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=-18.3 required=3.0 tests=BAYES_00,DKIMWL_WL_MED, DKIM_SIGNED,DKIM_VALID,DKIM_VALID_AU,HEADER_FROM_DIFFERENT_DOMAINS, INCLUDES_PATCH,MAILING_LIST_MULTI,SPF_HELO_NONE,SPF_PASS,USER_IN_DEF_DKIM_WL autolearn=ham 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 12B9EC433FE for ; Thu, 10 Dec 2020 18:30:15 +0000 (UTC) Received: from vger.kernel.org (vger.kernel.org [23.128.96.18]) by mail.kernel.org (Postfix) with ESMTP id C1EA623E1D for ; Thu, 10 Dec 2020 18:30:14 +0000 (UTC) Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S2390329AbgLJS36 (ORCPT ); Thu, 10 Dec 2020 13:29:58 -0500 Received: from lindbergh.monkeyblade.net ([23.128.96.19]:41742 "EHLO lindbergh.monkeyblade.net" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S2389128AbgLJS3l (ORCPT ); Thu, 10 Dec 2020 13:29:41 -0500 Received: from mail-io1-xd33.google.com (mail-io1-xd33.google.com [IPv6:2607:f8b0:4864:20::d33]) by lindbergh.monkeyblade.net (Postfix) with ESMTPS id 10251C0613D6 for ; Thu, 10 Dec 2020 10:29:01 -0800 (PST) Received: by mail-io1-xd33.google.com with SMTP id n4so6543730iow.12 for ; Thu, 10 Dec 2020 10:29:01 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20161025; h=mime-version:references:in-reply-to:from:date:message-id:subject:to :cc; bh=JNE/BHkW6DlVzWL1ezPxZSD39LGAT96NfgLIIKU+T+U=; b=oHo0cD8NjoCZBKGXficJtyifqk3JLHRnBAirkJiHOVsAKC1iTEi55ie1bpJXsqZTJX 6YrXMiYr/lcduxqFarqAdrNbJL0/Yzn3istkPZyG2Usfqn6dHZ+pDvSrfdyJpFJ4Xh2G GQbG7GNMdk0eUTmnuGxdJzqlSOz9ooJRwAgdpV9Hd/RP2xsZThAG1d6ZbcRGsfyiXHAA njJ3TFCyZwW/RfYud/Bv7/5lQ+P92ePKFvXKlmg2aY9PuUAF22FrXKU1MpmoEiBphONV oPerJv++Ipf23jOJbtjgNFcFr3OUN2F2GfJ4iMGbvekZxWdPbjulVDvqewn0EgRsc28U P6sA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20161025; h=x-gm-message-state:mime-version:references:in-reply-to:from:date :message-id:subject:to:cc; bh=JNE/BHkW6DlVzWL1ezPxZSD39LGAT96NfgLIIKU+T+U=; b=drNNsH5zPPHKYQ0v7ncya96BjIzlz71POSDFvSYZD8jpjdng2bmWtsgWhVIB/VwqAN sWt06zbGzTb5gHDXlBb+isEKLhj/dRNk4urLdGU0P34PisPO3yunXJuhFpDjMedIKKIG XeSfN6tIz6APQB6A04n//Hs3mEAgJUUy9VNnXI06K69yIPbOuAo8t1AlFHvOGiDg474n 9+ZKnl3Lj6aKy39wzb+MpGrNmWJkoNNjS+5OZnOf2n9WwvZMja9xq3HNAPzRWCUcH9Lu RdSHt2s6OV9pO7ZP5cjuUXRvG5gutayasHiEdOaz3pA3Xuam1bCCcpefrcksiCHD1A4r x1lg== X-Gm-Message-State: AOAM531IfZgZfnv+kdAkeASqZ+0EXN1a1SBQeq2nUF+qNepJeF1vZZYJ PgLeXqLPJ3oIb7ywl1P6tNOL7ZBCboqj2LAX1NMYtRT28Hp191am X-Google-Smtp-Source: ABdhPJzobFCStS8tNN/CX2ne9EpIsM7A8q5y44OZIAN8rhvbCzLi5V7rVgVsq0TFdNmZMHB0qZ3rDcy02I0P6YegPZU= X-Received: by 2002:a5e:a614:: with SMTP id q20mr9581890ioi.198.1607624940019; Thu, 10 Dec 2020 10:29:00 -0800 (PST) MIME-Version: 1.0 References: <20201210164315.GA184880@krava> In-Reply-To: From: Hao Luo Date: Thu, 10 Dec 2020 10:28:43 -0800 Message-ID: Subject: Re: Per-CPU variables in modules and pahole To: Andrii Nakryiko Cc: Jiri Olsa , bpf , dwarves@vger.kernel.org, Jiri Olsa , Arnaldo Carvalho de Melo Content-Type: text/plain; charset="UTF-8" Precedence: bulk List-ID: X-Mailing-List: bpf@vger.kernel.org On Thu, Dec 10, 2020 at 9:02 AM Andrii Nakryiko wrote: > > On Thu, Dec 10, 2020 at 8:43 AM Jiri Olsa wrote: > > > > On Wed, Dec 09, 2020 at 12:53:44PM -0800, Andrii Nakryiko wrote: > > > Hi, > > > > > > I'm working on supporting per-CPU symbols in BPF/libbpf, and the > > > prerequisite for that is BTF data for .data..percpu data section and > > > variables inside that. > > > > > > Turns out, pahole doesn't currently emit any BTF information for such > > > variables in kernel modules. And the reason why is quite confusing and > > > I can't figure it out myself, so was hoping someone else might be able > > > to help. > > > > > > To repro, you can take latest bpf-next tree and add this to > > > bpf_testmod/bpf_testmod.c inside selftests/bpf: > > > > > > $ git diff bpf_testmod/bpf_testmod.c > > > diff --git > > > a/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c > > > b/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c > > > index 2df19d73ca49..b2086b798019 100644 > > > --- a/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c > > > +++ b/tools/testing/selftests/bpf/bpf_testmod/bpf_testmod.c > > > @@ -3,6 +3,7 @@ > > > #include > > > #include > > > #include > > > +#include > > > #include > > > #include > > > #include "bpf_testmod.h" > > > @@ -10,6 +11,10 @@ > > > #define CREATE_TRACE_POINTS > > > #include "bpf_testmod-events.h" > > > > > > +DEFINE_PER_CPU(int, bpf_testmod_ksym_dummy1) = -1; > > > +DEFINE_PER_CPU(int, bpf_testmod_ksym_percpu) = 123; > > > +DEFINE_PER_CPU(int, bpf_testmod_ksym_dummy2) = -1; > > > + > > > noinline ssize_t > > > bpf_testmod_test_read(struct file *file, struct kobject *kobj, > > > struct bin_attribute *bin_attr, > > > > > > 1. So the very first issue (that I'm going to ignore for now) is that > > > if I just added bpf_testmod_ksym_percpu, it would get addr == 0 and > > > would be ignored by the current pahole logic. So we need to fix that > > > for modules. Adding dummy1 and dummy2 takes care of this for now, > > > bpf_testmod_ksym_percpu has offset 4. > > > > I removed that addr zero check in the modules changes but when > > collecting functions, but it's still there in collect_percpu_var > > Hao had some reason to skip per-cpu variables with offset 0, maybe he > can comment on that before we change it. > When I initially write that check, I see there are multiple symbols of the same name that associate with a single variable, but there is only one that has a non-zero address. Besides, there are symbols that don't associate to any variable and they have zero address. For example, those defined as __ADDRESSABLE(sym) and __UNIQUE_ID(prefix). They are quite a lot, I remember. So I filtered out the zero address for the purpose of accelerating encoding. I noticed that on x86_64, the first page of the percpu section is reserved, so I deem those symbols that are of normal interest should have positive addresses. > > > > > > > > > 2. Second issue is more interesting. Somehow, when pahole iterates > > > over DWARF variables, the address of bpf_testmod_ksym_percpu is > > > reported as 0x10e74, not 4. Which totally confuses pahole because > > > according to ELF symbols, bpf_testmod_ksym_percpu symbol has value 4. > > > I tracked this down to dwarf_getlocation() returning 10e74 as number > > > field in expr. > > > > in which place do you see that address? when I put displayed > > address from collect_percpu_var it shows 4 > > yes, ELF symbol's value is 4, but when iterating DWARF variables > (0x10e70 + 4) is returned. It does look like a special handling of > modules. I missed that libdw does some special things for specifically > modules. Further debugging yesterday showed that 0x10e70 roughly > corresponds to the offset of .data..per_cpu if you count all the > allocatable data sections that come before it. So I think you are > right. We should probably centralize the logic of kernel module > detection so that we can handle these module vs non-module differences > properly. > > > > > not sure this is related but looks like similar issue I had to > > solve for modules functions, as described in the changelog: > > (not merged yet) > > > > btf_encoder: Detect kernel module ftrace addresses > > > > ... > > There's one tricky point with kernel modules wrt Elf object, > > which we get from dwfl_module_getelf function. This function > > performs all possible relocations, including __mcount_loc > > section. > > > > So addrs array contains relocated values, which we need take > > into account when we compare them to functions values which > > are relative to their sections. > > ... > > > > The 0x10e74 value could be relocated 4.. but it's me guessing, > > because not sure where you see that address exactly > > > It comes up in cu__encode_btf(), var->ip.addr is not 4, as we expect it to be. > > > > > jirka > >