kallsyms: speed up /proc/kallsyms

Paulo Marques - www.grupopie.com

To err is human, but to really foul things up requires a computer.
Farmers' Almanac, 1978


Signed-Off-By: Paulo Marques <[email protected]>

 kernel/kallsyms.c  |   67 +++++++++++++------
 scripts/kallsyms.c |  185 ++++++++++++++++++++++++++++++++++++++++++++---------
 2 files changed, 202 insertions(+), 50 deletions(-)

diff -uprN -X ../dontdiff linux-2.6.9-rc1-mm2/kernel/kallsyms.c linux-2.6.9-rc1-mm2-kall/kernel/kallsyms.c
--- linux-2.6.9-rc1-mm2/kernel/kallsyms.c	2004-08-31 11:53:34.000000000 +0100
+++ linux-2.6.9-rc1-mm2-kall/kernel/kallsyms.c	2004-08-31 21:18:10.000000000 +0100
@@ -4,13 +4,12 @@
  * Rewritten and vastly simplified by Rusty Russell for in-kernel
  * module loader:
  *   Copyright 2002 Rusty Russell <[email protected]> IBM Corporation
- * Stem compression by Andi Kleen.
  *
  * ChangeLog:
  *
- * (25/Aug/2004) Paulo Marques
+ * (25/Aug/2004) Paulo Marques <[email protected]>
  *      Changed the compression method from stem compression to "table lookup"
- *      compression
+ *      compression (see scripts/kallsyms.c for a more complete description)
  */
 #include <linux/kallsyms.h>
 #include <linux/module.h>
@@ -48,40 +47,61 @@ static inline int is_kernel_text(unsigne
 	return 0;
 }

+/* expand a compressed symbol data into the resulting uncompressed string,
+   given the offset to where the symbol is in the compressed stream */
 static unsigned int kallsyms_expand_symbol(unsigned int off, char *result)
 {
-	int len, tlen;
+	int len;
 	u8 *tptr, *data;

+	/* get the compressed symbol length from the first symbol byte,
+	 * masking out the "is_exported" bit */
 	data = &kallsyms_names[off];
+	len = (*data) & 0x7F;
+	data++;

-	len=*data++;
+	/* update the offset to return the offset for the next symbol on
+	   the compressed stream */
 	off += len + 1;
+
+	/* for every byte on the compressed symbol data, copy the table
+	   entry for that byte */
 	while(len) {
-		tptr=&kallsyms_token_table[kallsyms_token_index[*data]];
+		tptr = &kallsyms_token_table[ kallsyms_token_index[*data] ];
 		data++;
 		len--;

-		tlen=*tptr++;
-		while(tlen) {
-			*result++=*tptr++;
-			tlen--;
+		while (*tptr) {
+			*result = *tptr;
+			result++;
+			tptr++;
 		}
 	}

-	*result = 0;
+	*result = '\0';

+	/* return to offset to the next symbol */
 	return off;
 }

+/* find the offset on the compressed stream given and index in the
+   kallsyms array */
 static unsigned int get_symbol_offset(unsigned long pos)
 {
 	u8 *name;
 	int i;

+        /* use the closest marker we have. We have markers every
+           256 positions, so that should be close enough */
 	name = &kallsyms_names[ kallsyms_markers[pos>>8] ];
+
+        /* sequentially scan all the symbols up to the point we're
+           searching for. Every symbol is stored in a
+	   [bit 7: is_exported | bits 6..0: <len>][<len> bytes of data]
+	   format, so we just need to add the len to the current
+	   pointer for every symbol we wish to skip */
 	for(i = 0; i < (pos&0xFF); i++)
-		name = name + (*name) + 1;
+		name = name + ((*name) & 0x7F) + 1;

 	return name - kallsyms_names;
 }
@@ -122,12 +142,16 @@ const char *kallsyms_lookup(unsigned lon
 		/* do a binary search on the sorted kallsyms_addresses array */
 		low = 0;
 		high = kallsyms_num_syms;
+
 		while (high-low > 1) {
 			mid = (low + high) / 2;
 			if (kallsyms_addresses[mid] <= addr) low = mid;
 			else high = mid;
 		}
-		while (low && kallsyms_addresses[low-1] == kallsyms_addresses[low])
+
+		/* search for the first aliased symbol. Aliased symbols are
+		   symbols with the same address */
+		while (low && kallsyms_addresses[low - 1] == kallsyms_addresses[low])
 			--low;

