[RFC PATCH v6 04/34] dyndbg: split struct _ddebug's display fields to new _ddebug_site

Jim Cromie
2021-05-29T20:02:30Z

Migrate 3 fields: module, function, file into a new struct _ddebug_site.
These reflect the code's hierarchy, and are inherently ordered by the
compile/link; they exhibit 90%,55%,45% repetition (on my fedora based,
localmodconfig'd, virtme-configkernel --update'd conf).

Since struct _ddebugs are placed in their own elf section, this means
splitting its section also. The new section is named "__dyndbg_sites"
(the plural name conveys the vector nature of the contents), and
"__dyndbg" is renamed "__dyndbgs" to reinforce this.

Struct _ddebug gets a new .site, pointing to a struct _ddebug_site.
The new ptr increases memory footprint, but it is temporary
scaffolding; once we can map from _ddebugs[N] --> _ddebug_sites[N]
indirectly, we can drop site pointer, regaining worst case memory
parity with master.

The indirection gives several advantages:

- site ptr lets us decouple the 2 arrays
we can properly isolate dependencies by allowing null site

- the moved display fields are inherently hierarchical, and the linker
section is ordered; so (module, file, function) have repeating
values (90%, 85%, 45%). This is readily compressible, even with a
simple field-wise run length encoding. Since I'm splitting the struct,
I also reordered the fields to match the hierarchy.

- the separate linker section sets up naturally for block compression.
section compression at build time is practical - how ?
include/linux/decompress/generic.h has no corresponding compression

- we could drop sites and their storage opportunistically.
this could reduce per-site mem by 24/56.

Subsystems may not need/want "module:func:line:" in their logs.
If they already use format-prefixes such as "drm:kms:",
they can select on those, and don't need the site info for that.
forex:
#> echo module drm format "^drm:kms: " +p >control
ie: dynamic_debug_exec_queries("format '^drm:kms: '", "drm");

Once we can map: ddebugs[N] -> ddebug_sites[N], we can:

- compress __dyndbg_sites during __init, and mark section __initdata
- store the compressed block instead.
- decompress on-demand, stream for `cat control`
- save chunks of decompressed buffer for enabled callsites
- free chunks on site disable, or on memory pressure.

Whats actually done here is ths rather mechanical, and preparatory.

dynamic_debug.h:

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

DEFINE_DYNAMIC_DEBUG_METADATA now declares and initializes a 2nd
static struct var holding the _ddebug_site, and refs _ddebug to it.

dynamic_debug.c:

dynamic_debug_init() mem-usage now also counts sites.

dynamic_emit_prefix() & ddebug_change() use those moved fields; they
get a new initialized auto-var, and the field refs get adjusted as
needed to follow the field moves from one struct to the other.

struct _ddebug_site *dc = dp->site;

ddebug_proc_show() differs slightly; it assigns to (not initializes)
the autovar, to avoid a panic when p == SEQ_START_TOKEN.

vmlinux.lds.h:

1st, move the original set of __dyndbg___start|stop directives into a
new macro; this puts a name (and a block of blamelog) on this and
future changes. Also, since compressed elf sections are supported,
maybe we can use it on our "__dyndbg_sites" section; the macro better
isolates the items involved in such a move.

Then add in new "__dyndbg_sites" section and its start|stop symbols.

I kept the same align(8) and KEEP as used in the "__dyndbgs" section,
but I think this can be relaxed; I suspect the align is there for the
static-key member, which has state encoded in 2 LSBs, and _ddebug_site
doesnt have a key. That said, I dont think anything is gained by
changing.

Signed-off-by: Jim Cromie <jim.cromie@gmail.com>
---
include/asm-generic/vmlinux.lds.h | 22 +++++++++++-----
include/linux/dynamic_debug.h | 37 +++++++++++++++++---------
kernel/module.c | 2 +-
lib/dynamic_debug.c | 44 ++++++++++++++++++-------------
4 files changed, 66 insertions(+), 39 deletions(-)

diff --git a/include/asm-generic/vmlinux.lds.h b/include/asm-generic/vmlinux.lds.h
index 0331d5d49551..f4e861282ed7 100644
--- a/include/asm-generic/vmlinux.lds.h
+++ b/include/asm-generic/vmlinux.lds.h
@@ -335,6 +335,20 @@
KEEP(*(.dtb.init.rodata)) \
__dtb_end = .;

