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When helping the bufdaemon from the buffer allocation context, there

is no sense to walk the whole dirty buffer queue.  We are only
interested in, and can operate on, the buffers owned by the current
vnode [1].  Instead of calling generic queue flush routine, do
VOP_FSYNC() if possible.

Holding the dirty buffer queue lock in the bufdaemon, without dropping
it, can cause starvation of buffer writes from other threads. This is
esp. easy to reproduce on the big memory machines, where large files
are written, causing almost all dirty buffers accumulating in several
big files, which vnodes are locked by writers. Bufdaemon cannot flush
any buffer, but is iterating over the whole dirty queue
continuously. Since dirty queue mutex is not dropped, bufdone() in
g_up thread is starved, usually deadlocking the machine [2]. Mitigate
this by dropping the queue lock after the vnode is locked, allowing
other queue lock contenders to make a progress.

Discussed with:	Jeff [1]
Reported by:	pho [2]
Tested by:	pho
Sponsored by:	The FreeBSD Foundation
MFC after:	2 weeks
Approved by:	re (hrs)
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kostikbel committed Oct 2, 2013
1 parent b029856 commit 031d824a492a5e1aa9303a5ead6dd94db7dd4597
Showing with 41 additions and 63 deletions.
  1. +41 −63 sys/kern/vfs_bio.c
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@@ -113,8 +113,8 @@ static void vfs_setdirty_locked_object(struct buf *bp);
static void vfs_vmio_release(struct buf *bp);
static int vfs_bio_clcheck(struct vnode *vp, int size,
daddr_t lblkno, daddr_t blkno);
-static int buf_flush(struct vnode *vp, int);
-static int flushbufqueues(struct vnode *, int, int);
+static int buf_flush(int);
+static int flushbufqueues(int, int);
static void buf_daemon(void);
static void bremfreel(struct buf *bp);
static __inline void bd_wakeup(void);
@@ -2048,7 +2048,7 @@ getnewbuf_bufd_help(struct vnode *vp, int gbflags, int slpflag, int slptimeo,
{
struct thread *td;
char *waitmsg;
- int fl, flags, norunbuf;
+ int cnt, error, flags, norunbuf, wait;
mtx_assert(&bqclean, MA_OWNED);
@@ -2072,10 +2072,13 @@ getnewbuf_bufd_help(struct vnode *vp, int gbflags, int slpflag, int slptimeo,
return;
td = curthread;
+ cnt = 0;
+ wait = MNT_NOWAIT;
mtx_lock(&nblock);
while (needsbuffer & flags) {
if (vp != NULL && (td->td_pflags & TDP_BUFNEED) == 0) {
mtx_unlock(&nblock);
+
/*
* getblk() is called with a vnode locked, and
* some majority of the dirty buffers may as
@@ -2084,15 +2087,20 @@ getnewbuf_bufd_help(struct vnode *vp, int gbflags, int slpflag, int slptimeo,
* cannot be achieved by the buf_daemon, that
* cannot lock the vnode.
*/
- norunbuf = ~(TDP_BUFNEED | TDP_NORUNNINGBUF) |
- (td->td_pflags & TDP_NORUNNINGBUF);
- /* play bufdaemon */
- td->td_pflags |= TDP_BUFNEED | TDP_NORUNNINGBUF;
- fl = buf_flush(vp, flushbufqtarget);
- td->td_pflags &= norunbuf;
+ if (cnt++ > 2)
+ wait = MNT_WAIT;
+ ASSERT_VOP_LOCKED(vp, "bufd_helper");
+ error = VOP_ISLOCKED(vp) == LK_EXCLUSIVE ? 0 :
+ vn_lock(vp, LK_TRYUPGRADE);
+ if (error == 0) {
+ /* play bufdaemon */
+ norunbuf = curthread_pflags_set(TDP_BUFNEED |
+ TDP_NORUNNINGBUF);
+ VOP_FSYNC(vp, wait, td);
+ atomic_add_long(&notbufdflushes, 1);
+ curthread_pflags_restore(norunbuf);
+ }
mtx_lock(&nblock);
- if (fl != 0)
- continue;
if ((needsbuffer & flags) == 0)
break;
}
@@ -2510,20 +2518,18 @@ static struct kproc_desc buf_kp = {
SYSINIT(bufdaemon, SI_SUB_KTHREAD_BUF, SI_ORDER_FIRST, kproc_start, &buf_kp);
static int
-buf_flush(struct vnode *vp, int target)
+buf_flush(int target)
{
int flushed;
- flushed = flushbufqueues(vp, target, 0);