 		/* Grab name */
@@ -141,8 +165,8 @@ const char *kallsyms_lookup(unsigned lon
 			}
 		}

+		/* if we found no next symbol, we use the end of the section */
 		if (!symbol_end) {
-			/* At worst, symbol ends at end of section. */
 			if (is_kernel_inittext(addr))
 				symbol_end = (unsigned long)_einittext;
 			else
@@ -182,7 +206,7 @@ void __print_symbol(const char *fmt, uns
 	printk(fmt, buffer);
 }

-/* To avoid O(n^2) iteration, we carry prefix along. */
+/* To avoid using get_symbol_offset for every symbol, we carry prefix along. */
 struct kallsym_iter
 {
 	loff_t pos;
@@ -217,16 +241,20 @@ static unsigned long get_ksymbol_core(st
 {
 	unsigned off = iter->nameoff;

-	off = kallsyms_expand_symbol(off, iter->name);
-
 	iter->owner = NULL;
 	iter->value = kallsyms_addresses[iter->pos];
+
 	if (is_kernel_text(iter->value) || is_kernel_inittext(iter->value))
 		iter->type = 't';
 	else
 		iter->type = 'd';

-	upcase_if_global(iter);
+	/* check the "is_exported" bit on the compressed stream */
+	if (kallsyms_names[off] & 0x80)
+		iter->type += 'A' - 'a';
+
+	off = kallsyms_expand_symbol(off, iter->name);
+
 	return off - iter->nameoff;
 }

@@ -306,7 +334,8 @@ struct seq_operations kallsyms_op = {
 static int kallsyms_open(struct inode *inode, struct file *file)
 {
 	/* We keep iterator in m->private, since normal case is to
-	 * s_start from where we left off, so we avoid O(N^2). */
+	 * s_start from where we left off, so we avoid doing
+	 * using get_symbol_offset for every symbol */
 	struct kallsym_iter *iter;
 	int ret;

diff -uprN -X ../dontdiff linux-2.6.9-rc1-mm2/scripts/kallsyms.c linux-2.6.9-rc1-mm2-kall/scripts/kallsyms.c
--- linux-2.6.9-rc1-mm2/scripts/kallsyms.c	2004-08-31 11:53:34.000000000 +0100
+++ linux-2.6.9-rc1-mm2-kall/scripts/kallsyms.c	2004-08-31 21:09:50.000000000 +0100
@@ -9,10 +9,19 @@
  *
  * ChangeLog:
  *
- * (25/Aug/2004) Paulo Marques
+ * (25/Aug/2004) Paulo Marques <[email protected]>
  *      Changed the compression method from stem compression to "table lookup"
  *      compression
  *
+ *      Table compression uses all the unused char codes on the symbols and
+ *  maps these to the most used substrings (tokens). For instance, it might
+ *  map char code 0xF7 to represent "write_" and then in every symbol where
+ *  "write_" appears it can be replaced by 0xF7, saving 5 bytes.
+ *      The used codes themselves are also placed in the table so that the
+ *  decompresion can work without "special cases".
+ *      Applied to kernel symbols, this usually produces a compression ratio
+ *  of about 50%.
+ *
  */

 #include <stdio.h>
@@ -20,28 +29,38 @@
 #include <string.h>
 #include <ctype.h>

-/* compression tunning settings */
+/* maximum token length used. It doesn't pay to increase it a lot, because
+ * very long substrings probably don't repeat themselves too often. */
 #define MAX_TOK_SIZE		11
 #define KSYM_NAME_LEN		127

