriin@fb.com,
arnaldo.melo@gmail.com, kafai@fb.com, songliubraving@fb.com,
john.fastabend@gmail.com, kpsingh@chromium.org,
linux@rasmusvillemoes.dk, joe@perches.com,
andriy.shevchenko@linux.intel.com, corbet@lwn.net,
bpf@vger.kernel.org, netdev@vger.kernel.org,
linux-kernel@vger.kernel.org, linux-doc@vger.kernel.org
Subject: Re: [PATCH v3 bpf-next 4/8] printk: add type-printing %pT format specifier which uses BTF
Date: Mon, 29 Jun 2020 11:43:50 +0200 [thread overview]
Message-ID: <20200629094349.GQ8444@alley> (raw)
In-Reply-To: <alpine.LRH.2.21.2006261147130.417@localhost>
On Fri 2020-06-26 12:37:19, Alan Maguire wrote:
>
> On Fri, 26 Jun 2020, Petr Mladek wrote:
>
> > On Tue 2020-06-23 13:07:07, Alan Maguire wrote:
> > >
> > > printk(KERN_INFO "%pT", BTF_PTR_TYPE(skb, struct sk_buff));
> > >
> > > struct sk_buff *skb = alloc_skb(64, GFP_KERNEL);
> > > pr_info("%pT", BTF_PTR_TYPE(skb, struct sk_buff));
> > >
> > > ...gives us:
> > >
> > > (struct sk_buff){
> > > .transport_header = (__u16)65535,
> > > .mac_header = (__u16)65535,
> > > .end = (sk_buff_data_t)192,
> > > .head = (unsigned char *)0x000000006b71155a,
> > > .data = (unsigned char *)0x000000006b71155a,
> > > .truesize = (unsigned int)768,
> > > .users = (refcount_t){
> > > .refs = (atomic_t){
> > > .counter = (int)1,
> > > },
> > > },
> > > .extensions = (struct skb_ext *)0x00000000f486a130,
> > > }
> > >
> > > printk output is truncated at 1024 bytes. For cases where overflow
> > > is likely, the compact/no type names display modes may be used.
> >
> > Hmm, this scares me:
> >
> > 1. The long message and many lines are going to stretch printk
> > design in another dimensions.
> >
> > 2. vsprintf() is important for debugging the system. It has to be
> > stable. But the btf code is too complex.
> >
>
> Right on both points, and there's no way around that really. Representing
> even small data structures will stretch us to or beyond the 1024 byte
> limit. This can be mitigated by using compact display mode and not
> printing field names, but the output becomes hard to parse then.
>
> I think a better approach might be to start small, adding the core
> btf_show functionality to BPF, allowing consumers to use it there,
> perhaps via a custom helper.
Sounds good to me.
> In the current model bpf_trace_printk() inherits the functionality
> to display data from core printk, so a different approach would
> be needed there.
BTW: Even the trace buffer has a limitation, see BUF_MAX_DATA_SIZE
in kernel/trace/ring_buffer.c. It is internally implemented as
a list of memory pages, see the comments above RB_BUFFER_OFF
definition.
It is typically 4k. I think that you might hit this limit as well.
We had to increase per-CPU buffers used by printk() in NMI context
because 4k was not enough for some backtraces.
So, using different approach would make sense even when using trace
buffer.
> Other consumers outside of BPF
> could potentially avail of the show functionality directly via the btf_show
> functions in the future, but at least it would have one consumer at the
> outset, and wouldn't present problems like these for printk.
Sounds good to me.
> > I would strongly prefer to keep this outside vsprintf and printk.
> > Please, invert the logic and convert it into using separate printk()
> > call for each printed line.
> >
>
> I think the above is in line with what you're suggesting?
Yes, as far as I understand it.
> Yep, no way round this either. I'll try a different approach. Thanks for
> taking a look!
Uff, thanks a lot for understanding. I hope that most of the code will
be reusable in some form.
Best Regards,
Petr
next prev parent reply other threads:[~2020-06-29 19:01 UTC|newest]
Thread overview: 18+ messages / expand[flat|nested] mbox.gz Atom feed top
2020-06-23 12:07 [PATCH v3 bpf-next 0/8] bpf, printk: add BTF-based type printing Alan Maguire
2020-06-23 12:07 ` [PATCH v3 bpf-next 1/8] bpf: provide function to get vmlinux BTF information Alan Maguire
2020-06-23 12:07 ` [PATCH v3 bpf-next 2/8] bpf: move to generic BTF show support, apply it to seq files/strings Alan Maguire
2020-06-23 12:07 ` [PATCH v3 bpf-next 3/8] checkpatch: add new BTF pointer format specifier Alan Maguire
2020-06-23 12:07 ` [PATCH v3 bpf-next 4/8] printk: add type-printing %pT format specifier which uses BTF Alan Maguire
2020-06-23 12:40 ` Andy Shevchenko
2020-06-23 13:11 ` Rasmus Villemoes
2020-06-26 10:15 ` Petr Mladek
2020-06-26 11:37 ` Alan Maguire
2020-06-29 9:43 ` Petr Mladek [this message]
2020-06-23 12:07 ` [PATCH v3 bpf-next 5/8] printk: initialize vmlinux BTF outside of printk in late_initcall() Alan Maguire
2020-06-23 12:07 ` [PATCH v3 bpf-next 6/8] printk: extend test_printf to test %pT BTF-based format specifier Alan Maguire
2020-06-23 13:02 ` Andy Shevchenko
2020-06-23 12:07 ` [PATCH v3 bpf-next 7/8] bpf: add support for %pT format specifier for bpf_trace_printk() helper Alan Maguire
2020-06-23 13:04 ` Andy Shevchenko
2020-06-23 12:07 ` [PATCH v3 bpf-next 8/8] bpf/selftests: add tests for %pT format specifier Alan Maguire
2020-06-23 18:16 ` Andrii Nakryiko
2020-06-30 11:31 ` [PATCH v3 bpf-next 0/8] bpf, printk: add BTF-based type printing Sergey Senozhatsky
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