MDEV-22850 Reduce buf_pool.page_hash latch contention

For reads, the buf_pool.page_hash is protected by buf_pool.mutex or
by the hash_lock. There is no need to compute or acquire hash_lock
if we are not modifying the buf_pool.page_hash.

However, the buf_pool.page_hash latch must be held exclusively
when changing buf_page_t::in_file(), or if we desire to prevent
buf_page_t::can_relocate() or buf_page_t::buf_fix_count()
from changing.

rw_lock_lock_word_decr(): Add a comment that explains the polling logic.

buf_page_t::set_state(): When in_file() is to be changed, assert that
an exclusive buf_pool.page_hash latch is being held. Unfortunately
we cannot assert this for set_state(BUF_BLOCK_REMOVE_HASH) because
set_corrupt_id() may already have been called.

buf_LRU_free_page(): Check buf_page_t::can_relocate() before
aqcuiring the hash_lock.

buf_block_t::initialise(): Initialize also page.buf_fix_count().

buf_page_create(): Initialize buf_fix_count while not holding
any mutex or hash_lock. Acquire the hash_lock only for the
duration of inserting the block to the buf_pool.page_hash.

buf_LRU_old_init(), buf_LRU_add_block(),
buf_page_t::belongs_to_unzip_LRU(): Do not assert buf_page_t::in_file(),
because buf_page_create() will invoke buf_LRU_add_block()
before acquiring hash_lock and buf_page_t::set_state().

buf_pool_t::validate(): Rely on the buf_pool.mutex and do not
unnecessarily acquire any buf_pool.page_hash latches.

buf_page_init_for_read(): Clarify that we must acquire the hash_lock
upfront in order to prevent a race with buf_pool_t::watch_remove().

storage/innobase/buf/buf0buf.cc

@@ -3776,15 +3776,16 @@ buf_page_try_get_func(
 }
 
 /** Initialize the block.
-@param page_id page id
-@param zip_size ROW_FORMAT=COMPRESSED page size, or 0 */
-void buf_block_t::initialise(const page_id_t page_id, ulint zip_size)
+@param page_id  page identifier
+@param zip_size ROW_FORMAT=COMPRESSED page size, or 0
+@param fix      initial buf_fix_count() */
+void buf_block_t::initialise(const page_id_t page_id, ulint zip_size,
+                             uint32_t fix)
 {
   ut_ad(page.state() != BUF_BLOCK_FILE_PAGE);
   buf_block_init_low(this);
   lock_hash_val= lock_rec_hash(page_id.space(), page_id.page_no());
-  page.init();
-  page.id_= page_id;
+  page.init(page_id, fix);
   page_zip_set_size(&page.zip, zip_size);
 }
 
@@ -3803,11 +3804,9 @@ buf_page_create(const page_id_t page_id, ulint zip_size, mtr_t *mtr)
   ut_ad(page_id.space() != 0 || !zip_size);
 
   buf_block_t *free_block= buf_LRU_get_free_block(false);
-  free_block->initialise(page_id, zip_size);
+  free_block->initialise(page_id, zip_size, 1);
 
-  rw_lock_t *hash_lock= buf_pool.hash_lock_get(page_id);
   mutex_enter(&buf_pool.mutex);
-  rw_lock_x_lock(hash_lock);
 
   buf_block_t *block= reinterpret_cast<buf_block_t*>
     (buf_pool.page_hash_get_low(page_id));
@@ -3816,7 +3815,6 @@ buf_page_create(const page_id_t page_id, ulint zip_size, mtr_t *mtr)
       !buf_pool.watch_is_sentinel(block->page))
   {
     /* Page can be found in buf_pool */
-    rw_lock_x_unlock(hash_lock);
     buf_LRU_block_free_non_file_page(free_block);
     mutex_exit(&buf_pool.mutex);
 
@@ -3846,11 +3844,17 @@ buf_page_create(const page_id_t page_id, ulint zip_size, mtr_t *mtr)
                         page_id.space(), page_id.page_no()));
 
   block= free_block;
-  buf_block_buf_fix_inc(block, __FILE__, __LINE__);
+
+  /* Duplicate buf_block_buf_fix_inc_func() */
+  ut_ad(block->page.buf_fix_count() == 1);
+  ut_ad(fsp_is_system_temporary(page_id.space()) ||
+        rw_lock_s_lock_nowait(block->debug_latch, __FILE__, __LINE__));
 
