linux-kernel - [RFC PATCH v3 18/18] dyndbg: shuffle ddebug

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Date:   Mon, 15 Mar 2021 23:08:01 -0600
From:   Jim Cromie <jim.cromie@...il.com>
To:     jbaron@...mai.com, gregkh@...uxfoundation.org,
        linux-kernel@...r.kernel.org
Cc:     Jim Cromie <jim.cromie@...il.com>
Subject: [RFC PATCH v3 18/18] dyndbg: shuffle ddebug_table fields

In preparation to unionize structs _ddebug & ddebug_table, shuffle
fields in latter so they match the layout of the former.  This MAY
simplify initialization of the header field, in particular by
preserving *sites.

It also sets up a later conversion to a flex-array ddebugs[].

This step is mostly to isolate/prove no breakage before HEAD++

Signed-off-by: Jim Cromie <jim.cromie@...il.com>
---
 include/linux/dynamic_debug.h | 29 +++++++++++++++++++++++++++++
 lib/dynamic_debug.c           |  2 +-
 2 files changed, 30 insertions(+), 1 deletion(-)

diff --git a/include/linux/dynamic_debug.h b/include/linux/dynamic_debug.h
index 229cfd81ffb3..22f11218047f 100644
--- a/include/linux/dynamic_debug.h
+++ b/include/linux/dynamic_debug.h
@@ -66,6 +66,35 @@ struct _ddebug {
 #endif
 } __aligned(8);
 
+/* this pair of header structs correspond quite deeply to struct
+ * _ddebug(|_site)s above; they are also created into __dyndbg*
+ * sections (by DEFINE_DYNAMIC_DEBUG_TABLE), and should be unionized
+ * with them to reinforce this.
+ *
+ * struct _ddebug_header is the important one, it has enough space to
+ * take struct ddebug_table's job, ie: link together into a list, and
+ * keep track of the modname & _ddebug(|_sites) vectors.
+ *
+ * Its other job is handled by its placement in the front of a
+ * module's _ddebug[N] entries.  Each _ddebug knows its N, so the
+ * header's address is computable, and its site pointer is available
+ * to get _ddebug_sites[N].  Then we can drop _ddebug.sites, regaining
+ * parity with original _ddebug footprint.
+ *
+ * Eventually, N will index a fetch from a compressed block, or for
+ * enabled callsites, a hash.  A global hash is probably adequate, if
+ * ~5k elements doesnt degrade access time.
+ */
+struct _ddebug_site_header {
+	/* we have 24 bytes total here, all currently unused */
+} __aligned(8);
+
+struct _ddebug_header {
+	struct _ddebug_site* sites;
+	struct list_head link;
+	const char* mod_name;
+	unsigned num_ddebugs;
+} __aligned(8);
 
 #if defined(CONFIG_DYNAMIC_DEBUG_CORE)
 
diff --git a/lib/dynamic_debug.c b/lib/dynamic_debug.c
index c1a7345277eb..5d1ce7f21c30 100644
--- a/lib/dynamic_debug.c
+++ b/lib/dynamic_debug.c
@@ -45,11 +45,11 @@ extern struct _ddebug_site __start___dyndbg_sites[];
 extern struct _ddebug_site __stop___dyndbg_sites[];
 
 struct ddebug_table {
+	struct _ddebug_site *sites;
 	struct list_head link;
 	const char *mod_name;
 	unsigned int num_ddebugs;
 	struct _ddebug *ddebugs;
-	struct _ddebug_site *sites;
 };
 
 struct ddebug_query {
-- 
2.29.2