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Date:      Mon, 9 Jun 2014 03:38:03 +0000 (UTC)
From:      Konstantin Belousov <kib@FreeBSD.org>
To:        src-committers@freebsd.org, svn-src-all@freebsd.org, svn-src-head@freebsd.org
Subject:   svn commit: r267255 - head/sys/kern
Message-ID:  <201406090338.s593c331040330@svn.freebsd.org>
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Author: kib
Date: Mon Jun  9 03:38:03 2014
New Revision: 267255
URL: http://svnweb.freebsd.org/changeset/base/267255

Log:
  Change the nblock mutex, protecting the needsbuffer buffer deficit
  flags, to rwlock.  Lock it in read mode when used from subroutines
  called from buffer release code paths.
  
  The needsbuffer is now updated using atomics, while read lock of
  nblock prevents loosing the wakeups from bufspacewakeup() and
  bufcountadd() in getnewbuf_bufd_help().
  
  In several interesting loads, needsbuffer flags are never set, while
  buffers are reused quickly.  This causes brelse() and bqrelse() from
  different threads to content on the nblock.  Now they take nblock in
  read mode, together with needsbuffer not needing an update, allowing
  higher parallelism.
  
  Tested by:	pho
  Sponsored by:	The FreeBSD Foundation
  MFC after:	2 weeks

Modified:
  head/sys/kern/vfs_bio.c

Modified: head/sys/kern/vfs_bio.c
==============================================================================
--- head/sys/kern/vfs_bio.c	Mon Jun  9 03:37:41 2014	(r267254)
+++ head/sys/kern/vfs_bio.c	Mon Jun  9 03:38:03 2014	(r267255)
@@ -254,7 +254,7 @@ static struct mtx_padalign rbreqlock;
 /*
  * Lock that protects needsbuffer and the sleeps/wakeups surrounding it.
  */
-static struct mtx_padalign nblock;
+static struct rwlock_padalign nblock;
 
 /*
  * Lock that protects bdirtywait.
@@ -299,7 +299,7 @@ static int runningbufreq;
  * Used in numdirtywakeup(), bufspacewakeup(), bufcountadd(), bwillwrite(),
  * getnewbuf(), and getblk().
  */
-static int needsbuffer;
+static volatile int needsbuffer;
 
 /*
  * Synchronization for bwillwrite() waiters.
@@ -457,18 +457,27 @@ bdirtyadd(void)
 static __inline void
 bufspacewakeup(void)
 {
+	int need_wakeup, on;
 
 	/*
 	 * If someone is waiting for BUF space, wake them up.  Even
 	 * though we haven't freed the kva space yet, the waiting
 	 * process will be able to now.
 	 */
-	mtx_lock(&nblock);
-	if (needsbuffer & VFS_BIO_NEED_BUFSPACE) {
-		needsbuffer &= ~VFS_BIO_NEED_BUFSPACE;
-		wakeup(&needsbuffer);
+	rw_rlock(&nblock);
+	for (;;) {
+		need_wakeup = 0;
+		on = needsbuffer;
+		if ((on & VFS_BIO_NEED_BUFSPACE) == 0)
+			break;
+		need_wakeup = 1;
+		if (atomic_cmpset_rel_int(&needsbuffer, on,
+		    on & ~VFS_BIO_NEED_BUFSPACE))
+			break;
 	}
-	mtx_unlock(&nblock);
+	if (need_wakeup)
+		wakeup((void *)&needsbuffer);
+	rw_runlock(&nblock);
 }
 
 /*
@@ -528,7 +537,7 @@ runningbufwakeup(struct buf *bp)
 static __inline void
 bufcountadd(struct buf *bp)
 {
-	int old;
+	int mask, need_wakeup, old, on;
 
