[PATCH 1/7] dyndbg: move struct _ddebug's display fields to new _ddebug_callsite

From: Jim Cromie
Date: Wed Nov 25 2020 - 14:37:41 EST

struct _ddebug has 5 fields used to display and select pr_debug
callsites, move these to a new struct _ddebug_callsite, and add ptr to
link 1st to 2nd. While this increases memory footprint by 1 ptr per
pr_debug, the indirection gives several advantages:

- we can allocate storage only for enabled callsites. Since pr_debugs
are 99% disabled, we should see savings.

- the display fields are inherently hierarchical, and the linker
section is ordered; so (module, file, function) have redundant
values (90%, 85%, 45%). This is readily compressible, even with a
simple field-wise run length encoding. Because theyre placed in a
separate linker section, theyre in a contiguous block of memory,
which should simplify that compression.

Looking forward, there are several approaches to get the advantages.

A - copy each callsite to zram, save to new .zhandle member, and
update site pointers. Must later retire __dyndbg_callsite section
afterwards to actually recover memory.

I did this (next patches), and get 3:1 zs_page:page compression. It
works when sites are zs_mapped in just for the print. But if I leave
them mapped in cuz the pr_debug is enabled, locking conflicts & panic
ensue. Patches follow.

B - compress __dyndbg_callsite linker section, using some format which
is good at random-index decompression.

I did objcopy --dump-sections .. vmlinux.o, got mostly empty data,
like Im getting values before the final link. Im missing some
understanding.

C - field-wise RLE. This is feeling increasingly suitable.

Whats actually done here:

dynamic_debug.h:

I cut struct _ddebug in half, renamed top-half (body), kept __align(8)
on both head & body, added a forward decl for a unified comment for
both head & body. And added head.site to point at body.

DECLARE_DYNAMIC_DEBUG_METADATA does the core of the work; it declares
and initializes both static struct vars together, and refs one to the
other.

And since Im rejiggering the structs:

- I moved static_key key to front of struct _ddebug; its the biggest
member, and most alignment sensitive, so moving it to front may
improve ambient pahole conditions.

- reorder display fields to match the hierarchy. This should help
improve compressability, particularly for field-wise RLE. With
this, consecutive records are tail-different.

Also
- reserved a flag bit for zram mapping (no use yet)
- I shrunk lineno member from 18 to 16 bits, and made it const.
No source file is near 64k-lines, I doubt any could get added.

dynamic_debug.c:

dynamic_debug_init() mem-usage now counts callsites.

The 3 funcs which use _ddebug* pointers (ddebug_change,
dynamic_emit_prefix, ddebug_proc_show) each get an auto-var, inited
with ->site, and s/dp/dc/ as needed. These once-per-func dp->site
derefs are also a setup for the next commit.

vmlinux.lds.h:

add __ddebug_callsites section, with the same align(8) and KEEP as
used in the __ddebug section.

TBD:

_align(8) may be unnecessary on struct _ddebug_callsite, I think its
there for the static_key member. I do wonder if its arch dependent, 8
seems big for i686 at least.

Signed-off-by: Jim Cromie <jim.cromie@xxxxxxxxx>
---
include/asm-generic/vmlinux.lds.h | 4 +++
include/linux/dynamic_debug.h | 41 +++++++++++++++---------
lib/dynamic_debug.c | 52 +++++++++++++++++--------------
3 files changed, 58 insertions(+), 39 deletions(-)

diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index b2b3d81b1535..1ef1efc73d20 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -340,6 +340,10 @@
*(__tracepoints) \
/* implement dynamic printk debug */ \
. = ALIGN(8); \
+ __start___dyndbg_callsites = .; \
+ KEEP(*(__dyndbg_callsites)) \
+ __stop___dyndbg_callsites = .; \
+ . = ALIGN(8); \
__start___dyndbg = .; \
KEEP(*(__dyndbg)) \
__stop___dyndbg = .; \
diff --git a/include/linux/dynamic_debug.h b/include/linux/dynamic_debug.h
index a57ee75342cf..0bf7036bcdb2 100644
--- a/include/linux/dynamic_debug.h
+++ b/include/linux/dynamic_debug.h
@@ -7,11 +7,14 @@
#endif

