qsp: add qsp_reset

I first implemented this by deleting all entries in the global
hash table. But doing that safely slows down profiling, since
we'd need to introduce rcu_read_lock/unlock in the fast path.

What's implemented here avoids messing with the thread-local
data in the global hash table. It achieves this by taking a snapshot
of the current state, so that subsequent reports present the delta
wrt to the snapshot.

Signed-off-by: Emilio G. Cota <cota@braap.org>
Signed-off-by: Paolo Bonzini <pbonzini@redhat.com>

include/qemu/qsp.h

@@ -24,5 +24,6 @@ void qsp_report(FILE *f, fprintf_function cpu_fprintf, size_t max,
 bool qsp_is_enabled(void);
 void qsp_enable(void);
 void qsp_disable(void);
+void qsp_reset(void);
 
 #endif /* QEMU_QSP_H */

util/qsp.c

@@ -47,6 +47,11 @@
  *   an intermediate hash table. This would simplify the code only slightly, but
  *   would perform badly if there were many threads and objects to track.
  *
+ * - Wrap operations on qsp entries with RCU read-side critical sections, so
+ *   that qsp_reset() can delete entries. Unfortunately, the overhead of calling
+ *   rcu_read_lock/unlock slows down atomic_add-bench -m by 24%. Having
+ *   a snapshot that is updated on qsp_reset() avoids this overhead.
+ *
  * Related Work:
  * - Lennart Poettering's mutrace: http://0pointer.de/blog/projects/mutrace.html
  * - Lozi, David, Thomas, Lawall and Muller. "Remote Core Locking: Migrating
@@ -57,6 +62,7 @@
 #include "qemu/thread.h"
 #include "qemu/timer.h"
 #include "qemu/qht.h"
+#include "qemu/rcu.h"
 #include "exec/tb-hash-xx.h"
 
 enum QSPType {
@@ -88,6 +94,12 @@ struct QSPEntry {
 };
 typedef struct QSPEntry QSPEntry;
 
+struct QSPSnapshot {
+    struct rcu_head rcu;
+    struct qht ht;
+};
+typedef struct QSPSnapshot QSPSnapshot;
+
 /* initial sizing for hash tables */
 #define QSP_INITIAL_SIZE 64
 
@@ -107,6 +119,7 @@ static __thread int qsp_thread;
 static struct qht qsp_callsite_ht;
 
 static struct qht qsp_ht;
+static QSPSnapshot *qsp_snapshot;
 static bool qsp_initialized, qsp_initializing;
 
 static const char * const qsp_typenames[] = {
@@ -505,15 +518,69 @@ static void qsp_aggregate(struct qht *global_ht, void *p, uint32_t h, void *up)
     qsp_entry_aggregate(agg, e);
 }
 
+static void qsp_iter_diff(struct qht *orig, void *p, uint32_t hash, void *htp)
+{
+    struct qht *ht = htp;
+    QSPEntry *old = p;
+    QSPEntry *new;
+
+    new = qht_lookup(ht, old, hash);
+    /* entries are never deleted, so we must have this one */
+    g_assert(new != NULL);
+    /* our reading of the stats happened after the snapshot was taken */
+    g_assert(new->n_acqs >= old->n_acqs);
+    g_assert(new->ns >= old->ns);
+
+    new->n_acqs -= old->n_acqs;
+    new->ns -= old->ns;
+
+    /* No point in reporting an empty entry */
+    if (new->n_acqs == 0 && new->ns == 0) {
+        bool removed = qht_remove(ht, new, hash);
+
+        g_assert(removed);
+        g_free(new);
+    }
+}
+
+static void qsp_diff(struct qht *orig, struct qht *new)
+{
+    qht_iter(orig, qsp_iter_diff, new);
+}
+
+static void qsp_ht_delete(struct qht *ht, void *p, uint32_t h, void *htp)
+{
+    g_free(p);
+}
+
 static void qsp_mktree(GTree *tree)
 {
+    QSPSnapshot *snap;
     struct qht ht;
 
+    /*
+     * First, see if there's a prior snapshot, so that we read the global hash
+     * table _after_ the snapshot has been created, which guarantees that
+     * the entries we'll read will be a superset of the snapshot's entries.
+     *
+     * We must remain in an RCU read-side critical section until we're done
+     * with the snapshot.
+     */
+    rcu_read_lock();
+    snap = atomic_rcu_read(&qsp_snapshot);
+
     /* Aggregate all results from the global hash table into a local one */
     qht_init(&ht, qsp_entry_no_thread_cmp, QSP_INITIAL_SIZE,
              QHT_MODE_AUTO_RESIZE | QHT_MODE_RAW_MUTEXES);
     qht_iter(&qsp_ht, qsp_aggregate, &ht);
 
+    /* compute the difference wrt the snapshot, if any */
+    if (snap) {
+        qsp_diff(&snap->ht, &ht);
+    }
+    /* done with the snapshot; RCU can reclaim it */
+    rcu_read_unlock();
+
     /* sort the hash table elements by using a tree */
     qht_iter(&ht, qsp_sort, tree);
 
@@ -652,3 +719,30 @@ void qsp_report(FILE *f, fprintf_function cpu_fprintf, size_t max,
     pr_report(&rep, f, cpu_fprintf);
     report_destroy(&rep);
 }
+
+static void qsp_snapshot_destroy(QSPSnapshot *snap)
+{
+    qht_iter(&snap->ht, qsp_ht_delete, NULL);
+    qht_destroy(&snap->ht);
+    g_free(snap);
+}
+
+void qsp_reset(void)
+{
+    QSPSnapshot *new = g_new(QSPSnapshot, 1);
+    QSPSnapshot *old;
+
+    qsp_init();
+
+    qht_init(&new->ht, qsp_entry_cmp, QSP_INITIAL_SIZE,
+             QHT_MODE_AUTO_RESIZE | QHT_MODE_RAW_MUTEXES);
+
+    /* take a snapshot of the current state */
+    qht_iter(&qsp_ht, qsp_aggregate, &new->ht);
+
+    /* replace the previous snapshot, if any */
+    old = atomic_xchg(&qsp_snapshot, new);
+    if (old) {
+        call_rcu(old, qsp_snapshot_destroy, rcu);
+    }
+}