   /* The block must be put to the LRU list */
-  block->page.set_state(BUF_BLOCK_FILE_PAGE);
   buf_LRU_add_block(&block->page, false);
+  rw_lock_t *hash_lock= buf_pool.hash_lock_get(page_id);
+  rw_lock_x_lock(hash_lock);
+  block->page.set_state(BUF_BLOCK_FILE_PAGE);
   ut_d(block->page.in_page_hash= true);
   HASH_INSERT(buf_page_t, hash, buf_pool.page_hash, page_id.fold(),
               &block->page);
@@ -4401,7 +4405,6 @@ void buf_pool_t::validate()
 	ulint		n_zip		= 0;
 
 	mutex_enter(&mutex);
-	page_hash_lock_all();
 
 	chunk_t* chunk = chunks;
 
@@ -4492,7 +4495,6 @@ void buf_pool_t::validate()
 
 	ut_ad(UT_LIST_GET_LEN(flush_list) == n_flushing);
 
-	page_hash_unlock_all();
 	mutex_exit(&flush_list_mutex);
 
 	if (curr_size == old_size

storage/innobase/buf/buf0lru.cc

@@ -940,7 +940,6 @@ static void buf_LRU_old_init()
 	     bpage = UT_LIST_GET_PREV(LRU, bpage)) {
 
 		ut_ad(bpage->in_LRU_list);
-		ut_ad(bpage->in_file());
 
 		/* This loop temporarily violates the
 		assertions of buf_page_t::set_old(). */
@@ -1066,7 +1065,6 @@ buf_LRU_add_block(
 				the start, regardless of this parameter */
 {
 	ut_ad(mutex_own(&buf_pool.mutex));
-	ut_a(bpage->in_file());
 	ut_ad(!bpage->in_LRU_list);
 
 	if (!old || (UT_LIST_GET_LEN(buf_pool.LRU) < BUF_LRU_OLD_MIN_LEN)) {
@@ -1153,10 +1151,18 @@ bool buf_LRU_free_page(buf_page_t *bpage, bool zip)
 	ut_ad(bpage->in_file());
 	ut_ad(bpage->in_LRU_list);
 
+	/* First, perform a quick check before we acquire hash_lock. */
+	if (!bpage->can_relocate()) {
+		return false;
+	}
+
+	/* We must hold an exclusive hash_lock to prevent
+	bpage->can_relocate() from changing due to a concurrent
+	execution of buf_page_get_low(). */
 	rw_lock_t* hash_lock = buf_pool.hash_lock_get(id);
 	rw_lock_x_lock(hash_lock);
 
-	if (!bpage->can_relocate()) {
+	if (UNIV_UNLIKELY(!bpage->can_relocate())) {
 		/* Do not free buffer fixed and I/O-fixed blocks. */
 		goto func_exit;
 	}

storage/innobase/buf/buf0rea.cc

@@ -121,6 +121,8 @@ static buf_page_t* buf_page_init_for_read(ulint mode, const page_id_t page_id,
 
   mutex_enter(&buf_pool.mutex);
 
+  /* We must acquire hash_lock this early to prevent
+  a race condition with buf_pool_t::watch_remove() */
   rw_lock_t *hash_lock= buf_pool.hash_lock_get(page_id);
   rw_lock_x_lock(hash_lock);
 
@@ -151,8 +153,8 @@ static buf_page_t* buf_page_init_for_read(ulint mode, const page_id_t page_id,
       buf_pool.watch_remove(hash_page);
     }
 
-    block->page.set_state(BUF_BLOCK_FILE_PAGE);
     block->page.set_io_fix(BUF_IO_READ);
+    block->page.set_state(BUF_BLOCK_FILE_PAGE);
     ut_ad(!block->page.in_page_hash);
     ut_d(block->page.in_page_hash= true);
     HASH_INSERT(buf_page_t, hash, buf_pool.page_hash, page_id.fold(), bpage);
@@ -202,8 +204,8 @@ static buf_page_t* buf_page_init_for_read(ulint mode, const page_id_t page_id,
       {
         /* The block was added by some other thread. */
         rw_lock_x_unlock(hash_lock);
-	buf_buddy_free(data, zip_size);
-	goto func_exit;
+        buf_buddy_free(data, zip_size);
+        goto func_exit;
       }
     }
 