+ flushed = flushbufqueues(target, 0);
if (flushed == 0) {
/*
* Could not find any buffers without rollback
* dependencies, so just write the first one
* in the hopes of eventually making progress.
*/
- if (vp != NULL && target > 2)
- target /= 2;
- flushbufqueues(vp, target, 1);
+ flushed = flushbufqueues(target, 1);
}
return (flushed);
}
@@ -2560,7 +2566,7 @@ buf_daemon()
* the I/O system.
*/
while (numdirtybuffers > lodirty) {
- if (buf_flush(NULL, numdirtybuffers - lodirty) == 0)
+ if (buf_flush(numdirtybuffers - lodirty) == 0)
break;
kern_yield(PRI_USER);
}
@@ -2615,7 +2621,7 @@ SYSCTL_INT(_vfs, OID_AUTO, flushwithdeps, CTLFLAG_RW, &flushwithdeps,
0, "Number of buffers flushed with dependecies that require rollbacks");
static int
-flushbufqueues(struct vnode *lvp, int target, int flushdeps)
+flushbufqueues(int target, int flushdeps)
{
struct buf *sentinel;
struct vnode *vp;
@@ -2625,7 +2631,6 @@ flushbufqueues(struct vnode *lvp, int target, int flushdeps)
int flushed;
int queue;
int error;
- bool unlock;
flushed = 0;
queue = QUEUE_DIRTY;
@@ -2634,27 +2639,24 @@ flushbufqueues(struct vnode *lvp, int target, int flushdeps)
sentinel->b_qindex = QUEUE_SENTINEL;
mtx_lock(&bqdirty);
TAILQ_INSERT_HEAD(&bufqueues[queue], sentinel, b_freelist);
+ mtx_unlock(&bqdirty);
while (flushed != target) {
+ maybe_yield();
+ mtx_lock(&bqdirty);
bp = TAILQ_NEXT(sentinel, b_freelist);
if (bp != NULL) {
TAILQ_REMOVE(&bufqueues[queue], sentinel, b_freelist);
TAILQ_INSERT_AFTER(&bufqueues[queue], bp, sentinel,
b_freelist);
- } else
+ } else {
+ mtx_unlock(&bqdirty);
break;
- /*
- * Skip sentinels inserted by other invocations of the
- * flushbufqueues(), taking care to not reorder them.
- */
- if (bp->b_qindex == QUEUE_SENTINEL)
- continue;
- /*
- * Only flush the buffers that belong to the
- * vnode locked by the curthread.
- */
- if (lvp != NULL && bp->b_vp != lvp)
- continue;
- if (BUF_LOCK(bp, LK_EXCLUSIVE | LK_NOWAIT, NULL) != 0)
+ }
+ KASSERT(bp->b_qindex != QUEUE_SENTINEL,
+ ("parallel calls to flushbufqueues() bp %p", bp));
+ error = BUF_LOCK(bp, LK_EXCLUSIVE | LK_NOWAIT, NULL);
+ mtx_unlock(&bqdirty);
+ if (error != 0)
continue;
if (bp->b_pin_count > 0) {
BUF_UNLOCK(bp);
@@ -2670,11 +2672,9 @@ flushbufqueues(struct vnode *lvp, int target, int flushdeps)
continue;
}
if (bp->b_flags & B_INVAL) {
- bremfreel(bp);
- mtx_unlock(&bqdirty);
+ bremfreef(bp);
brelse(bp);
flushed++;
- mtx_lock(&bqdirty);
continue;
}
@@ -2701,45 +2701,23 @@ flushbufqueues(struct vnode *lvp, int target, int flushdeps)
BUF_UNLOCK(bp);
continue;
}
- if (lvp == NULL) {
- unlock = true;
- error = vn_lock(vp, LK_EXCLUSIVE | LK_NOWAIT);
- } else {
- ASSERT_VOP_LOCKED(vp, "getbuf");
- unlock = false;
- error = VOP_ISLOCKED(vp) == LK_EXCLUSIVE ? 0 :
- vn_lock(vp, LK_TRYUPGRADE);
- }
+ error = vn_lock(vp, LK_EXCLUSIVE | LK_NOWAIT);
if (error == 0) {
- mtx_unlock(&bqdirty);
CTR3(KTR_BUF, "flushbufqueue(%p) vp %p flags %X",
bp, bp->b_vp, bp->b_flags);
- if (curproc == bufdaemonproc)
- vfs_bio_awrite(bp);
- else {
- bremfree(bp);
- bwrite(bp);
- notbufdflushes++;
- }
+ vfs_bio_awrite(bp);
vn_finished_write(mp);
- if (unlock)
- VOP_UNLOCK(vp, 0);
+ VOP_UNLOCK(vp, 0);
flushwithdeps += hasdeps;
flushed++;
-
- /*
- * Sleeping on runningbufspace while holding
- * vnode lock leads to deadlock.
- */
- if (curproc == bufdaemonproc &&
- runningbufspace > hirunningspace)
+ if (runningbufspace > hirunningspace)
waitrunningbufspace();
- mtx_lock(&bqdirty);
continue;
}
vn_finished_write(mp);
BUF_UNLOCK(bp);
}
+ mtx_lock(&bqdirty);
TAILQ_REMOVE(&bufqueues[queue], sentinel, b_freelist);
mtx_unlock(&bqdirty);
free(sentinel, M_TEMP);

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