[PATCH] kallsyms data size reduction / lookup speedup

This patch is an improvement over my first kallsyms speedup patch posted about
2 weeks ago.

It changes scripts/kallsyms as to produce a different format for
kallsyms_names and extra data to speedup lookups.  The compression algorithm
is quite simple: it uses all the char codes not actually used in symbols to
build a lookup table that translates these codes into small strings.  For
instance, in my test runs the code 0xFE was being translated into "acpi_"
giving a 4 byte save on every translation.

The advantage of this algorithm is that to translate a symbol we only require
information that is stored on that symbol position, and never need to go back
on the compressed stream to get information from other symbols.

To give an idea about the benefits of this algorithm here are some benchmark
results on a P4 2.8GHz with a symbol table with 10000 entries:

kallsyms_lookup average time:
  vanilla           1346.0 us
  speedup             14.4 us
  with this patch      0.5 us

total data produced by scripts/kallsyms:
  uncompressed         169 Kb
  vanilla              134 Kb
  with this patch       91 Kb

(speedup was my latest patch, that only changed the way kallsyms_lookup worked
and not the data format)

I removed a cond_resched() from the proc/kallsyms handling code path, because
using stem compression, if the current position went backwards, the hole
stream would be uncompressed up to the current position.  It seemed that by
removing this loop it would be safe to remove the conditional reschedule
altogether.

There is just one catch with this patch: the time it takes to compile the
kernel goes up just a bit (about 0.8s on a P4 2.8GHz with defconfig).  If this
delay is not acceptable I can change the compression algorithm so that it can
use the previous table (calculating a new table is what consumes most of the
time, and not doing the actual compression) and check to see if it obtains a
similar compression ratio.  If it does, then this is a sign that the symbol
patterns haven't changed that much and this table is still good to use.  This
would not only cut the time down to half on any compilation (because of the 2
pass symbol build method), but in frequent cases where a developer is
compiling a single file and linking everything over and over again, the table
optimization process would never run.

I'm CC'ing Brent Casavant on this email, because last june he sent a patch
trying a different approach that used a 32 entry symbol cache, because there
was a problem with the time "top" took to read "proc/<pid>/wchan".  I was
hopping he would be willing to test this patch and comment on the results.
Signed-off-by: Paulo Marques <pmarques@grupopie.com>
Signed-off-by: Andrew Morton <akpm@osdl.org>
Signed-off-by: Linus Torvalds <torvalds@osdl.org>

kernel/kallsyms.c

@@ -4,7 +4,12 @@
  * Rewritten and vastly simplified by Rusty Russell for in-kernel
  * module loader:
  *   Copyright 2002 Rusty Russell <rusty@rustcorp.com.au> IBM Corporation
- * Stem compression by Andi Kleen.
+ *
+ * ChangeLog:
+ *
+ * (25/Aug/2004) Paulo Marques <pmarques@grupopie.com>
+ *      Changed the compression method from stem compression to "table lookup"
+ *      compression (see scripts/kallsyms.c for a more complete description)
  */
 #include <linux/kallsyms.h>
 #include <linux/module.h>
@@ -17,7 +22,12 @@
 /* These will be re-linked against their real values during the second link stage */
 extern unsigned long kallsyms_addresses[] __attribute__((weak));
 extern unsigned long kallsyms_num_syms __attribute__((weak));
-extern char kallsyms_names[] __attribute__((weak));
+extern u8 kallsyms_names[] __attribute__((weak));
+
+extern u8 kallsyms_token_table[] __attribute__((weak));
+extern u16 kallsyms_token_index[] __attribute__((weak));
+
+extern unsigned long kallsyms_markers[] __attribute__((weak));
 
 /* Defined by the linker script. */
 extern char _stext[], _etext[], _sinittext[], _einittext[];
@@ -37,21 +47,88 @@ static inline int is_kernel_text(unsigned long addr)
 	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, skipped_first = 0;
+	u8 *tptr, *data;
+
+	/* get the compressed symbol length from the first symbol byte */
+	data = &kallsyms_names[off];
+	len = *data;
+	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] ];
+		data++;
+		len--;
+
+		while (*tptr) {
+			if(skipped_first) {
+				*result = *tptr;
+				result++;
+			} else
+				skipped_first = 1;
+			tptr++;
+		}
+	}
+
+	*result = '\0';
+
+	/* return to offset to the next symbol */
+	return off;
+}
+
+/* get symbol type information. This is encoded as a single char at the
+ * begining of the symbol name */
+static char kallsyms_get_symbol_type(unsigned int off)
+{
+	/* get just the first code, look it up in the token table, and return the
+	 * first char from this token */
+	return kallsyms_token_table[ kallsyms_token_index[ kallsyms_names[off+1] ] ];
+}
+
+
+/* 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 [<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;
+
+	return name - kallsyms_names;
+}
+
 /* Lookup the address for this symbol. Returns 0 if not found. */
 unsigned long kallsyms_lookup_name(const char *name)
 {
 	char namebuf[KSYM_NAME_LEN+1];
 	unsigned long i;
-	char *knames;
+	unsigned int off;
 
-	for (i = 0, knames = kallsyms_names; i < kallsyms_num_syms; i++) {
-		unsigned prefix = *knames++;
+	for (i = 0, off = 0; i < kallsyms_num_syms; i++) {
+		off = kallsyms_expand_symbol(off, namebuf);
 