/*
- * An instance of this structure is created in a special
- * ELF section at every dynamic debug callsite. At runtime,
- * the special section is treated as an array of these.
+ * a pair of these structs are created in 2 special ELF sections
+ * (__dyndbg, __dyndbg_callsites) for every dynamic debug callsite.
+ * During init, __dyndbg_callsites is copied to zram, and links to
+ * them in _ddebug are updated. At runtime, the __dyndbg section is
+ * treated as an array of struct _ddebugs.
*/
-struct _ddebug {
+struct _ddebug;
+struct _ddebug_callsite {
/*
* These fields are used to drive the user interface
* for selecting and displaying debug callsites.
@@ -20,7 +23,17 @@ struct _ddebug {
const char *function;
const char *filename;
const char *format;
- unsigned int lineno:18;
+ const unsigned int lineno:16;
+} __aligned(8);
+
+struct _ddebug {
+#ifdef CONFIG_JUMP_LABEL
+ union {
+ struct static_key_true dd_key_true;
+ struct static_key_false dd_key_false;
+ } key;
+#endif
+ struct _ddebug_callsite *site;
/*
* The flags field controls the behaviour at the callsite.
* The bits here are changed dynamically when the user
@@ -32,20 +45,14 @@ struct _ddebug {
#define _DPRINTK_FLAGS_INCL_FUNCNAME (1<<2)
#define _DPRINTK_FLAGS_INCL_LINENO (1<<3)
#define _DPRINTK_FLAGS_INCL_TID (1<<4)
+#define _DPRINTK_FLAGS_MAPPED (1<<7) /* reserved */
#if defined DEBUG
#define _DPRINTK_FLAGS_DEFAULT _DPRINTK_FLAGS_PRINT
#else
#define _DPRINTK_FLAGS_DEFAULT 0
#endif
unsigned int flags:8;
-#ifdef CONFIG_JUMP_LABEL
- union {
- struct static_key_true dd_key_true;
- struct static_key_false dd_key_false;
- } key;
-#endif
-} __attribute__((aligned(8)));
-
+} __aligned(8);


#if defined(CONFIG_DYNAMIC_DEBUG_CORE)
@@ -83,13 +90,17 @@ void __dynamic_ibdev_dbg(struct _ddebug *descriptor,
const char *fmt, ...);

#define DEFINE_DYNAMIC_DEBUG_METADATA(name, fmt) \
- static struct _ddebug __aligned(8) \
- __section("__dyndbg") name = { \
+ static struct _ddebug_callsite __aligned(8) \
+ __section("__dyndbg_callsites") name##_site = { \
.modname = KBUILD_MODNAME, \
.function = __func__, \
.filename = __FILE__, \
.format = (fmt), \
.lineno = __LINE__, \
+ }; \
+ static struct _ddebug __aligned(8) \
+ __section("__dyndbg") name = { \
+ .site = &name##_site, \
.flags = _DPRINTK_FLAGS_DEFAULT, \
_DPRINTK_KEY_INIT \
}
diff --git a/lib/dynamic_debug.c b/lib/dynamic_debug.c
index 711a9def8c83..2e4a39c349a5 100644
--- a/lib/dynamic_debug.c
+++ b/lib/dynamic_debug.c
@@ -165,19 +165,20 @@ static int ddebug_change(const struct ddebug_query *query,

for (i = 0; i < dt->num_ddebugs; i++) {
struct _ddebug *dp = &dt->ddebugs[i];
+ struct _ddebug_callsite *dc = dp->site;

/* match against the source filename */
if (query->filename &&
- !match_wildcard(query->filename, dp->filename) &&
+ !match_wildcard(query->filename, dc->filename) &&
!match_wildcard(query->filename,
- kbasename(dp->filename)) &&
+ kbasename(dc->filename)) &&
!match_wildcard(query->filename,
- trim_prefix(dp->filename)))
+ trim_prefix(dc->filename)))
continue;

/* match against the function */
if (query->function &&
- !match_wildcard(query->function, dp->function))
+ !match_wildcard(query->function, dc->function))
continue;

/* match against the format */
@@ -185,19 +186,19 @@ static int ddebug_change(const struct ddebug_query *query,
if (*query->format == '^') {
char *p;
/* anchored search. match must be at beginning */
- p = strstr(dp->format, query->format+1);
- if (p != dp->format)
+ p = strstr(dc->format, query->format+1);
+ if (p != dc->format)
continue;
- } else if (!strstr(dp->format, query->format))
+ } else if (!strstr(dc->format, query->format))
continue;
}