 /* we use only a subset of the complete symbol table to gather the token count,
-   to speed up compression, at the expense of a little compression ratio
-*/
+ * to speed up compression, at the expense of a little compression ratio */
 #define WORKING_SET		1024
+
+/* first find the best token only on the list of tokens that would profit more
+ * than GOOD_BAD_THRESHOLD. Only if this list is empty go to the "bad" list.
+ * Increasing this value will put less tokens on the "good" list, so the search
+ * is faster. However, if the good list runs out of tokens, we must painfully
+ * search the bad list. */
 #define GOOD_BAD_THRESHOLD	10

+/* token hash parameters */
 #define HASH_BITS		18
 #define HASH_TABLE_SIZE		(1 << HASH_BITS)
 #define HASH_MASK		(HASH_TABLE_SIZE - 1)
 #define HASH_BASE_OFFSET	2166136261U
 #define HASH_FOLD(a)		((a)&(HASH_MASK))

+/* flags to mark symbols */
+#define SYM_FLAG_VALID		1
+#define SYM_FLAG_SAMPLED	2
+#define SYM_FLAG_EXPORTED	4

 struct sym_entry {
 	unsigned long long addr;
 	char type;
-	char sample;
-	char valid;
+	unsigned char flags;
 	unsigned char len;
 	unsigned char *sym;
 };
@@ -49,23 +68,28 @@ struct sym_entry {

 static struct sym_entry *table;
 static int size, cnt;
-static unsigned long long _stext, _etext, _sinittext, _einittext;
+static unsigned long long _stext, _etext, _sinittext, _einittext, _start_ksymtab, _stop_ksymtab;
 static int all_symbols = 0;

+/* aray of pointers into the symbol table sorted by name */
+static struct sym_entry **sorted_table;

 struct token {
 	unsigned char data[MAX_TOK_SIZE];
 	unsigned char len;
+	/* profit: the number of bytes that could be saved by inserting this
+	 * token into the table */
 	int profit;
-	struct token *next;
-	struct token *right;
-	struct token *left;
-	struct token *smaller;
+	struct token *next;	/* next token on the hash list */
+	struct token *right;	/* next token on the good/bad list */
+	struct token *left;    /* previous token on the good/bad list */
+	struct token *smaller; /* token that is less one letter than this one */
 	};

 struct token bad_head, good_head;
 struct token *hash_table[HASH_TABLE_SIZE];

+/* the table that holds the result of the compression */
 unsigned char best_table[256][MAX_TOK_SIZE+1];
 unsigned char best_table_len[256];

@@ -101,6 +125,10 @@ read_symbol(FILE *in, struct sym_entry *
 		_sinittext = s->addr;
 	else if (strcmp(str, "_einittext") == 0)
 		_einittext = s->addr;
+	else if (strcmp(str, "__start___ksymtab") == 0)
+		_start_ksymtab = s->addr;
+	else if (strcmp(str, "__stop___ksymtab") == 0)
+		_stop_ksymtab = s->addr;
 	else if (toupper(s->type) == 'A' || toupper(s->type) == 'U')
 		return -1;

@@ -126,7 +154,10 @@ symbol_valid(struct sym_entry *s)
 	if (strstr(s->sym, "_compiled.") ||
 	    strcmp(s->sym, "kallsyms_addresses") == 0 ||
 	    strcmp(s->sym, "kallsyms_num_syms") == 0 ||
-	    strcmp(s->sym, "kallsyms_names") == 0)
+	    strcmp(s->sym, "kallsyms_names") == 0 ||
+	    strcmp(s->sym, "kallsyms_markers") == 0 ||
+	    strcmp(s->sym, "kallsyms_token_table") == 0 ||
+	    strcmp(s->sym, "kallsyms_token_index") == 0)
 		return 0;

 	/* Exclude linker generated symbols which vary between passes */
@@ -161,16 +192,21 @@ static void output_label(char *label)
 	printf("%s:\n",label);
 }

