MDEV-33053 InnoDB LRU flushing does not run before running out of buffer pool

buf_flush_LRU(): Display a warning if no pages could be evicted and
no writes initiated.

buf_pool_t::need_LRU_eviction(): Renamed from buf_pool_t::ran_out().
Check if the amount of free pages is smaller than innodb_lru_scan_depth
instead of checking if it is 0.

buf_flush_page_cleaner(): For the final LRU flush after a checkpoint
flush, use a "budget" of innodb_io_capacity_max, like we do in the
case when we are not in "furious" checkpoint flushing.

Co-developed by: Debarun Banerjee
Reviewed by: Debarun Banerjee
Tested by: Matthias Leich

Author: dr-m

storage/innobase/buf/buf0flu.cc

@@ -1797,6 +1797,28 @@ ulint buf_flush_LRU(ulint max_n, bool evict)
     buf_pool.try_LRU_scan= true;
     pthread_cond_broadcast(&buf_pool.done_free);
   }
+  else if (!pages && !buf_pool.try_LRU_scan &&
+           buf_pool.LRU_warned.test_and_set(std::memory_order_acquire))
+  {
+    /* For example, with the minimum innodb_buffer_pool_size=5M and
+    the default innodb_page_size=16k there are only a little over 316
+    pages in the buffer pool. The buffer pool can easily be exhausted
+    by a workload of some dozen concurrent connections. The system could
+    reach a deadlock like the following:
+
+    (1) Many threads are waiting in buf_LRU_get_free_block()
+    for buf_pool.done_free.
+    (2) Some threads are waiting for a page latch which is held by
+    another thread that is waiting in buf_LRU_get_free_block().
+    (3) This thread is the only one that could make progress, but
+    we fail to do so because all the pages that we scanned are
+    buffer-fixed or latched by some thread. */
+    sql_print_warning("InnoDB: Could not free any blocks in the buffer pool!"
+                      " %zu blocks are in use and %zu free."
+                      " Consider increasing innodb_buffer_pool_size.",
+                      UT_LIST_GET_LEN(buf_pool.LRU),
+                      UT_LIST_GET_LEN(buf_pool.free));
+  }
 
   return pages;
 }
@@ -2287,6 +2309,16 @@ static ulint page_cleaner_flush_pages_recommendation(ulint last_pages_in,
 	goto func_exit;
 }
 
+TPOOL_SUPPRESS_TSAN
+bool buf_pool_t::need_LRU_eviction() const
+{
+  /* try_LRU_scan==false means that buf_LRU_get_free_block() is waiting
+  for buf_flush_page_cleaner() to evict some blocks */
+  return UNIV_UNLIKELY(!try_LRU_scan ||
+                       (UT_LIST_GET_LEN(LRU) > BUF_LRU_MIN_LEN &&
+                        UT_LIST_GET_LEN(free) < srv_LRU_scan_depth / 2));
+}
+
 /** page_cleaner thread tasked with flushing dirty pages from the buffer
 pools. As of now we'll have only one coordinator. */
 static void buf_flush_page_cleaner()
@@ -2319,21 +2351,24 @@ static void buf_flush_page_cleaner()
     }
 
     mysql_mutex_lock(&buf_pool.flush_list_mutex);
-    if (buf_pool.ran_out())
-      goto no_wait;
-    else if (srv_shutdown_state > SRV_SHUTDOWN_INITIATED)
-      break;
+    if (!buf_pool.need_LRU_eviction())
+    {
+      if (srv_shutdown_state > SRV_SHUTDOWN_INITIATED)
+        break;
 
-    if (buf_pool.page_cleaner_idle() &&
-        (!UT_LIST_GET_LEN(buf_pool.flush_list) ||
-         srv_max_dirty_pages_pct_lwm == 0.0))
-      /* We are idle; wait for buf_pool.page_cleaner_wakeup() */
-      my_cond_wait(&buf_pool.do_flush_list,
-                   &buf_pool.flush_list_mutex.m_mutex);
-    else
-      my_cond_timedwait(&buf_pool.do_flush_list,
-                        &buf_pool.flush_list_mutex.m_mutex, &abstime);
-  no_wait:
+      if (buf_pool.page_cleaner_idle() &&
+          (!UT_LIST_GET_LEN(buf_pool.flush_list) ||
+           srv_max_dirty_pages_pct_lwm == 0.0))
+      {
+        buf_pool.LRU_warned.clear(std::memory_order_release);
+        /* We are idle; wait for buf_pool.page_cleaner_wakeup() */
+        my_cond_wait(&buf_pool.do_flush_list,
+                     &buf_pool.flush_list_mutex.m_mutex);
+      }
+      else
+        my_cond_timedwait(&buf_pool.do_flush_list,
+                          &buf_pool.flush_list_mutex.m_mutex, &abstime);
+    }
     set_timespec(abstime, 1);
 
     lsn_limit= buf_flush_sync_lsn;
@@ -2365,7 +2400,7 @@ static void buf_flush_page_cleaner()
       }
       while (false);
 
-      if (!buf_pool.ran_out())
+      if (!buf_pool.need_LRU_eviction())
         continue;
       mysql_mutex_lock(&buf_pool.flush_list_mutex);
       oldest_lsn= buf_pool.get_oldest_modification(0);
@@ -2394,7 +2429,7 @@ static void buf_flush_page_cleaner()
       if (oldest_lsn >= soft_lsn_limit)
         buf_flush_async_lsn= soft_lsn_limit= 0;
     }
-    else if (buf_pool.ran_out())
+    else if (buf_pool.need_LRU_eviction())
     {
       buf_pool.page_cleaner_set_idle(false);
       buf_pool.n_flush_inc();
@@ -2509,9 +2544,11 @@ static void buf_flush_page_cleaner()
                                    MONITOR_FLUSH_ADAPTIVE_PAGES,
                                    n_flushed);
     }
-    else if (buf_flush_async_lsn <= oldest_lsn)
+    else if (buf_flush_async_lsn <= oldest_lsn &&
+             !buf_pool.need_LRU_eviction())
       goto check_oldest_and_set_idle;
 