+/* implement dynamic printk debug */
+#if defined(CONFIG_DYNAMIC_DEBUG) || defined(CONFIG_DYNAMIC_DEBUG_CORE)
+#define DYNAMIC_DEBUG_DATA() \
+ . = ALIGN(8); \
+ __start___dyndbg_sites = .; \
+ KEEP(*(__dyndbg_sites)) \
+ __stop___dyndbg_sites = .; \
+ __start___dyndbg = .; \
+ KEEP(*(__dyndbgs)) \
+ __stop___dyndbg = .;
+#else
+#define DYNAMIC_DEBUG_DATA()
+#endif
+
/*
* .data section
*/
@@ -351,12 +365,8 @@
__end_once = .; \
STRUCT_ALIGN(); \
*(__tracepoints) \
- /* implement dynamic printk debug */ \
- . = ALIGN(8); \
- __start___dyndbg = .; \
- KEEP(*(__dyndbg)) \
- __stop___dyndbg = .; \
- LIKELY_PROFILE() \
+ DYNAMIC_DEBUG_DATA() \
+ LIKELY_PROFILE() \
BRANCH_PROFILE() \
TRACE_PRINTKS() \
BPF_RAW_TP() \
diff --git a/include/linux/dynamic_debug.h b/include/linux/dynamic_debug.h
index dce631e678dd..130d5b36e08e 100644
--- a/include/linux/dynamic_debug.h
+++ b/include/linux/dynamic_debug.h
@@ -7,20 +7,28 @@
#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_sites) for every dynamic debug callsite.
+ * At runtime, the sections are treated as arrays.
*/
-struct _ddebug {
+struct _ddebug;
+struct _ddebug_site {
/*
- * These fields are used to drive the user interface
- * for selecting and displaying debug callsites.
+ * These fields (and lineno) are used to:
+ * - decorate log messages per _ddebug.flags
+ * - select callsites for modification via >control
+ * - display callsites & settings in `cat control`
*/
const char *modname;
- const char *function;
const char *filename;
+ const char *function;
+} __aligned(8);
+
+struct _ddebug {
+ struct _ddebug_site *site;
+ /* format is always needed, lineno shares word with flags */
const char *format;
- unsigned int lineno:18;
+ const unsigned lineno:18;
/*
* The flags field controls the behaviour at the callsite.
* The bits here are changed dynamically when the user
@@ -49,8 +57,7 @@ struct _ddebug {
struct static_key_false dd_key_false;
} key;
#endif
-} __attribute__((aligned(8)));
-
+} __aligned(8);

#if defined(CONFIG_DYNAMIC_DEBUG_CORE)
@@ -88,11 +95,15 @@ 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_site __aligned(8) \
+ __section("__dyndbg_sites") name##_site = { \
.modname = KBUILD_MODNAME, \
- .function = __func__, \
.filename = __FILE__, \
+ .function = __func__, \
+ }; \
+ static struct _ddebug __aligned(8) \
+ __section("__dyndbgs") name = { \
+ .site = &name##_site, \
.format = (fmt), \
.lineno = __LINE__, \
.flags = _DPRINTK_FLAGS_DEFAULT, \
diff --git a/kernel/module.c b/kernel/module.c
index 30479355ab85..592e71f9f99d 100644
--- a/kernel/module.c
+++ b/kernel/module.c
@@ -3331,7 +3331,7 @@ static int find_module_sections(struct module *mod, struct load_info *info)
if (section_addr(info, "__obsparm"))
pr_warn("%s: Ignoring obsolete parameters\n", mod->name);

- info->debug = section_objs(info, "__dyndbg",
+ info->debug = section_objs(info, "__dyndbgs",
sizeof(*info->debug), &info->num_debug);

return 0;
diff --git a/lib/dynamic_debug.c b/lib/dynamic_debug.c
index ab8fc16911ae..691e43d58eef 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_site *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 */
@@ -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), dp->lineno,
+ dt->mod_name, dc->function,
ddebug_describe_flags(dp->flags, &fbuf));
}
}
@@ -586,12 +587,13 @@ 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_site *desc = dp->site;

- 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
@@ -599,15 +601,15 @@ 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);
+ dp->lineno);
if (pos - pos_after_tid)
pos += snprintf(buf + pos, remaining(pos), " ");
if (pos >= PREFIX_SIZE)
@@ -884,6 +886,7 @@ static int ddebug_proc_show(struct seq_file *m, void *p)
{
struct ddebug_iter *iter = m->private;
struct _ddebug *dp = p;
+ struct _ddebug_site *dc;
struct flagsbuf flags;

if (p == SEQ_START_TOKEN) {
@@ -892,9 +895,11 @@ static int ddebug_proc_show(struct seq_file *m, void *p)
return 0;
}

+ dc = dp->site;
+
seq_printf(m, "%s:%u [%s]%s =%s \"",
- trim_prefix(dp->filename), dp->lineno,
- iter->table->mod_name, dp->function,
+ trim_prefix(dc->filename), dp->lineno,
+ iter->table->mod_name, dc->function,
ddebug_describe_flags(dp->flags, &flags));
seq_escape(m, dp->format, "\t\r\n\"");
seq_puts(m, "\"\n");
@@ -1097,17 +1102,17 @@ 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++;
@@ -1117,9 +1122,10 @@ static int __init dynamic_debug_init(void)
goto out_err;

ddebug_init_success = 1;
- vpr_info("%d prdebugs in %d modules, %d KiB in ddebug tables, %d kiB in __dyndbg section\n",
+ vpr_info("%d prdebugs in %d modules, %d KiB in ddebug tables, %d KiB in __dyndbg section, %d KiB in __dyndbg_sites section\n",
entries, modct, (int)((modct * sizeof(struct ddebug_table)) >> 10),
- (int)((entries * sizeof(struct _ddebug)) >> 10));
+ (int)((entries * sizeof(struct _ddebug)) >> 10),
+ (int)((entries * sizeof(struct _ddebug_site)) >> 10));

/* apply ddebug_query boot param, dont unload tables on err */
if (ddebug_setup_string[0] != '\0') {
--
2.31.1