+/* uncompress a compressed symbol. When this function is called, the best table
+ * might still be compressed itself, so the function needs to be recursive */
 static int expand_symbol(unsigned char *data, int len, char *result)
 {
 	int c, rlen, total=0;

 	while (len) {
 		c = *data;
+		/* if the table holds a single char that is the same as the one
+		 * we are looking for, then end the search */
 		if (best_table[c][0]==c && best_table_len[c]==1) {
 			*result++ = c;
 			total++;
 		} else {
+			/* if not, recurse and expand */
 			rlen = expand_symbol(best_table[c], best_table_len[c], result);
 			total += rlen;
 			result += rlen;
@@ -205,7 +241,7 @@ write_src(void)
 	output_label("kallsyms_addresses");
 	valid = 0;
 	for (i = 0; i < cnt; i++) {
-		if (table[i].valid) {
+		if (table[i].flags & SYM_FLAG_VALID) {
 			printf("\tPTR\t%#llx\n", table[i].addr);
 			valid++;
 		}
@@ -216,6 +252,8 @@ write_src(void)
 	printf("\tPTR\t%d\n", valid);
 	printf("\n");

+	/* table of offset markers, that give the offset in the compressed stream
+	 * every 256 symbols */
 	markers = (unsigned int *) malloc(sizeof(unsigned int)*((valid + 255) / 256));

 	output_label("kallsyms_names");
@@ -223,13 +261,15 @@ write_src(void)
 	off = 0;
 	for (i = 0; i < cnt; i++) {

-		if (!table[i].valid)
+		if (!table[i].flags & SYM_FLAG_VALID)
 			continue;

 		if ((valid & 0xFF) == 0)
 			markers[valid >> 8] = off;

-		printf("\t.byte 0x%02x", table[i].len);
+		k = table[i].len;
+		if (table[i].flags & SYM_FLAG_EXPORTED) k |= 0x80;
+		printf("\t.byte 0x%02x", k);
 		for (k = 0; k < table[i].len; k++)
 			printf(", 0x%02x", table[i].sym[k]);
 		printf("\n");
@@ -244,14 +284,15 @@ write_src(void)
 		printf("\tPTR\t%d\n", markers[i]);
 	printf("\n");

+	free(markers);
+
 	output_label("kallsyms_token_table");
 	off = 0;
 	for (i = 0; i < 256; i++) {
 		best_idx[i] = off;
 		expand_symbol(best_table[i],best_table_len[i],buf);
-		k = strlen(buf);
-		printf("\t.byte 0x%02x\n\t.ascii\t\"%s\"\n", k, buf);
-		off += k + 1;
+		printf("\t.asciz\t\"%s\"\n", buf);
+		off += strlen(buf) + 1;
 	}
 	printf("\n");

@@ -280,6 +321,7 @@ static unsigned int hash_token(unsigned
 	return HASH_FOLD(hash);
 }

+/* find a token given its data and hash value */
 static struct token *find_token_hash(unsigned char *data, int len, unsigned int hash)
 {
 	struct token *ptr;
@@ -309,6 +351,9 @@ static inline void remove_token_from_gro
 	ptr->right->left = ptr->left;
 }

+
+/* build the counts for all the tokens that start with "data", and have lenghts
+ * from 2 to "len" */
 static void learn_token(unsigned char *data, int len)
 {
 	struct token *ptr,*last_ptr;
@@ -319,6 +364,7 @@ static void learn_token(unsigned char *d
 	if (len > MAX_TOK_SIZE)
 		len = MAX_TOK_SIZE;

+	/* calculate and store the hash values for all the sub-tokens */
 	hash = rehash_token(hash, data[0]);
 	for (i = 2; i <= len; i++) {
 		hash = rehash_token(hash, data[i-1]);
@@ -334,10 +380,19 @@ static void learn_token(unsigned char *d
 		if (!ptr) ptr = find_token_hash(data, i, hash);

 		if (!ptr) {
+			/* create a new token entry */
 			ptr = (struct token *) malloc(sizeof(*ptr));
+
 			memcpy(ptr->data, data, i);
 			ptr->len = i;
+
+			/* when we create an entry, it's profit is 0 because
+			 * we also take into account the size of the token on
+			 * the compressed table. We then subtract GOOD_BAD_THRESHOLD
+			 * so that the test to see if this token belongs to
+			 * the good or bad list, is a comparison to zero */
 			ptr->profit = -GOOD_BAD_THRESHOLD;
+
 			ptr->next = hash_table[hash];
 			hash_table[hash] = ptr;

