[RFC PATCH v3 12/18] dyndbg: allow deleting site info via control interface

[Jim Cromie <jim.cromie@gmail.com>]

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 aa4a3f5d8d35..d811273c8484 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_ANYSITE		\
 	(_DPRINTK_FLAGS_INCL_MODNAME		\
diff --git a/lib/dynamic_debug.c b/lib/dynamic_debug.c
index 6fbd9099c2fa..fdc2d0e15731 100644
--- a/lib/dynamic_debug.c
+++ b/lib/dynamic_debug.c
@@ -92,6 +92,7 @@ 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]; };
@@ -201,6 +202,14 @@ 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;
+		}
+	}
 }
 
 /*
-- 
2.29.2