/* match against the line number range */
if (query->first_lineno &&
- dp->lineno < query->first_lineno)
+ dc->lineno < query->first_lineno)
continue;
if (query->last_lineno &&
- dp->lineno > query->last_lineno)
+ dc->lineno > query->last_lineno)
continue;

nfound++;
@@ -214,8 +215,8 @@ static int ddebug_change(const struct ddebug_query *query,
#endif
dp->flags = newflags;
v2pr_info("changed %s:%d [%s]%s =%s\n",
- trim_prefix(dp->filename), dp->lineno,
- dt->mod_name, dp->function,
+ trim_prefix(dc->filename), dc->lineno,
+ dt->mod_name, dc->function,
ddebug_describe_flags(dp->flags, &fbuf));
}
}
@@ -587,14 +588,15 @@ static int remaining(int wrote)
return 0;
}

-static char *dynamic_emit_prefix(const struct _ddebug *desc, char *buf)
+static char *dynamic_emit_prefix(const struct _ddebug *dp, char *buf)
{
int pos_after_tid;
int pos = 0;
+ const struct _ddebug_callsite *desc = dp->site;

*buf = '\0';

- if (desc->flags & _DPRINTK_FLAGS_INCL_TID) {
+ if (dp->flags & _DPRINTK_FLAGS_INCL_TID) {
if (in_interrupt())
pos += snprintf(buf + pos, remaining(pos), "<intr> ");
else
@@ -602,13 +604,13 @@ static char *dynamic_emit_prefix(const struct _ddebug *desc, char *buf)
task_pid_vnr(current));
}
pos_after_tid = pos;
- if (desc->flags & _DPRINTK_FLAGS_INCL_MODNAME)
+ if (dp->flags & _DPRINTK_FLAGS_INCL_MODNAME)
pos += snprintf(buf + pos, remaining(pos), "%s:",
desc->modname);
- if (desc->flags & _DPRINTK_FLAGS_INCL_FUNCNAME)
+ if (dp->flags & _DPRINTK_FLAGS_INCL_FUNCNAME)
pos += snprintf(buf + pos, remaining(pos), "%s:",
desc->function);
- if (desc->flags & _DPRINTK_FLAGS_INCL_LINENO)
+ if (dp->flags & _DPRINTK_FLAGS_INCL_LINENO)
pos += snprintf(buf + pos, remaining(pos), "%d:",
desc->lineno);
if (pos - pos_after_tid)
@@ -880,6 +882,7 @@ static int ddebug_proc_show(struct seq_file *m, void *p)
{
struct ddebug_iter *iter = m->private;
struct _ddebug *dp = p;
+ struct _ddebug_callsite *dc = dp->site;
struct flagsbuf flags;

if (p == SEQ_START_TOKEN) {
@@ -889,10 +892,10 @@ static int ddebug_proc_show(struct seq_file *m, void *p)
}

seq_printf(m, "%s:%u [%s]%s =%s \"",
- trim_prefix(dp->filename), dp->lineno,
- iter->table->mod_name, dp->function,
+ trim_prefix(dc->filename), dc->lineno,
+ iter->table->mod_name, dc->function,
ddebug_describe_flags(dp->flags, &flags));
- seq_escape(m, dp->format, "\t\r\n\"");
+ seq_escape(m, dc->format, "\t\r\n\"");
seq_puts(m, "\"\n");

return 0;
@@ -1094,17 +1097,17 @@ static int __init dynamic_debug_init(void)
return 0;
}
iter = __start___dyndbg;
- modname = iter->modname;
+ modname = iter->site->modname;
iter_start = iter;
for (; iter < __stop___dyndbg; iter++) {
entries++;
- if (strcmp(modname, iter->modname)) {
+ if (strcmp(modname, iter->site->modname)) {
modct++;
ret = ddebug_add_module(iter_start, n, modname);
if (ret)
goto out_err;
n = 0;
- modname = iter->modname;
+ modname = iter->site->modname;
iter_start = iter;
}
n++;
@@ -1114,9 +1117,10 @@ static int __init dynamic_debug_init(void)
goto out_err;

ddebug_init_success = 1;
- vpr_info("%d modules, %d entries and %d bytes in ddebug tables, %d bytes in __dyndbg section\n",
+ vpr_info("%d modules, %d entries and %d bytes in ddebug tables, %d bytes in __dyndbg section, %d bytes in __dyndbg_callsites section\n",
modct, entries, (int)(modct * sizeof(struct ddebug_table)),
- (int)(entries * sizeof(struct _ddebug)));
+ (int)(entries * sizeof(struct _ddebug)),
+ (int)(entries * sizeof(struct _ddebug_callsite)));

/* apply ddebug_query boot param, dont unload tables on err */
if (ddebug_setup_string[0] != '\0') {
--
2.28.0