Re: accessing global and per-cpu vars

From: Andrii Nakryiko <andrii.nakryiko@gmail.com>
To: Hao Luo <haoluo@google.com>
Cc: Arnaldo Carvalho de Melo <acme@kernel.org>,
	Alexei Starovoitov <alexei.starovoitov@gmail.com>,
	Andrii Nakryiko <andriin@fb.com>,
	Daniel Borkmann <daniel@iogearbox.net>, bpf <bpf@vger.kernel.org>,
	Oleg Rombakh <olegrom@google.com>,
	Martin KaFai Lau <kafai@fb.com>
Subject: Re: accessing global and per-cpu vars
Date: Tue, 26 May 2020 11:20:53 -0700	[thread overview]
Message-ID: <CAEf4Bza9TP50Rtdg1s2qZ7t4547wQr=E-72_6m81ZX8vwZOPEA@mail.gmail.com> (raw)
In-Reply-To: <CA+khW7j=ejncVYgY=hKEnkrkwA=Wjwa6Y2PFWgzrV1EV_8rvpw@mail.gmail.com>

On Tue, May 26, 2020 at 12:58 AM Hao Luo <haoluo@google.com> wrote:
>
> Hi, Arnaldo and Andrii,
>
> Thanks for taking a look and checking.
>
> On Fri, May 22, 2020 at 7:28 AM Arnaldo Carvalho de Melo <acme@kernel.org> wrote:
>>
>> Em Thu, May 21, 2020 at 11:58:47AM -0700, Andrii Nakryiko escreveu:
>> > On Thu, May 21, 2020 at 10:07 AM Alexei Starovoitov <alexei.starovoitov@gmail.com> wrote:
>> > > 2. teach pahole to store ' A ' annotated kallsyms into vmlinux BTF as
>> > > BTF_KIND_VAR.
>> > > There are ~300 of them, so should be minimal increase in size.
>> >
>> > I thought we'd do that based on section name? Or we will actually
>> > teach pahole to extract kallsyms from vmlinux image?
>>
>> No need to touch kallsyms:
>>
>>   net/core/filter.c
>>
>>   DEFINE_PER_CPU(struct bpf_redirect_info, bpf_redirect_info);
>>
>>   # grep -w bpf_redirect_info /proc/kallsyms
>>   000000000002a160 A bpf_redirect_info
>>   #
>>   # readelf -s ~acme/git/build/v5.7-rc2+/vmlinux | grep bpf_redirect_info
>>   113637: 000000000002a2e0    32 OBJECT  GLOBAL DEFAULT   34 bpf_redirect_info
>>   #
>>
>> Its in the ELF symtab.
>>
>> [root@quaco ~]# grep ' A ' /proc/kallsyms | wc -l
>> 351
>> [root@quaco ~]# readelf -s ~acme/git/build/v5.7-rc2+/vmlinux | grep "OBJECT  GLOBAL" | wc -l
>> 3221
>> [root@quaco ~]#
>>
>> So ' A ' in kallsyms needs some extra info from the symtab in addition
>>
>> to being OBJECT GLOBAL, checking...
>
>
> After playing a bit, I found 'A' symbols in kallsyms include the per_cpu variables (e.g. runqueues and sched_clock_data), either global or local. An example of the global var is 'runqueues' and the example of local one is 'sched_clock_data'.
>
> The OBJECT GLOBAL symbols in vmlinux include the global variables such as runqueues. It also includes those symbols annotated as other capital letters such as 'R' or 'B' in kallsyms. For example, __per_cpu_offset is OBJECT GLOBAL in vmlinux and it's annotated as 'R', implying a global const variable.
>
> I think either the vmlinux approach or the kallsyms approach is good enough. I will continue experimenting while working on step 1.
>

/proc/kallsyms is available in runtime (if configured, of course),
while vmlinux image might not be available at runtime at all in some
environments. This is one of the reasons for BTF to be exposed in
runtime through /sys/kernel/btf/vmlinux, instead of just keeping it in
vmlinux image. So I think kallsyms approach is better and more
reliable.

As for 'A', 'R', 'B', etc. Can we please look at source code of
whatever in kernel defines those lettera in ksyms, instead of guessing
based on a subset of symbols? Guessing like this makes me nervous :)

> Thanks,
> Hao
>
>>
>> > There was step 1.5 (or even 0.5) to see if it's feasible to add not
>> > just per-CPU variables as well.
>>
>> - Arnaldo