linux-kernel - [RFC][PATCH 16/16] objtool: Optimize !vmlinux.o again

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Date:   Thu, 12 Mar 2020 14:41:23 +0100
From:   Peter Zijlstra <peterz@...radead.org>
To:     tglx@...utronix.de, jpoimboe@...hat.com
Cc:     linux-kernel@...r.kernel.org, x86@...nel.org, peterz@...radead.org
Subject: [RFC][PATCH 16/16] objtool: Optimize !vmlinux.o again

When doing kbuild tests to see if the objtool changes affected those I
found that there was a measurable regression:

          pre		  post

  real    1m13.594        1m16.488s
  user    34m58.246s      35m23.947s
  sys     4m0.393s        4m27.312s

Perf showed that for small files the increased hash-table sizes were a
measurable difference. Since we already have -l "vmlinux" to
distinguish between the modes, make it also use a smaller portion of
the hash-tables.

This flips it into a small win:

  real    1m14.143s
  user    34m49.292s
  sys     3m44.746s

Signed-off-by: Peter Zijlstra (Intel) <peterz@...radead.org>
---
 tools/objtool/elf.c |   40 ++++++++++++++++++++++++++++------------
 tools/objtool/elf.h |    4 ++--
 2 files changed, 30 insertions(+), 14 deletions(-)

--- a/tools/objtool/elf.c
+++ b/tools/objtool/elf.c
@@ -27,6 +27,21 @@ static inline u32 str_hash(const char *s
 	return jhash(str, strlen(str), 0);
 }
 
+static inline int elf_hash_bits(void)
+{
+	return vmlinux ? 20 : 16;
+}
+
+static inline void elf_hash_add(struct hlist_head *table, struct hlist_node *node, u32 key)
+{
+	hlist_add_head(node, &table[hash_32(key, elf_hash_bits())]);
+}
+
+static void elf_hash_init(struct hlist_head *table)
+{
+	__hash_init(table, 1U << elf_hash_bits());
+}
+
 static void rb_add(struct rb_root *tree, struct rb_node *node,
 		   int (*cmp)(struct rb_node *, const struct rb_node *))
 {
@@ -300,8 +315,8 @@ static int read_sections(struct elf *elf
 		}
 		sec->len = sec->sh.sh_size;
 
-		hash_add(elf->section_hash, &sec->hash, sec->idx);
-		hash_add(elf->section_name_hash, &sec->name_hash, str_hash(sec->name));
+		elf_hash_add(elf->section_hash, &sec->hash, sec->idx);
+		elf_hash_add(elf->section_name_hash, &sec->name_hash, str_hash(sec->name));
 	}
 
 	if (stats)
@@ -387,8 +402,8 @@ static int read_symbols(struct elf *elf)
 			entry = &sym->sec->symbol_list;
 		list_add(&sym->list, entry);
 
-		hash_add(elf->symbol_hash, &sym->hash, sym->idx);
-		hash_add(elf->symbol_name_hash, &sym->name_hash, str_hash(sym->name));
+		elf_hash_add(elf->symbol_hash, &sym->hash, sym->idx);
+		elf_hash_add(elf->symbol_name_hash, &sym->name_hash, str_hash(sym->name));
 	}
 
 	if (stats)
@@ -497,7 +512,7 @@ static int read_relas(struct elf *elf)
 			}
 
 			list_add_tail(&rela->list, &sec->rela_list);
-			hash_add(elf->rela_hash, &rela->hash, rela_hash(rela));
+			elf_hash_add(elf->rela_hash, &rela->hash, rela_hash(rela));
 			nr_rela++;
 		}
 		max_rela = max(max_rela, nr_rela);
@@ -524,15 +539,16 @@ struct elf *elf_read(const char *name, i
 		perror("malloc");
 		return NULL;
 	}
-	memset(elf, 0, sizeof(*elf));
+	memset(elf, 0, offsetof(struct elf, sections));
 
-	hash_init(elf->symbol_hash);
-	hash_init(elf->symbol_name_hash);
-	hash_init(elf->section_hash);
-	hash_init(elf->section_name_hash);
-	hash_init(elf->rela_hash);
 	INIT_LIST_HEAD(&elf->sections);
 
+	elf_hash_init(elf->symbol_hash);
+	elf_hash_init(elf->symbol_name_hash);
+	elf_hash_init(elf->section_hash);
+	elf_hash_init(elf->section_name_hash);
+	elf_hash_init(elf->rela_hash);
+
 	elf->fd = open(name, flags);
 	if (elf->fd == -1) {
 		fprintf(stderr, "objtool: Can't open '%s': %s\n",
@@ -671,7 +687,7 @@ struct section *elf_create_section(struc
 	shstrtab->len += strlen(name) + 1;
 	shstrtab->changed = true;
 
-	hash_add(elf->section_hash, &sec->hash, sec->idx);
+	elf_hash_add(elf->section_hash, &sec->hash, sec->idx);
 
 	return sec;
 }
--- a/tools/objtool/elf.h
+++ b/tools/objtool/elf.h
@@ -81,8 +81,8 @@ struct elf {
 	struct list_head sections;
 	DECLARE_HASHTABLE(symbol_hash, 20);
 	DECLARE_HASHTABLE(symbol_name_hash, 20);
-	DECLARE_HASHTABLE(section_hash, 16);
-	DECLARE_HASHTABLE(section_name_hash, 16);
+	DECLARE_HASHTABLE(section_hash, 20);
+	DECLARE_HASHTABLE(section_name_hash, 20);
 	DECLARE_HASHTABLE(rela_hash, 20);
 };