Re: [RFC/PATCHES 00/12] pahole: Reproducible parallel DWARF loading/serial BTF encoding

Arnaldo Carvalho de Melo
2024-04-12T20:36:17Z

On Mon, Apr 08, 2024 at 11:39:32AM -0300, Arnaldo Carvalho de Melo wrote:
> On Mon, Apr 08, 2024 at 01:00:59PM +0100, Alan Maguire wrote:
> > On 04/04/2024 09:58, Alan Maguire wrote:
> > > Program terminated with signal SIGSEGV, Segmentation fault.
> > > #0 0x00007f8c8260a58c in ptr_table__entry (pt=0x7f8c60001e70, id=77)
> > > at /home/almagui/src/dwarves/dwarves.c:612
> > > 612 return id >= pt->nr_entries ? NULL : pt->entries[id];
> > > [Current thread is 1 (Thread 0x7f8c65400700 (LWP 624441))]
> > > (gdb) print *(struct ptr_table *)0x7f8c60001e70
> > > $1 = {entries = 0x0, nr_entries = 2979, allocated_entries = 4096}
> > > (gdb)
>
> > > So it looks like the ptr_table has 2979 entries but entries is NULL;
> > > could there be an issue where CU initialization is not yet complete
> > > for some threads (it also happens very early in processing)? Can you
> > > reproduce this failure at your end? Thanks!
>
> > the following (when applied on top of the series) resolves the
> > segmentation fault for me:
>
> > diff --git a/pahole.c b/pahole.c
> > index 6c7e738..5ff0eaf 100644
> > --- a/pahole.c
> > +++ b/pahole.c
> > @@ -3348,8 +3348,8 @@ static enum load_steal_kind pahole_stealer(struct
> > cu *cu,
> > if (conf_load->reproducible_build) {
> > ret = LSK__KEEPIT; // we're not processing the
> > cu passed to this function, so keep it.
> > - // Equivalent to LSK__DELETE since we processed
> > this
> > - cus__remove(cus, cu);
> > - cu__delete(cu);
> > }
> > out_btf:
> > if (!thr_data) // See comment about reproducibe_build above
> >
>
> Yeah, Jiri also pointed out this call to cu__delete() was new, I was
> trying to avoid having unprocessed 'struct cu' using too much memory, so
> after processing it, delete them, but as you found out there are
> references to that memory...
>
> > In other words, the problem is we remove/delete CUs when finished with
> > them in each thread (when BTF is generated). However because the
> > save/add_saved_funcs stashes CU references in the associated struct
> > function * (to allow prototype comparison for the same function in
> > different CUs), we end up with stale CU references and in this case the
> > freed/nulled ptr_table caused an issue. As far as I can see we need to
> > retain CUs until all BTF has been merged from threads.
>
> > With the fix in place, I'm seeing less then 100msec difference between
> > reproducible/non-reproducible vmlinux BTF generation; that's great!
>
> Excellent!
>
> I'll remove it and add a note crediting you with the removal and having
> the explanation about why its not possibe to delete it at that point
> (references to the associated 'struct function').

So I removed that cus__remove + cu__delete and also the other one at the
flush operation, leaving all cleaning up to cus__delete() time:

⬢[acme@toolbox pahole]$ git diff
diff --git a/dwarves.c b/dwarves.c
index fbc8d8aa0060b7d0..1ec259f50dbd3778 100644
--- a/dwarves.c
+++ b/dwarves.c
@@ -489,8 +489,12 @@ struct cu *cus__get_next_processable_cu(struct cus *cus)
cu->state = CU__PROCESSING;
goto found;
case CU__PROCESSING:
- // This will only happen when we get to parallel
- // reproducible BTF encoding, libbpf dedup work needed here.
+ // This will happen when we get to parallel
+ // reproducible BTF encoding, libbpf dedup work needed
+ // here. The other possibility is when we're flushing
+ // the DWARF processed CUs when the parallel DWARF
+ // loading stoped and we still have CUs to encode to
+ // BTF because of ordering requirements.
continue;
case CU__UNPROCESSED:
// The first entry isn't loaded, signal the
diff --git a/pahole.c b/pahole.c
index 6c7e73835b3e9139..77772bb42bb443ce 100644
--- a/pahole.c
+++ b/pahole.c
@@ -3347,9 +3347,9 @@ static enum load_steal_kind pahole_stealer(struct cu *cu,

if (conf_load->reproducible_build) {
ret = LSK__KEEPIT; // we're not processing the cu passed to this function, so keep it.
- // Equivalent to LSK__DELETE since we processed this
- cus__remove(cus, cu);
- cu__delete(cu);
+ // Kinda equivalent to LSK__DELETE since we processed this, but we can't delete it
+ // as we stash references to entries in CUs for 'struct function' in btf_encoder__add_saved_funcs()
+ // and btf_encoder__save_func(), so we can't delete them here. - Alan Maguire
}
out_btf:
if (!thr_data) // See comment about reproducibe_build above
@@ -3667,9 +3667,6 @@ static int cus__flush_reproducible_build(struct cus *cus, struct btf_encoder *en
err = btf_encoder__encode_cu(encoder, cu, conf_load);
if (err < 0)
break;
-
- cus__remove(cus, cu);
- cu__delete(cu);
⬢[acme@toolbox pahole]$

It ends up taking a bit more time on this 14700K with 32Gb, I'll later
try to remove that need to keep everything in memory and also double
check this hunch that this is due to keeping everyuthing in memory.

Can I take this (with the above patch, that is a bit bigger than yours)
as a Tested-by + Reviewed-by you?

- Arnald