 	KASSERT((bp->b_flags & B_INFREECNT) == 0,
 	    ("buf %p already counted as free", bp));
@@ -536,14 +545,22 @@ bufcountadd(struct buf *bp)
 	old = atomic_fetchadd_int(&numfreebuffers, 1);
 	KASSERT(old >= 0 && old < nbuf,
 	    ("numfreebuffers climbed to %d", old + 1));
-	mtx_lock(&nblock);
-	if (needsbuffer) {
-		needsbuffer &= ~VFS_BIO_NEED_ANY;
-		if (numfreebuffers >= hifreebuffers)
-			needsbuffer &= ~VFS_BIO_NEED_FREE;
-		wakeup(&needsbuffer);
+	mask = VFS_BIO_NEED_ANY;
+	if (numfreebuffers >= hifreebuffers)
+		mask |= VFS_BIO_NEED_FREE;
+	rw_rlock(&nblock);
+	for (;;) {
+		need_wakeup = 0;
+		on = needsbuffer;
+		if (on == 0)
+			break;
+		need_wakeup = 1;
+		if (atomic_cmpset_rel_int(&needsbuffer, on, on & ~mask))
+			break;
 	}
-	mtx_unlock(&nblock);
+	if (need_wakeup)
+		wakeup((void *)&needsbuffer);
+	rw_runlock(&nblock);
 }
 
 /*
@@ -787,7 +804,7 @@ bufinit(void)
 	mtx_init(&bqclean, "bufq clean lock", NULL, MTX_DEF);
 	mtx_init(&bqdirty, "bufq dirty lock", NULL, MTX_DEF);
 	mtx_init(&rbreqlock, "runningbufspace lock", NULL, MTX_DEF);
-	mtx_init(&nblock, "needsbuffer lock", NULL, MTX_DEF);
+	rw_init(&nblock, "needsbuffer lock");
 	mtx_init(&bdlock, "buffer daemon lock", NULL, MTX_DEF);
 	mtx_init(&bdirtylock, "dirty buf lock", NULL, MTX_DEF);
 
@@ -2085,9 +2102,7 @@ getnewbuf_bufd_help(struct vnode *vp, in
 		waitmsg = "newbuf";
 		flags = VFS_BIO_NEED_ANY;
 	}
-	mtx_lock(&nblock);
-	needsbuffer |= flags;
-	mtx_unlock(&nblock);
+	atomic_set_int(&needsbuffer, flags);
 	mtx_unlock(&bqclean);
 
 	bd_speedup();	/* heeeelp */
@@ -2097,12 +2112,11 @@ getnewbuf_bufd_help(struct vnode *vp, in
 	td = curthread;
 	cnt = 0;
 	wait = MNT_NOWAIT;
-	mtx_lock(&nblock);
-	while (needsbuffer & flags) {
+	rw_wlock(&nblock);
+	while ((needsbuffer & flags) != 0) {
 		if (vp != NULL && vp->v_type != VCHR &&
 		    (td->td_pflags & TDP_BUFNEED) == 0) {
-			mtx_unlock(&nblock);
-
+			rw_wunlock(&nblock);
 			/*
 			 * getblk() is called with a vnode locked, and
 			 * some majority of the dirty buffers may as
@@ -2124,15 +2138,16 @@ getnewbuf_bufd_help(struct vnode *vp, in
 				atomic_add_long(&notbufdflushes, 1);
 				curthread_pflags_restore(norunbuf);
 			}
-			mtx_lock(&nblock);
+			rw_wlock(&nblock);
 			if ((needsbuffer & flags) == 0)
 				break;
 		}
-		if (msleep(&needsbuffer, &nblock, (PRIBIO + 4) | slpflag,
-		    waitmsg, slptimeo))
+		error = rw_sleep((void *)&needsbuffer, &nblock, (PRIBIO + 4) |
+		    slpflag, waitmsg, slptimeo);
+		if (error != 0)
 			break;
 	}
-	mtx_unlock(&nblock);
+	rw_wunlock(&nblock);
 }
 
 static void

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