-		strlcpy(namebuf + prefix, knames, KSYM_NAME_LEN - prefix);
 		if (strcmp(namebuf, name) == 0)
 			return kallsyms_addresses[i];
-
-		knames += strlen(knames) + 1;
 	}
 	return module_kallsyms_lookup_name(name);
 }
@@ -62,7 +139,7 @@ const char *kallsyms_lookup(unsigned long addr,
 			    unsigned long *offset,
 			    char **modname, char *namebuf)
 {
-	unsigned long i, best = 0;
+	unsigned long i, low, high, mid;
 
 	/* This kernel should never had been booted. */
 	BUG_ON(!kallsyms_addresses);
@@ -71,40 +148,45 @@ const char *kallsyms_lookup(unsigned long addr,
 	namebuf[0] = 0;
 
 	if (is_kernel_text(addr) || is_kernel_inittext(addr)) {
-		unsigned long symbol_end;
-		char *name = kallsyms_names;
-
-		/* They're sorted, we could be clever here, but who cares? */
-		for (i = 0; i < kallsyms_num_syms; i++) {
-			if (kallsyms_addresses[i] > kallsyms_addresses[best] &&
-			    kallsyms_addresses[i] <= addr)
-				best = i;
-		}
+		unsigned long symbol_end=0;
 
-		/* Grab name */
-		for (i = 0; i <= best; i++) { 
-			unsigned prefix = *name++;
-			strncpy(namebuf + prefix, name, KSYM_NAME_LEN - prefix);
-			name += strlen(name) + 1;
+		/* 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;
 		}
 
-		/* At worst, symbol ends at end of section. */
-		if (is_kernel_inittext(addr))
-			symbol_end = (unsigned long)_einittext;
-		else
-			symbol_end = (unsigned long)_etext;
+		/* 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 */
+		kallsyms_expand_symbol(get_symbol_offset(low), namebuf);
 
 		/* Search for next non-aliased symbol */
-		for (i = best+1; i < kallsyms_num_syms; i++) {
-			if (kallsyms_addresses[i] > kallsyms_addresses[best]) {
+		for (i = low + 1; i < kallsyms_num_syms; i++) {
+			if (kallsyms_addresses[i] > kallsyms_addresses[low]) {
 				symbol_end = kallsyms_addresses[i];
 				break;
 			}
 		}
 
-		*symbolsize = symbol_end - kallsyms_addresses[best];
+		/* if we found no next symbol, we use the end of the section */
+		if (!symbol_end) {
+			if (is_kernel_inittext(addr))
+				symbol_end = (unsigned long)_einittext;
+			else
+				symbol_end = (unsigned long)_etext;
+		}
+
+		*symbolsize = symbol_end - kallsyms_addresses[low];
 		*modname = NULL;
-		*offset = addr - kallsyms_addresses[best];
+		*offset = addr - kallsyms_addresses[low];
 		return namebuf;
 	}
 
@@ -135,7 +217,7 @@ void __print_symbol(const char *fmt, unsigned long address)
 	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;
@@ -168,31 +250,23 @@ static int get_ksymbol_mod(struct kallsym_iter *iter)
 /* Returns space to next name. */
 static unsigned long get_ksymbol_core(struct kallsym_iter *iter)
 {
-	unsigned stemlen, off = iter->nameoff;
-
-	/* First char of each symbol name indicates prefix length
-	   shared with previous name (stem compression). */
-	stemlen = kallsyms_names[off++];
+	unsigned off = iter->nameoff;
 
-	strlcpy(iter->name+stemlen, kallsyms_names + off,
-		KSYM_NAME_LEN+1-stemlen);
-	off += strlen(kallsyms_names + off) + 1;
 	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);
+	iter->type = kallsyms_get_symbol_type(off);
+
+	off = kallsyms_expand_symbol(off, iter->name);
+
 	return off - iter->nameoff;
 }
 
-static void reset_iter(struct kallsym_iter *iter)
+static void reset_iter(struct kallsym_iter *iter, loff_t new_pos)
 {
 	iter->name[0] = '\0';
-	iter->nameoff = 0;
-	iter->pos = 0;
+	iter->nameoff = get_symbol_offset(new_pos);
+	iter->pos = new_pos;
 }
 
 /* Returns false if pos at or past end of file. */
@@ -204,16 +278,13 @@ static int update_iter(struct kallsym_iter *iter, loff_t pos)
 		return get_ksymbol_mod(iter);
 	}
 	
-	/* If we're past the desired position, reset to start. */
-	if (pos < iter->pos)
-		reset_iter(iter);
-
-	/* We need to iterate through the previous symbols: can be slow */
-	for (; iter->pos != pos; iter->pos++) {
-		iter->nameoff += get_ksymbol_core(iter);
-		cond_resched();
-	}
-	get_ksymbol_core(iter);
+	/* If we're not on the desired position, reset to new position. */
+	if (pos != iter->pos)
+		reset_iter(iter, pos);
+
+	iter->nameoff += get_ksymbol_core(iter);
+	iter->pos++;
+
 	return 1;
 }
 
@@ -267,14 +338,15 @@ 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;
 
 	iter = kmalloc(sizeof(*iter), GFP_KERNEL);
 	if (!iter)
 		return -ENOMEM;
-	reset_iter(iter);
+	reset_iter(iter, 0);
 
 	ret = seq_open(file, &kallsyms_op);
 	if (ret == 0)