[RFC PATCH v6 14/34] dyndbg: allow deleting site info via control interface

Jim Cromie
2021-05-29T20:02:52Z

Allow users & subsystems to selectively delete callsite info for
pr-debug callsites. Hopefully, this can lead to actual recovery of
memory.

DRM is a potential user which would drop the sites:

- has distinct categories for logging, and can map them over to a
format prefix, like: "drm:core:", "drm:kms:", etc.

- are happy with group control of all the callsites in a class/cateory.
individual control is still possible using queries including line numbers

- don't need dynamic "module:function:line:" prefixes in log messages

- don't care about loss of context in /proc/dynamic_debug/control

before:

init/initramfs.c:485 [initramfs]unpack_to_rootfs =_ "Detected %s compressed data\012"
init/main.c:1337 [main]run_init_process =pm " %s\012"
init/main.c:1335 [main]run_init_process =pm " with environment:\012"
init/main.c:1334 [main]run_init_process =pm " %s\012"
init/main.c:1332 [main]run_init_process =pm " with arguments:\012"
init/main.c:1121 [main]initcall_blacklisted =pm "initcall %s blacklisted\012"
init/main.c:1082 [main]initcall_blacklist =pm "blacklisting initcall %s\012"

then:
bash-5.0# echo file init/main.c +D > /proc/dynamic_debug/control

after:

init/initramfs.c:485 [initramfs]unpack_to_rootfs =_ "Detected %s compressed data\012"
[main]:1337 =pmD " %s\012"
[main]:1335 =pmD " with environment:\012"
[main]:1334 =pmD " %s\012"
[main]:1332 =pmD " with arguments:\012"
[main]:1121 =pmD "initcall %s blacklisted\012"
[main]:1082 =pmD "blacklisting initcall %s\012"

Notes:

If Drm adopted dyndbg, i915 + drm* would add ~1600 prdebugs, amdgpu +
drm* would add ~3200 callsites, so the additional memory costs are
substantial. In trade, drm and drivers would avoid lots of calls to
drm_debug_enabled(). This patch should reduce the costs.

Using this interface, drm could drop site info for all categories /
prefixes controlled by bits in drm.debug, while preserving site info
and individual selectivity for any uncategorized prdebugs, and for all
other modules.

Lastly, because lineno field was not moved into _ddebug_callsite, it
can be used to modify a single[*] callsite even if drm has dropped all
the callsite data:

echo module $mod format ^$prefix line $line +p >control

Dropping site info is a one-way, information losing operation, so
minor misuse is possible. Worst case is maybe (depending upon
previous settings) some loss of logging context/decorations.

echo +D > /proc/dynamic_debug/control

[*] amdgpu has some macros invoking clusters of pr_debugs; each use of
them creates a cluster of pr-debugs with the same line number.

Signed-off-by: Jim Cromie <jim.cromie@gmail.com>
---
include/linux/dynamic_debug.h | 1 +
lib/dynamic_debug.c | 9 +++++++++
2 files changed, 10 insertions(+)

diff --git a/include/linux/dynamic_debug.h b/include/linux/dynamic_debug.h
index 40ea086853ff..f789608ab935 100644
--- a/include/linux/dynamic_debug.h
+++ b/include/linux/dynamic_debug.h
@@ -40,6 +40,7 @@ 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_DELETE_SITE (1<<7) /* drop site info to save ram */

#define _DPRINTK_FLAGS_INCL_ANY \
(_DPRINTK_FLAGS_INCL_MODNAME | _DPRINTK_FLAGS_INCL_FUNCNAME |\
diff --git a/lib/dynamic_debug.c b/lib/dynamic_debug.c
index ad7cda840733..a4cb048357fb 100644
--- a/lib/dynamic_debug.c
+++ b/lib/dynamic_debug.c
@@ -93,6 +93,7 @@ static struct { unsigned flag:8; char opt_char; } opt_array[] = {
{ _DPRINTK_FLAGS_INCL_LINENO, 'l' },
{ _DPRINTK_FLAGS_INCL_TID, 't' },
{ _DPRINTK_FLAGS_NONE, '_' },
+ { _DPRINTK_FLAGS_DELETE_SITE, 'D' },
};

struct flagsbuf { char buf[ARRAY_SIZE(opt_array)+1]; };
@@ -202,6 +203,14 @@ static void ddebug_alter_site(struct _ddebug *dp,
} else if (modifiers->flags & _DPRINTK_FLAGS_PRINT)
static_branch_enable(&dp->key.dd_key_true);
#endif
+ /* delete site info for this callsite */
+ if (modifiers->flags & _DPRINTK_FLAGS_DELETE_SITE) {
+ if (dp->site) {
+ vpr_info("dropping site info %s.%s.%d\n", dp->site->filename,
+ dp->site->function, dp->lineno);
+ dp->site = NULL;
+ }
+ }
}

/*
--
2.31.1