On Wed, Nov 24, 2021 at 7:02 AM Jiri Olsa <jolsa@xxxxxxxxxx> wrote:
>
> On Wed, Nov 24, 2021 at 12:39:00PM +0100, Jiri Olsa wrote:
> > On Thu, Nov 18, 2021 at 02:49:35PM -0800, Andrii Nakryiko wrote:
> > > On Thu, Nov 18, 2021 at 6:27 AM Jiri Olsa <jolsa@xxxxxxxxxx> wrote:
> > > >
> > > > On Wed, Nov 17, 2021 at 11:42:58AM -0800, Andrii Nakryiko wrote:
> > > > > On Wed, Nov 17, 2021 at 11:41 AM Andrii Nakryiko <andrii@xxxxxxxxxx> wrote:
> > > > > >
> > > > > > According to [0], compilers sometimes might produce duplicate DWARF
> > > > > > definitions for exactly the same struct/union within the same
> > > > > > compilation unit (CU). We've had similar issues with identical arrays
> > > > > > and handled them with a similar workaround in 6b6e6b1d09aa ("libbpf:
> > > > > > Accomodate DWARF/compiler bug with duplicated identical arrays"). Do the
> > > > > > same for struct/union by ensuring that two structs/unions are exactly
> > > > > > the same, down to the integer values of field referenced type IDs.
> > > > >
> > > > > Jiri, can you please try this in your setup and see if that handles
> > > > > all situations or there are more complicated ones still. We'll need a
> > > > > test for more complicated ones in that case :( Thanks.
> > > >
> > > > it seems to help largely, but I still see few modules (67 out of 780)
> > > > that keep 'struct module' for some reason.. their struct module looks
> > > > completely the same as is in vmlinux
> > >
> > > Curious, what's the size of all the module BTFs now?
> >
> > sorry for delay, I was waiting for s390x server
> >
> > so with 'current' fedora kernel rawhide I'm getting slightly different
> > total size number than before, so something has changed after the merge
> > window..
> >
> > however the increase with BTF enabled in modules is now from 16M to 18M,
> > so the BTF data adds just about 2M, which I think we can live with
> >
16MB for vmlinux BTF is quite a lot. Is it a allmodyesconfig or something?
> > > And yes, please
> > > try to narrow down what is causing the bloat this time. I think this
> >
> > I'm on it
>
> I'm seeing vmlinux BTF having just FWD record for sctp_mib struct,
> while the kernel module has the full definition
>
> kernel:
>
> [2798] STRUCT 'netns_sctp' size=296 vlen=46
> 'sctp_statistics' type_id=2800 bits_offset=0
>
> [2799] FWD 'sctp_mib' fwd_kind=struct
> [2800] PTR '(anon)' type_id=2799
>
>
> module before dedup:
>
> [78928] STRUCT 'netns_sctp' size=296 vlen=46
> 'sctp_statistics' type_id=78930 bits_offset=0
>
> [78929] STRUCT 'sctp_mib' size=272 vlen=1
> 'mibs' type_id=80518 bits_offset=0
> [78930] PTR '(anon)' type_id=78929
>
>
> this field is referenced from within 'struct module' so it won't
> match its kernel version and as a result extra 'struct module'
> stays in the module's BTF
>
Yeah, not much we could do about that short of just blindly matching
FWD to STRUCT/UNION/ENUM by name, which sounds bad to me, I avoided
doing that in BTF dedup algorithm. We only resolve FWD to
STRUCT/UNION/ENUM when we have some containing struct with a field
that points to FWD and (in another instance of the containing struct)
to STRUCT/UNION/ENUM. That way we have sort of a proof that we are
resolving the right FWD. While in this case it would be a blind guess
based on name alone.
> I'll need to check debuginfo/pahole if that FWD is correct, but
> I guess it's normal that some structs might end up unwinded only
> in modules and not necessarily in vmlinux
It can happen, yes. If that's a very common case, ideally we should
make sure that modules keep FWD or that vmlinux BTF does have a full
struct instead of FWD.
>
> jirka
>
