[tip: objtool/core] objtool: Improve reloc hash size guestimate

[tip-bot2 for Peter Zijlstra]

The following commit has been merged into the objtool/core branch of tip:

Commit-ID:     d33b9035e14a35f6f2a5f067f0b156a93581811d
Gitweb:        https://git.kernel.org/tip/d33b9035e14a35f6f2a5f067f0b156a93581811d
Author:        Peter Zijlstra <peterz@xxxxxxxxxxxxx>
AuthorDate:    Fri, 11 Jun 2021 08:33:36 +02:00
Committer:     Peter Zijlstra <peterz@xxxxxxxxxxxxx>
CommitterDate: Mon, 14 Jun 2021 14:05:36 +02:00

objtool: Improve reloc hash size guestimate

Nathan reported that LLVM ThinLTO builds have a performance regression
with commit 25cf0d8aa2a3 ("objtool: Rewrite hashtable sizing"). Sami
was quick to note that this is due to their use of -ffunction-sections.

As a result the .text section is small and basing the number of relocs
off of that no longer works. Instead have read_sections() compute the
sum of all SHF_EXECINSTR sections and use that.

Fixes: 25cf0d8aa2a3 ("objtool: Rewrite hashtable sizing")
Reported-by: Nathan Chancellor <nathan@xxxxxxxxxx>
Debugged-by: Sami Tolvanen <samitolvanen@xxxxxxxxxx>
Signed-off-by: Peter Zijlstra (Intel) <peterz@xxxxxxxxxxxxx>
Tested-by: Nathan Chancellor <nathan@xxxxxxxxxx>
Link: https://lkml.kernel.org/r/YMJpGLuGNsGtA5JJ@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx
---
 tools/objtool/elf.c                 | 11 ++++-------
 tools/objtool/include/objtool/elf.h |  1 +
 2 files changed, 5 insertions(+), 7 deletions(-)

diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c
index a8a0ee2..2371ccc 100644
--- a/tools/objtool/elf.c
+++ b/tools/objtool/elf.c
@@ -288,6 +288,9 @@ static int read_sections(struct elf *elf)
 		}
 		sec->len = sec->sh.sh_size;
 
+		if (sec->sh.sh_flags & SHF_EXECINSTR)
+			elf->text_size += sec->len;
+
 		list_add_tail(&sec->list, &elf->sections);
 		elf_hash_add(section, &sec->hash, sec->idx);
 		elf_hash_add(section_name, &sec->name_hash, str_hash(sec->name));
@@ -581,13 +584,7 @@ static int read_relocs(struct elf *elf)
 	unsigned int symndx;
 	unsigned long nr_reloc, max_reloc = 0, tot_reloc = 0;
 
-	sec = find_section_by_name(elf, ".text");
-	if (!sec) {
-		WARN("no .text");
-		return -1;
-	}
-
-	if (!elf_alloc_hash(reloc, sec->len / 16))
+	if (!elf_alloc_hash(reloc, elf->text_size / 16))
 		return -1;
 
 	list_for_each_entry(sec, &elf->sections, list) {
diff --git a/tools/objtool/include/objtool/elf.h b/tools/objtool/include/objtool/elf.h
index 9008275..e343950 100644
--- a/tools/objtool/include/objtool/elf.h
+++ b/tools/objtool/include/objtool/elf.h
@@ -83,6 +83,7 @@ struct elf {
 	int fd;
 	bool changed;
 	char *name;
+	unsigned int text_size;
 	struct list_head sections;
 
 	int symbol_bits;