From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: X-Spam-Checker-Version: SpamAssassin 3.4.0 (2014-02-07) on aws-us-west-2-korg-lkml-1.web.codeaurora.org X-Spam-Level: X-Spam-Status: No, score=-2.5 required=3.0 tests=HEADER_FROM_DIFFERENT_DOMAINS, MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_AGENT_SANE_1 autolearn=no autolearn_force=no version=3.4.0 Received: from mail.kernel.org (mail.kernel.org [198.145.29.99]) by smtp.lore.kernel.org (Postfix) with ESMTP id 7C95AC433E3 for ; Mon, 29 Jun 2020 19:01:30 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id 687A320663 for ; Mon, 29 Jun 2020 19:01:30 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1730438AbgF2TB2 (ORCPT ); Mon, 29 Jun 2020 15:01:28 -0400 Received: from mx2.suse.de ([195.135.220.15]:48448 "EHLO mx2.suse.de" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1728101AbgF2TBY (ORCPT ); Mon, 29 Jun 2020 15:01:24 -0400 X-Virus-Scanned: by amavisd-new at test-mx.suse.de Received: from relay2.suse.de (unknown [195.135.221.27]) by mx2.suse.de (Postfix) with ESMTP id 7BA11AD8D; Mon, 29 Jun 2020 09:44:40 +0000 (UTC) Date: Mon, 29 Jun 2020 11:43:50 +0200 From: Petr Mladek To: Alan Maguire Cc: rostedt@goodmis.org, sergey.senozhatsky@gmail.com, Linus Torvalds , 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 Message-ID: <20200629094349.GQ8444@alley> References: <1592914031-31049-1-git-send-email-alan.maguire@oracle.com> <1592914031-31049-5-git-send-email-alan.maguire@oracle.com> <20200626101523.GM8444@alley> MIME-Version: 1.0 Content-Type: text/plain; charset=us-ascii Content-Disposition: inline In-Reply-To: User-Agent: Mutt/1.10.1 (2018-07-13) Sender: linux-kernel-owner@vger.kernel.org Precedence: bulk List-ID: X-Mailing-List: linux-kernel@vger.kernel.org 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