Re: [PATCH v2 bpf-next 3/4] selftests/bpf: Add profiler test

[Andrii Nakryiko <andrii.nakryiko@xxxxxxxxx>]

On Tue, Oct 13, 2020 at 2:03 PM Alexei Starovoitov
<alexei.starovoitov@xxxxxxxxx> wrote:
>
> On Tue, Oct 13, 2020 at 12:56 PM Jiri Olsa <jolsa@xxxxxxxxxx> wrote:
> >
> > On Thu, Oct 08, 2020 at 06:12:39PM -0700, Alexei Starovoitov wrote:
> >
> > SNIP
> >
> > > +
> > > +#ifdef UNROLL
> > > +#pragma unroll
> > > +#endif
> > > +     for (int i = 0; i < MAX_CGROUPS_PATH_DEPTH; i++) {
> > > +             filepart_length =
> > > +                     bpf_probe_read_str(payload, MAX_PATH, BPF_CORE_READ(cgroup_node, name));
> > > +             if (!cgroup_node)
> > > +                     return payload;
> > > +             if (cgroup_node == cgroup_root_node)
> > > +                     *root_pos = payload - payload_start;
> > > +             if (filepart_length <= MAX_PATH) {
> > > +                     barrier_var(filepart_length);
> > > +                     payload += filepart_length;
> > > +             }
> > > +             cgroup_node = BPF_CORE_READ(cgroup_node, parent);
> > > +     }
> > > +     return payload;
> > > +}
> > > +
> > > +static ino_t get_inode_from_kernfs(struct kernfs_node* node)
> > > +{
> > > +     struct kernfs_node___52* node52 = (void*)node;
> > > +
> > > +     if (bpf_core_field_exists(node52->id.ino)) {
> > > +             barrier_var(node52);
> > > +             return BPF_CORE_READ(node52, id.ino);
> > > +     } else {
> > > +             barrier_var(node);
> > > +             return (u64)BPF_CORE_READ(node, id);
> > > +     }
> > > +}
> > > +
> > > +int pids_cgrp_id = 1;
> >
> >
> > hi,
> > I'm getting compilation failure with this:
> >
> >           CLNG-LLC [test_maps] profiler2.o
> >         In file included from progs/profiler2.c:6:
> >         progs/profiler.inc.h:246:5: error: redefinition of 'pids_cgrp_id' as different kind of symbol
> >         int pids_cgrp_id = 1;
> >             ^
> >         /home/jolsa/linux-qemu/tools/testing/selftests/bpf/tools/include/vmlinux.h:14531:2: note: previous definition is here
> >                 pids_cgrp_id = 11,
>
> Interesting.
> You probably have CONFIG_CGROUP_PIDS in your .config?
> I don't and bpf CI doesn't have it either, so this issue wasn't spotted earlier.
>
> I can hard code 11, of course, but
> but maybe Andrii has a cool way to use co-re to deal with this?

Cool or not, but we do have a way to deal with it. :)

> I think
> "extern bool CONFIG_CGROUP_PIDS __kconfig"
> won't work.
> A good opportunity to try to use bpf_core_enum_value_exists() ?

There are fews parts here.

First, we can't rely that vmlinux.h has pids_cgrp_id enum defined, so
we need to define our own. But in such a way that it doesn't collide
with enum cgroup_subsys_id in vmlinux.h. ___suffix for the rescue
here:

enum cgroup_subsys_id___local {
    pids_cgrp_id___local = 1, /* anything but zero */
};

If value is zero, built-in will complain. Need to check with Yonghong
on why that is. For all things CO-RE, ___local suffix is going to be
ignored.

Second, detecting if the kernel even has this pids_cgrp_id defined.
That could be done with __kconfig as you mentioned, actually (but need
__weak as well). But one can also use bpf_core_enum_value_exists as
well. So it's either:

extern bool CONFIG_CGROUP_PIDS __kconfig __weak;

...

if (ENABLE_CGROUP_V1_RESOLVER && CONFIG_CGROUP_PIDS) {
   ...
}


Or a bit more verbose way with relos:

if (ENABLE_CGROUP_V1_RESOLVER &&
    bpf_core_enum_value_exists(enum cgroup_subsys_id___local,
pids_cgrp_id___local)) {
   ...
}

Third, actually getting the value of enum. This one would be
impossible without CO-RE reloc, but that's exactly what
bpf_core_enum_value() exists for:


int cgrp_id = bpf_core_enum_value(enum cgroup_subsys_id___local,
pids_cgrp_id___local);


I'd go with Kconfig + bpf_core_enum_value(), as it's shorter and
nicer. This compiles and works with my Kconfig, but I haven't checked
with CONFIG_CGROUP_PIDS defined.


diff --git a/tools/testing/selftests/bpf/progs/profiler.inc.h
b/tools/testing/selftests/bpf/progs/profiler.inc.h
index 00578311a423..79b8d2860a5c 100644
--- a/tools/testing/selftests/bpf/progs/profiler.inc.h
+++ b/tools/testing/selftests/bpf/progs/profiler.inc.h
@@ -243,7 +243,11 @@ static ino_t get_inode_from_kernfs(struct
kernfs_node* node)
        }
 }

-int pids_cgrp_id = 1;
+extern bool CONFIG_CGROUP_PIDS __kconfig __weak;
+
+enum cgroup_subsys_id___local {
+       pids_cgrp_id___local = 1, /* anything but zero */
+};

 static INLINE void* populate_cgroup_info(struct cgroup_data_t* cgroup_data,
                                         struct task_struct* task,
@@ -253,7 +257,9 @@ static INLINE void* populate_cgroup_info(struct
cgroup_data_t* cgroup_data,
                BPF_CORE_READ(task, nsproxy, cgroup_ns, root_cset,
dfl_cgrp, kn);
        struct kernfs_node* proc_kernfs = BPF_CORE_READ(task, cgroups,
dfl_cgrp, kn);

-       if (ENABLE_CGROUP_V1_RESOLVER) {
+       if (ENABLE_CGROUP_V1_RESOLVER && CONFIG_CGROUP_PIDS) {
+               int cgrp_id = bpf_core_enum_value(enum
cgroup_subsys_id___local, pids_cgrp_id___local);
+
 #ifdef UNROLL
 #pragma unroll
 #endif
@@ -262,7 +268,7 @@ static INLINE void* populate_cgroup_info(struct
cgroup_data_t* cgroup_data,
                                BPF_CORE_READ(task, cgroups, subsys[i]);
                        if (subsys != NULL) {
                                int subsys_id = BPF_CORE_READ(subsys, ss, id);
-                               if (subsys_id == pids_cgrp_id) {
+                               if (subsys_id == cgrp_id) {
                                        proc_kernfs =
BPF_CORE_READ(subsys, cgroup, kn);
                                        root_kernfs =
BPF_CORE_READ(subsys, ss, root, kf_root, kn);
                                        break;