storage/innobase/include/buf0buf.h

@@ -995,6 +995,14 @@ class buf_page_t
     buf_fix_count_= buf_fix_count;
   }
 
+  /** Initialize some more fields */
+  void init(page_id_t id, uint32_t buf_fix_count= 0)
+  {
+    init();
+    id_= id;
+    buf_fix_count_= buf_fix_count;
+  }
+
 public:
   const page_id_t &id() const { return id_; }
   buf_page_state state() const { return state_; }
@@ -1010,8 +1018,7 @@ class buf_page_t
   /** @return if this belongs to buf_pool.unzip_LRU */
   bool belongs_to_unzip_LRU() const
   {
-    ut_ad(in_file());
-    return zip.data && state() == BUF_BLOCK_FILE_PAGE;
+    return zip.data && state() != BUF_BLOCK_ZIP_PAGE;
   }
 
   inline void add_buf_fix_count(uint32_t count);
@@ -1277,9 +1284,10 @@ struct buf_block_t{
   ulint zip_size() const { return page.zip_size(); }
 
   /** Initialize the block.
-  @param page_id page id
-  @param zip_size ROW_FORMAT=COMPRESSED page size, or 0 */
-  void initialise(const page_id_t page_id, ulint zip_size);
+  @param page_id  page identifier
+  @param zip_size ROW_FORMAT=COMPRESSED page size, or 0
+  @param fix      initial buf_fix_count() */
+  void initialise(const page_id_t page_id, ulint zip_size, uint32_t fix= 0);
 };
 
 /**********************************************************************//**
@@ -2134,6 +2142,29 @@ inline void buf_page_t::set_buf_fix_count(uint32_t count)
 inline void buf_page_t::set_state(buf_page_state state)
 {
   ut_ad(mutex_own(&buf_pool.mutex));
+#ifdef UNIV_DEBUG
+  switch (state) {
+  case BUF_BLOCK_REMOVE_HASH:
+    /* buf_pool_t::corrupted_evict() invokes set_corrupt_id()
+    before buf_LRU_free_one_page(), so we cannot assert that
+    we are holding the hash_lock. */
+    break;
+  case BUF_BLOCK_MEMORY:
+    if (!in_file()) break;
+    /* fall through */
+  case BUF_BLOCK_FILE_PAGE:
+    ut_ad(rw_lock_own(buf_pool.hash_lock_get(id_), RW_LOCK_X));
+    break;
+  case BUF_BLOCK_NOT_USED:
+    if (!in_file()) break;
+    /* fall through */
+  case BUF_BLOCK_ZIP_PAGE:
+    ut_ad((this >= &buf_pool.watch[0] &&
+           this <= &buf_pool.watch[UT_ARR_SIZE(buf_pool.watch)]) ||
+          rw_lock_own(buf_pool.hash_lock_get(id_), RW_LOCK_X));
+    break;
+  }
+#endif
   state_= state;
 }
 
@@ -2212,7 +2243,6 @@ inline bool buf_page_t::is_old() const
 /** Set whether a block is old in buf_pool.LRU */
 inline void buf_page_t::set_old(bool old)
 {
-  ut_ad(in_file());
   ut_ad(mutex_own(&buf_pool.mutex));
   ut_ad(in_LRU_list);
 

storage/innobase/include/sync0rw.ic

@@ -220,8 +220,31 @@ rw_lock_lock_word_decr(
 
 			return(true);
 		}
+
+		/* Note that lock_copy was reloaded above. We will
+		keep trying if a spurious conflict occurred, typically
+		caused by concurrent executions of
+		rw_lock_s_lock(). */
+
+#if 1 /* FIXME: MDEV-22871 Spurious contention between rw_lock_s_lock() */
+
+		/* When the number of concurrently executing threads
+		exceeds the number of available processor cores,
+		multiple buf_pool.page_hash S-latch requests would
+		conflict here, mostly in buf_page_get_low(). We should
+		implement a simpler rw-lock where the S-latch
+		acquisition would be a simple fetch_add(1) followed by
+		either an optional load() loop to wait for the X-latch
+		to be released, or a fetch_sub(1) and a retry.
+
+		For now, we work around the problem with a delay in
+		this loop. It helped a little on some systems and was
+		reducing performance on others. */
 		(void) LF_BACKOFF();
+#endif
 	}
+
+	/* A real conflict was detected. */
 	return(false);
 }