fscache: Add a tracepoint for cookie use/unuse

Add a tracepoint to track fscache_use/unuse_cookie().

Signed-off-by: David Howells <dhowells@redhat.com>
Reviewed-by: Jeff Layton <jlayton@kernel.org>
cc: linux-cachefs@redhat.com
Link: https://lore.kernel.org/r/164021588628.640689.12942919367404043608.stgit@warthog.procyon.org.uk/ # v4

Author: dhowells

fs/fscache/cookie.c

@@ -556,6 +556,7 @@ void __fscache_use_cookie(struct fscache_cookie *cookie, bool will_modify)
 {
 	enum fscache_cookie_state state;
 	bool queue = false;
+	int n_active;
 
 	_enter("c=%08x", cookie->debug_id);
 
@@ -565,7 +566,11 @@ void __fscache_use_cookie(struct fscache_cookie *cookie, bool will_modify)
 
 	spin_lock(&cookie->lock);
 
-	atomic_inc(&cookie->n_active);
+	n_active = atomic_inc_return(&cookie->n_active);
+	trace_fscache_active(cookie->debug_id, refcount_read(&cookie->ref),
+			     n_active, atomic_read(&cookie->n_accesses),
+			     will_modify ?
+			     fscache_active_use_modify : fscache_active_use);
 
 again:
 	state = fscache_cookie_state(cookie);
@@ -638,13 +643,29 @@ static void fscache_unuse_cookie_locked(struct fscache_cookie *cookie)
 void __fscache_unuse_cookie(struct fscache_cookie *cookie,
 			    const void *aux_data, const loff_t *object_size)
 {
+	unsigned int debug_id = cookie->debug_id;
+	unsigned int r = refcount_read(&cookie->ref);
+	unsigned int a = atomic_read(&cookie->n_accesses);
+	unsigned int c;
+
 	if (aux_data || object_size)
 		__fscache_update_cookie(cookie, aux_data, object_size);
 
-	if (atomic_dec_and_lock(&cookie->n_active, &cookie->lock)) {
-		fscache_unuse_cookie_locked(cookie);
-		spin_unlock(&cookie->lock);
+	/* Subtract 1 from counter unless that drops it to 0 (ie. it was 1) */
+	c = atomic_fetch_add_unless(&cookie->n_active, -1, 1);
+	if (c != 1) {
+		trace_fscache_active(debug_id, r, c - 1, a, fscache_active_unuse);
+		return;
 	}
+
+	spin_lock(&cookie->lock);
+	r = refcount_read(&cookie->ref);
+	a = atomic_read(&cookie->n_accesses);
+	c = atomic_dec_return(&cookie->n_active);
+	trace_fscache_active(debug_id, r, c, a, fscache_active_unuse);
+	if (c == 0)
+		fscache_unuse_cookie_locked(cookie);
+	spin_unlock(&cookie->lock);
 }
 EXPORT_SYMBOL(__fscache_unuse_cookie);
 

include/trace/events/fscache.h

@@ -71,6 +71,12 @@ enum fscache_cookie_trace {
 	fscache_cookie_see_work,
 };
 
+enum fscache_active_trace {
+	fscache_active_use,
+	fscache_active_use_modify,
+	fscache_active_unuse,
+};
+
 enum fscache_access_trace {
 	fscache_access_acquire_volume,
 	fscache_access_acquire_volume_end,
@@ -146,6 +152,11 @@ enum fscache_access_trace {
 	EM(fscache_cookie_see_withdraw,		"-   x-wth")		\
 	E_(fscache_cookie_see_work,		"-   work ")
 
+#define fscache_active_traces		\
+	EM(fscache_active_use,			"USE          ")	\
+	EM(fscache_active_use_modify,		"USE-m        ")	\
+	E_(fscache_active_unuse,		"UNUSE        ")
+
 #define fscache_access_traces		\
 	EM(fscache_access_acquire_volume,	"BEGIN acq_vol")	\
 	EM(fscache_access_acquire_volume_end,	"END   acq_vol")	\
@@ -264,6 +275,39 @@ TRACE_EVENT(fscache_cookie,
 		      __entry->ref)
 	    );
 
+TRACE_EVENT(fscache_active,
+	    TP_PROTO(unsigned int cookie_debug_id,
+		     int ref,
+		     int n_active,
+		     int n_accesses,
+		     enum fscache_active_trace why),
+
+	    TP_ARGS(cookie_debug_id, ref, n_active, n_accesses, why),
+
+	    TP_STRUCT__entry(
+		    __field(unsigned int,		cookie		)
+		    __field(int,			ref		)
+		    __field(int,			n_active	)
+		    __field(int,			n_accesses	)
+		    __field(enum fscache_active_trace,	why		)
+			     ),
+
+	    TP_fast_assign(
+		    __entry->cookie	= cookie_debug_id;
+		    __entry->ref	= ref;
+		    __entry->n_active	= n_active;
+		    __entry->n_accesses	= n_accesses;
+		    __entry->why	= why;
+			   ),
+
+	    TP_printk("c=%08x %s r=%d a=%d c=%d",
+		      __entry->cookie,
+		      __print_symbolic(__entry->why, fscache_active_traces),
+		      __entry->ref,
+		      __entry->n_accesses,
+		      __entry->n_active)
+	    );
+
 TRACE_EVENT(fscache_access_cache,
 	    TP_PROTO(unsigned int cache_debug_id,
 		     int ref,