@@ -346,11 +401,13 @@ static void learn_token(unsigned char *d
 			ptr->smaller = NULL;
 		} else {
 			newprofit = ptr->profit + (ptr->len - 1);
+			/* check to see if this token needs to be moved to a
+			 * different list */
 			if((ptr->profit < 0) && (newprofit >= 0)) {
 				remove_token_from_group(ptr);
 				insert_token_in_group(&good_head,ptr);
 			}
-		ptr->profit = newprofit;
+			ptr->profit = newprofit;
 		}

 		if (last_ptr) last_ptr->smaller = ptr;
@@ -360,6 +417,10 @@ static void learn_token(unsigned char *d
 	}
 }

+/* decrease the counts for all the tokens that start with "data", and have lenghts
+ * from 2 to "len". This function is much simpler than learn_token because we have
+ * more guarantees (tho tokens exist, the ->smaller pointer is set, etc.)
+ * The two separate functions exist only because of compression performance */
 static void forget_token(unsigned char *data, int len)
 {
 	struct token *ptr;
@@ -384,6 +445,7 @@ static void forget_token(unsigned char *
 	}
 }

+/* count all the possible tokens in a symbol */
 static void learn_symbol(unsigned char *symbol, int len)
 {
 	int i;
@@ -392,6 +454,7 @@ static void learn_symbol(unsigned char *
 		learn_token(symbol + i, len - i);
 }

+/* decrease the count for all the possible tokens in a symbol */
 static void forget_symbol(unsigned char *symbol, int len)
 {
 	int i;
@@ -400,49 +463,98 @@ static void forget_symbol(unsigned char
 		forget_token(symbol + i, len - i);
 }

+static int symbol_sort(const void *a, const void *b)
+{
+	return strcmp( (*((struct sym_entry **) a))->sym,
+				(*((struct sym_entry **) b))->sym );
+}
+
+
+/* find out if a symbol is exported. Exported symbols have a corresponding
+ * __ksymtab_<symbol> entry and their addresses are between __start___ksymtab
+ * and __stop___ksymtab */
+static int is_exported(char *name)
+{
+	struct sym_entry key, *ksym, **result;
+	char buf[KSYM_NAME_LEN+32];
+
+	sprintf(buf, "__ksymtab_%s", name);
+	key.sym = buf;
+
+	ksym = &key;
+	result = bsearch(&ksym, sorted_table, cnt,
+				sizeof(struct sym_entry *), symbol_sort);
+
+	if(!result) return 0;
+
+	ksym = *result;
+
+	return ((ksym->addr >= _start_ksymtab) && (ksym->addr < _stop_ksymtab));
+}
+
+/* set all the symbol flags and do the initial token count */
 static void build_initial_tok_table(void)
 {
 	int i, use_it, valid;

+	/* build a sorted symbol pointer array so that searching a particular
+	 * symbol is faster */
+	sorted_table = (struct sym_entry **) malloc(sizeof(struct sym_entry *) * cnt);
+	for (i = 0; i < cnt; i++)
+		sorted_table[i] = &table[i];
+	qsort(sorted_table, cnt, sizeof(struct sym_entry *), symbol_sort);
+
 	valid = 0;
 	for (i = 0; i < cnt; i++) {
-		table[i].valid = symbol_valid(&table[i]);
-		if (table[i].valid) valid++;
+		table[i].flags = 0;
+		if ( symbol_valid(&table[i]) ) {
+			table[i].flags |= SYM_FLAG_VALID;
+			valid++;
+		}
 	}

 	use_it = 0;
 	for (i = 0; i < cnt; i++) {
-		table[i].sample = 0;
-		if (table[i].valid) {
+		if (table[i].flags & SYM_FLAG_VALID) {
+
 			use_it += WORKING_SET;
+
 			if (use_it >= valid) {
-				table[i].sample = 1;
+				table[i].flags |= SYM_FLAG_SAMPLED;
 				use_it -= valid;
 			}
+
+			if( is_exported(table[i].sym) )
+				table[i].flags |= SYM_FLAG_EXPORTED;
 		}
-		if (table[i].sample)
+		if (table[i].flags & SYM_FLAG_SAMPLED)
 			learn_symbol(table[i].sym, table[i].len);
 	}
 }

