mds: do not duplicate journaler write heads

The MDS can unnecessarily write out the journaler head multiple times. Check
the last_written Header to see if it's necessary to write and gather waiters.

Signed-off-by: Patrick Donnelly <[email protected]>

Author: batrick

src/mds/MDLog.cc

@@ -146,13 +146,63 @@ class C_MDL_WriteError : public MDSIOContextBase {
 };
 
 
+class C_MDL_WriteHead : public MDSIOContextBase {
+public:
+  explicit C_MDL_WriteHead(MDLog* m)
+    : MDSIOContextBase(true)
+    , mdlog(m)
+    {}
+  void print(ostream& out) const override {
+    out << "mdlog_write_head";
+  }
+protected:
+  void finish(int r) override {
+    mdlog->finish_head_waiters();
+  }
+  MDSRank *get_mds() override {return mdlog->mds;}
+
+  MDLog *mdlog;
+};
+
+void MDLog::finish_head_waiters()
+{
+  ceph_assert(ceph_mutex_is_locked_by_me(mds->mds_lock));
+
+  auto&& last_committed = journaler->get_last_committed();
+  auto& expire_pos = last_committed.expire_pos;
+
+  dout(20) << __func__ << " expire_pos=" << std::hex << expire_pos << dendl;
+
+  {
+    auto last = waiting_for_expire.upper_bound(expire_pos);
+    for (auto it = waiting_for_expire.begin(); it != last; it++) {
+      finish_contexts(g_ceph_context, it->second);
+    }
+    waiting_for_expire.erase(waiting_for_expire.begin(), last);
+  }
+}
+
 void MDLog::write_head(MDSContext *c) 
 {
-  Context *fin = NULL;
-  if (c != NULL) {
-    fin = new C_IO_Wrapper(mds, c);
+  ceph_assert(ceph_mutex_is_locked_by_me(mds->mds_lock));
+
+  auto&& last_written = journaler->get_last_written();
+  auto expire_pos = journaler->get_expire_pos();
+  dout(10) << __func__ << " last_written=" << last_written << " current expire_pos=" << std::hex << expire_pos << dendl;
+
+  if (last_written.expire_pos < expire_pos) {
+    if (c != NULL) {
+      dout(25) << __func__ << " queueing waiter " << c << dendl;
+      waiting_for_expire[expire_pos].push_back(c);
+    }
+
+    auto* fin = new C_MDL_WriteHead(this);
+    journaler->write_head(fin);
+  } else {
+    if (c) {
+      c->complete(0);
+    }
   }
-  journaler->write_head(fin);
 }
 
 uint64_t MDLog::get_read_pos() const
@@ -174,6 +224,8 @@ uint64_t MDLog::get_safe_pos() const
 
 void MDLog::create(MDSContext *c)
 {
+  ceph_assert(ceph_mutex_is_locked_by_me(mds->mds_lock));
+
   dout(5) << "create empty log" << dendl;
 
   C_GatherBuilder gather(g_ceph_context);
@@ -845,7 +897,6 @@ void MDLog::_trim_expired_segments(auto& locker, MDSContext* ctx)
   ceph_assert(locker.owns_lock());
 
   // trim expired segments?
-  bool trimmed = false;
   uint64_t end = 0;
   for (auto it = segments.begin(); it != segments.end(); ++it) {
     auto& [seq, ls] = *it;
@@ -881,7 +932,6 @@ void MDLog::_trim_expired_segments(auto& locker, MDSContext* ctx)
       } else {
         logger->set(l_mdl_expos, jexpire_pos);
       }
-      trimmed = true;
     }
 
     if (!expired_segments.count(ls)) {
@@ -895,13 +945,7 @@ void MDLog::_trim_expired_segments(auto& locker, MDSContext* ctx)
 
   locker.unlock();
 
-  if (trimmed) {
-    write_head(ctx);
-  } else {
-    if (ctx) {
-      ctx->complete(0);
-    }
-  }
+  write_head(ctx);
 }
 
 void MDLog::_expired(LogSegment *ls)

src/mds/MDLog.h

@@ -133,6 +133,8 @@ class MDLog {
   void kick_submitter();
   void shutdown();
 
+  void finish_head_waiters();
+
   void submit_entry(LogEvent *e, MDSLogContextBase* c = 0) {
     std::lock_guard l(submit_mutex);
     _submit_entry(e, c);
@@ -320,5 +322,7 @@ class MDLog {
   // guarded by mds_lock
   std::condition_variable_any cond;
   std::atomic<bool> upkeep_log_trim_shutdown{false};
+
+  std::map<uint64_t, std::vector<Context*>> waiting_for_expire; // protected by mds_lock
 };
 #endif