Re: [PATCH] setlocalversion: Add workaround for "git describe" performance issue

>
> On Wed, Oct 30 2024, Josh Poimboeuf <jpoimboe@kernel.org> wrote:
>
> > If HEAD isn't associated with an annotated tag, a bug (or feature?) in
> > "git describe --match" causes it to search every commit in the entire
> > repository looking for additional match candidates.  Instead of it
> > taking a fraction of a second, it adds 10-15 seconds to the beginning of
> > every kernel build.
> >
> > Fix it by adding an additional dummy match which is slightly further
> > away from the most recent one, along with setting the max candidate
> > count to 1 (not 2, apparently another bug).
> >
>
> cc += git list
>
> Hm, I tried looking at the git describe source code, and while I can't
> claim I understand it very well, I think the main problem is around
> this part:
>
>                         if (!tags && !all && n->prio < 2) {
>                                 unannotated_cnt++;
>                         } else if (match_cnt < max_candidates) {
>                                 struct possible_tag *t = &all_matches[match_cnt++];
>                                 t->name = n;
>                                 t->depth = seen_commits - 1;
>                                 t->flag_within = 1u << match_cnt;
>                                 t->found_order = match_cnt;
>                                 c->object.flags |= t->flag_within;
>                                 if (n->prio == 2)
>                                         annotated_cnt++;
>                         }
>                         else {
>                                 gave_up_on = c;
>                                 break;
>                         }
>
> So in the case where one doesn't pass any --match, we get something like
>
> git describe --debug 5f78aec0d7e9
> describe 5f78aec0d7e9
> No exact match on refs or tags, searching to describe
>  annotated        243 v4.19-rc5
>  annotated        485 v4.19-rc4
>  annotated        814 v4.19-rc3
>  annotated       1124 v4.19-rc2
>  annotated       1391 v4.19-rc1
>  annotated      10546 v4.18
>  annotated      10611 v4.18-rc8
>  annotated      10819 v4.18-rc7
>  annotated      11029 v4.18-rc6
>  annotated      11299 v4.18-rc5
> traversed 11400 commits
> more than 10 tags found; listed 10 most recent
> gave up search at 1e4b044d22517cae7047c99038abb444423243ca
> v4.19-rc5-243-g5f78aec0d7e9
>
> and that "gave up" commit is v4.18-rc4, the eleventh commit
> encountered. That also explains why you have to add a "dummy" second
> --match to make --candidates=1 have the expected behaviour.
>
> Perhaps the logic should instead be that as soon as match_cnt hits
> max_candidates (i.e. all the tags we're going to consider have actually
> been visited), we break out. That is, the last "else" above should
> instead be replaced by
>
>   if (match_cnt == max_candidates) {
>     ... /* ? , gave_up_on is now a misnomer */
>     break;
>   }
>
> Then as a further DWIM aid, wherever the initialization logic is could
> be updated so that, after expanding all the --match= wildcards, if the
> number of tags is less than max_candidates, automatically lower
> max_candidates to that number (which in the setlocalversion case will
> always be 1 because we're not actually passing a wildcard).
>
> Or, we could explicitly on the kernel side pass that --candidates=1, but
> yes, I agree it looks like a bug that the loop requires encountering +1
> tag to hit that break;.
>
> Rasmus
>