Message-ID: <CAK7LNAS6aU4L+4JyDXzncMVsY+6XRYTD=RkhcXSUXTRh_WxWWw@mail.gmail.com>
Date: Sat, 23 Nov 2024 17:06:50 +0900
From: Masahiro Yamada <masahiroy@...nel.org>
To: Rasmus Villemoes <linux@...musvillemoes.dk>
Cc: linux-kbuild@...r.kernel.org, linux-kernel@...r.kernel.org, 
	Jeff King <peff@...f.net>, Sean Christopherson <seanjc@...gle.com>, Josh Poimboeuf <jpoimboe@...nel.org>
Subject: Re: [PATCH v4] setlocalversion: work around "git describe" performance

On Sat, Nov 23, 2024 at 12:01 AM Rasmus Villemoes
<linux@...musvillemoes.dk> wrote:
>
> Contrary to expectations, passing a single candidate tag to "git
> describe" is slower than not passing any --match options.
>
>   $ time git describe --debug
>   ...
>   traversed 10619 commits
>   ...
>   v6.12-rc5-63-g0fc810ae3ae1
>
>   real    0m0.169s
>
>   $ time git describe --match=v6.12-rc5 --debug
>   ...
>   traversed 1310024 commits
>   v6.12-rc5-63-g0fc810ae3ae1
>
>   real    0m1.281s
>
> In fact, the --debug output shows that git traverses all or most of
> history. For some repositories and/or git versions, those 1.3s are
> actually 10-15 seconds.
>
> This has been acknowledged as a performance bug in git [1], and a fix
> is on its way [2]. However, no solution is yet in git.git, and even
> when one lands, it will take quite a while before it finds its way to
> a release and for $random_kernel_developer to pick that up.
>
> So rewrite the logic to use plumbing commands. For each of the
> candidate values of $tag, we ask: (1) is $tag even an annotated
> tag? (2) Is it eligible to describe HEAD, i.e. an ancestor of
> HEAD? (3) If so, how many commits are in $tag..HEAD?
>
> I have tested that this produces the same output as the current script
> for ~700 random commits between v6.9..v6.10. For those 700 commits,
> and in my git repo, the 'make -s kernelrelease' command is on average
> ~4 times faster with this patch applied (geometric mean of ratios).
>
> For the commit mentioned in Josh's original report [3], the
> time-consuming part of setlocalversion goes from
>
> $ time git describe --match=v6.12-rc5 c1e939a21eb1
> v6.12-rc5-44-gc1e939a21eb1
>
> real    0m1.210s
>
> to
>
> $ time git rev-list --count --left-right v6.12-rc5..c1e939a21eb1
> 0       44
>
> real    0m0.037s
>
> [1] https://lore.kernel.org/git/20241101113910.GA2301440@coredump.intra.peff.net/
> [2] https://lore.kernel.org/git/20241106192236.GC880133@coredump.intra.peff.net/
> [3] https://lore.kernel.org/lkml/309549cafdcfe50c4fceac3263220cc3d8b109b2.1730337435.git.jpoimboe@kernel.org/
>
> Reported-by: Sean Christopherson <seanjc@...gle.com>
> Closes: https://lore.kernel.org/lkml/ZPtlxmdIJXOe0sEy@google.com/
> Reported-by: Josh Poimboeuf <jpoimboe@...nel.org>
> Closes: https://lore.kernel.org/lkml/309549cafdcfe50c4fceac3263220cc3d8b109b2.1730337435.git.jpoimboe@kernel.org/
> Signed-off-by: Rasmus Villemoes <linux@...musvillemoes.dk>
> ---
> v4:
>
> - Switch the logic to make use of the return values from try_tag,
>   instead of asking whether $count has been set.


No, please do not do this.

As I replied in v3, my plan is to set -e, because otherwise
the shell script is fragile.

With this version, -e will not work in try_tag()
because it is used in the if condition.


> +try_tag() {
> +       tag="$1"
> +
> +       # Is $tag an annotated tag?
> +       [ "$(git cat-file -t "$tag" 2> /dev/null)" = tag ] || return 1
> +
> +       # Is it an ancestor of HEAD, and if so, how many commits are in $tag..HEAD?
> +       # shellcheck disable=SC2046 # word splitting is the point here
> +       set -- $(git rev-list --count --left-right "$tag"...HEAD 2> /dev/null)
> +
> +       # $1 is 0 if and only if $tag is an ancestor of HEAD. Use
> +       # string comparison, because $1 is empty if the 'git rev-list'
> +       # command somehow failed.
> +       [ "$1" = 0 ] || return 1
> +
> +       # $2 is the number of commits in the range $tag..HEAD, possibly 0.
> +       count="$2"

Redundant double-quotes.



--
Best Regards
Masahiro Yamada

Powered by blists - more mailing lists