Re: About perf tool set exclude_guest = 1 on guest environment

[Arnaldo Carvalho de Melo <acme@kernel.org>]

Em Thu, Jan 20, 2022 at 09:35:59PM +0800, Eric Lin escreveu:
> On Wed, Jan 19, 2022 at 1:02 PM Namhyung Kim <namhyung@kernel.org> wrote:
> > On Mon, Jan 17, 2022 at 8:05 PM Eric Lin <eric.lin@sifive.com> wrote:
> > > Currently, I run the perf command 'perf record -e branches:u ls'  and
> > > 'perf record -e branches ls'. It will set exclude_guest = 1 as below:
> > >
> > > # perf record -e branches:u ls
> > > # perf evlist -v
> > > branches:u: .., exclude_kernel: 1, exclude_hv: 1,, exclude_guest: 1, ..
> > >
> > > # perf record -e branches ls
> > > # perf evlist -v
> > > branches: ..., exclude_guest: 1, ...
> > >
> > > As I understand it, set exclude_guest =1 will not include guest counting.
> > > May I ask if I run the above commands as a guest user, should the
> > > architecture implementation ignore this exclude_guest flags and still
> > > make perf can count guest events in Linux kernel?
> >
> > So do you want to run the command in the host?
> > Then you can use both 'H' and 'G' modifiers like
> >
> >   # perf record -e branches:HG ls
> >
> > or (for user-mode only)
> >
> >   # perf record -e branches:uHG ls
> >
> 
> Actually, I want to run the commands in the guest.
> But when I run the commands "perf record -e branches:u ls" and "perf
> record -e branches ls", it will set exclude_guet = 1.
> According to the documentation[1] exclude_guest definition, it should
> not count guest events.
> So, if I run these commands as a guest user can I get the profile data?
> 
> [1] https://man7.org/linux/man-pages/man2/perf_event_open.2.html

The tool shouldn't know that it is running in a guest, right?

And the kernel, running on a guest should exclude its guests if
attr.exclude_guest is set, not itself and its non-guest userland.

So, before delving into how the kernel running on guest _actually_
handles attr.exclude_guest=1 behaves, what are the results you are
getting from running perf on a guest?

Lemme see...

On a fedora:35 guest:

[root@fedora ~]# perf record ls
anaconda-ks.cfg  perf.data
[ perf record: Woken up 1 times to write data ]
[ perf record: Captured and wrote 0.014 MB perf.data (3 samples) ]
[root@fedora ~]# perf evlist -v
cpu-clock:pppH: type: 1, size: 128, { sample_period, sample_freq }: 4000, sample_type: IP|TID|TIME|PERIOD, read_format: ID, disabled: 1, inherit: 1, mmap: 1, comm: 1, freq: 1, enable_on_exec: 1, task: 1, precise_ip: 3, sample_id_all: 1, exclude_guest: 1, mmap2: 1, comm_exec: 1, ksymbol: 1, bpf_event: 1
[root@fedora ~]#

IIRC there is some extra magic to allow 'cycles' to be seen on the
guest, but apart from that it seems to work.

Eduardo, can you help us here?

- Arnaldo