+    n= srv_max_io_capacity;
     n= n >= n_flushed ? n - n_flushed : 0;
     goto LRU_flush;
   }

storage/innobase/buf/buf0lru.cc

@@ -60,10 +60,6 @@ static constexpr ulint BUF_LRU_OLD_TOLERANCE = 20;
 frames in the buffer pool, we set this to TRUE */
 static bool buf_lru_switched_on_innodb_mon = false;
 
-/** True if diagnostic message about difficult to find free blocks
-in the buffer bool has already printed. */
-static bool	buf_lru_free_blocks_error_printed;
-
 /******************************************************************//**
 These statistics are not 'of' LRU but 'for' LRU.  We keep count of I/O
 and page_zip_decompress() operations.  Based on the statistics,
@@ -408,6 +404,7 @@ buf_block_t *buf_LRU_get_free_block(bool have_mutex)
 	buf_LRU_check_size_of_non_data_objects();
 	buf_block_t* block;
 
+	IF_DBUG(static bool buf_lru_free_blocks_error_printed,);
 	DBUG_EXECUTE_IF("ib_lru_force_no_free_page",
 		if (!buf_lru_free_blocks_error_printed) {
 			n_iterations = 21;
@@ -417,9 +414,25 @@ buf_block_t *buf_LRU_get_free_block(bool have_mutex)
 	/* If there is a block in the free list, take it */
 	if ((block = buf_LRU_get_free_only()) != nullptr) {
 got_block:
+		const ulint LRU_size = UT_LIST_GET_LEN(buf_pool.LRU);
+		const ulint available = UT_LIST_GET_LEN(buf_pool.free);
+		const ulint scan_depth = srv_LRU_scan_depth / 2;
+		ut_ad(LRU_size <= BUF_LRU_MIN_LEN || available >= scan_depth
+		      || buf_pool.need_LRU_eviction());
+
 		if (!have_mutex) {
 			mysql_mutex_unlock(&buf_pool.mutex);
 		}
+
+		if (UNIV_UNLIKELY(available < scan_depth)
+		    && LRU_size > BUF_LRU_MIN_LEN) {
+			mysql_mutex_lock(&buf_pool.flush_list_mutex);
+			if (!buf_pool.page_cleaner_active()) {
+				buf_pool.page_cleaner_wakeup(true);
+			}
+			mysql_mutex_unlock(&buf_pool.flush_list_mutex);
+		}
+
 		block->page.zip.clear();
 		return block;
 	}
@@ -445,10 +458,11 @@ buf_block_t *buf_LRU_get_free_block(bool have_mutex)
 		if ((block = buf_LRU_get_free_only()) != nullptr) {
 			goto got_block;
 		}
+		const bool wake = buf_pool.need_LRU_eviction();
 		mysql_mutex_unlock(&buf_pool.mutex);
 		mysql_mutex_lock(&buf_pool.flush_list_mutex);
 		const auto n_flush = buf_pool.n_flush();
-		if (!buf_pool.try_LRU_scan) {
+		if (wake && !buf_pool.page_cleaner_active()) {
 			buf_pool.page_cleaner_wakeup(true);
 		}
 		mysql_mutex_unlock(&buf_pool.flush_list_mutex);
@@ -467,9 +481,10 @@ buf_block_t *buf_LRU_get_free_block(bool have_mutex)
 		MONITOR_INC( MONITOR_LRU_GET_FREE_WAITS );
 	}
 
-	if (n_iterations == 21 && !buf_lru_free_blocks_error_printed
-	    && srv_buf_pool_old_size == srv_buf_pool_size) {
-		buf_lru_free_blocks_error_printed = true;
+	if (n_iterations == 21
+	    && srv_buf_pool_old_size == srv_buf_pool_size
+	    && buf_pool.LRU_warned.test_and_set(std::memory_order_acquire)) {
+		IF_DBUG(buf_lru_free_blocks_error_printed = true,);
 		mysql_mutex_unlock(&buf_pool.mutex);
 		ib::warn() << "Difficult to find free blocks in the buffer pool"
 			" (" << n_iterations << " search iterations)! "

storage/innobase/include/buf0buf.h

@@ -1488,10 +1488,8 @@ class buf_pool_t
         n_chunks_new / 4 * chunks->size;
   }
 
-  /** @return whether the buffer pool has run out */
-  TPOOL_SUPPRESS_TSAN
-  bool ran_out() const
-  { return UNIV_UNLIKELY(!try_LRU_scan || !UT_LIST_GET_LEN(free)); }
+  /** @return whether the buffer pool is running low */
+  bool need_LRU_eviction() const;
 
   /** @return whether the buffer pool is shrinking */
   inline bool is_shrinking() const
@@ -1811,6 +1809,9 @@ class buf_pool_t
   Set whenever the free list grows, along with a broadcast of done_free.
   Protected by buf_pool.mutex. */
   Atomic_relaxed<bool> try_LRU_scan;
+  /** Whether we have warned to be running out of buffer pool */
+  std::atomic_flag LRU_warned;
+
 	/* @} */
 
 	/** @name LRU replacement algorithm fields */