linux-kernel - Re: [PATCH 01/13] objtool: Rewrite hashtable sizing

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Message-ID: <YMJpGLuGNsGtA5JJ@hirez.programming.kicks-ass.net>
Date:   Thu, 10 Jun 2021 21:33:44 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Sami Tolvanen <samitolvanen@...gle.com>
Cc:     Nathan Chancellor <nathan@...nel.org>, X86 ML <x86@...nel.org>,
        Josh Poimboeuf <jpoimboe@...hat.com>,
        Jason Baron <jbaron@...mai.com>,
        Steven Rostedt <rostedt@...dmis.org>,
        Ard Biesheuvel <ardb@...nel.org>,
        LKML <linux-kernel@...r.kernel.org>,
        Nick Desaulniers <ndesaulniers@...gle.com>,
        clang-built-linux <clang-built-linux@...glegroups.com>
Subject: Re: [PATCH 01/13] objtool: Rewrite hashtable sizing

On Thu, Jun 10, 2021 at 11:50:36AM -0700, Sami Tolvanen wrote:
> On Thu, Jun 10, 2021 at 11:14 AM Nathan Chancellor <nathan@...nel.org> wrote:
> > Adding Sami because I am not sure why this patch would have much of an impact
> > in relation to LTO. https://git.kernel.org/tip/25cf0d8aa2a3 is the patch in
> > question.
> 
> It's because LLVM enables -ffunction-sections with LTO, so using .text
> section size to estimate the reloc hash table size isn't going to be
> accurate, as confirmed by objtool output with --stats:
> 
>   OBJTOOL vmlinux.o
> nr_sections: 141481
> section_bits: 17
> nr_symbols: 215262
> symbol_bits: 17
> max_reloc: 24850
> tot_reloc: 590890
> reloc_bits: 10

Bah. Would something like the *completely* untested below help with that?

---
diff --git a/tools/objtool/elf.c b/tools/objtool/elf.c
index 25f6d293bc86..8676c7598728 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 90082751f851..e34395047530 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;