+/* replace a given token in all the valid symbols. Use the sampled symbols
+ * to update the counts */
 static void compress_symbols(unsigned char *str, int tlen, int idx)
 {
 	int i, len, learn, size;
 	unsigned char *p;

 	for (i = 0; i < cnt; i++) {
-		if (!table[i].valid) continue;
+
+		if (!(table[i].flags & SYM_FLAG_VALID)) continue;

 		len = table[i].len;
 		learn = 0;
 		p = table[i].sym;

 		do {
+			/* find the token on the symbol */
 			p = (unsigned char *) strstr((char *) p, (char *) str);
 			if (!p) break;

 			if (!learn) {
-				if (table[i].sample)
+				/* if this symbol was used to count, decrease it */
+				if (table[i].flags & SYM_FLAG_SAMPLED)
 					forget_symbol(table[i].sym, len);
 				learn = 1;
 			}
@@ -457,11 +569,14 @@ static void compress_symbols(unsigned ch

 		if(learn) {
 			table[i].len = len;
-			if(table[i].sample) learn_symbol(table[i].sym, len);
+			/* if this symbol was used to count, learn it again */
+			if(table[i].flags & SYM_FLAG_SAMPLED)
+				learn_symbol(table[i].sym, len);
 		}
 	}
 }

+/* search the token with the maximum profit */
 static struct token *find_best_token(void)
 {
 	struct token *ptr,*best,*head;
@@ -486,29 +601,37 @@ static struct token *find_best_token(voi
 	return best;
 }

+/* this is the core of the algorithm: calculate the "best" table */
 static void optimize_result(void)
 {
 	struct token *best;
 	int i;

 	/* using the '\0' symbol last allows compress_symbols to use standard
-	   fast string functions
-	*/
+	 * fast string functions */
 	for (i = 255; i >= 0; i--) {
+
+		/* if this table slot is empty (it is not used by an actual
+		 * original char code */
 		if (!best_table_len[i]) {
+
+			/* find the token with the breates profit value */
 			best = find_best_token();

+			/* place it in the "best" table */
 			best_table_len[i] = best->len;
 			memcpy(best_table[i], best->data, best_table_len[i]);
 			/* zero terminate the token so that we can use strstr
 			   in compress_symbols */
 			best_table[i][best_table_len[i]]='\0';

+			/* replace this token in all the valid symbols */
 			compress_symbols(best_table[i], best_table_len[i], i);
 		}
 	}
 }

+/* start by placing the symbols that are actually used on the table */
 static void insert_real_symbols_in_table(void)
 {
 	int i, j, c;
@@ -517,7 +640,7 @@ static void insert_real_symbols_in_table
 	memset(best_table_len, 0, sizeof(best_table_len));

 	for (i = 0; i < cnt; i++) {
-		if (table[i].valid) {
+		if (table[i].flags & SYM_FLAG_VALID) {
 			for (j = 0; j < table[i].len; j++) {
 				c = table[i].sym[j];
 				best_table[c][0]=c;

Anyone who quotes me in their signature is an idiot -- Rusty Russell

Paulo Marques - www.grupopie.com

To err is human, but to really foul things up requires a computer.
Farmers' Almanac, 1978

Paulo Marques - www.grupopie.com

To err is human, but to really foul things up requires a computer.
Farmers' Almanac, 1978

Paulo Marques - www.grupopie.com

To err is human, but to really foul things up requires a computer.
Farmers' Almanac, 1978

Paulo Marques - www.grupopie.com

To err is human, but to really foul things up requires a computer.
Farmers' Almanac, 1978

Paulo Marques - www.grupopie.com

To err is human, but to really foul things up requires a computer.
Farmers' Almanac, 1978