| Message ID | orobfdqhbu.fsf@livre.home (mailing list archive) |
|---|---|
| State | New, archived |
| Headers | show |
On Thu, Feb 21, 2013 at 06:15:49PM -0700, Alexandre Oliva wrote: > On Feb 21, 2013, Alexandre Oliva <oliva@gnu.org> wrote: > > > What I saw in that function also happens to explain why in some cases I > > see filesystems allocate a huge number of chunks that remain unused > > (leading to the scenario above, of not having more chunks to allocate). > > It happens for data and metadata, but not necessarily both. I'm > > guessing some thread sets the force_alloc flag on the corresponding > > space_info, and then several threads trying to get disk space end up > > attempting to allocate a new chunk concurrently. All of them will see > > the force_alloc flag and bump their local copy of force up to the level > > they see first, and they won't clear it even if another thread succeeds > > in allocating a chunk, thus clearing the force flag. Then each thread > > that observed the force flag will, on its turn, force the allocation of > > a new chunk. And any threads that come in while it does that will see > > the force flag still set and pick it up, and so on. This sounds like a > > problem to me, but... what should the correct behavior be? Clear > > force_flag once we copy it to a local force? Reset force to the > > incoming value on every loop? > > I think a slight variant of the following makes the most sense, so I > implemented it in the patch below. > > > Set the flag to our incoming force if we have it at first, clear our > > local flag, and move it from the space_info when we determined that we > > are the thread that's going to perform the allocation? > > > From: Alexandre Oliva <oliva@gnu.org> > > btrfs: consume force_alloc in the first thread to chunk_alloc > > Even if multiple threads in do_chunk_alloc look at force_alloc and see > a force flag, it suffices that one of them consumes the flag. Arrange > for an incoming force argument to make to force_alloc in case of > concurrent calls, so that it is used only by the first thread to get > to allocation after the initial request. > > Signed-off-by: Alexandre Oliva <oliva@gnu.org> > --- > fs/btrfs/extent-tree.c | 8 ++++++++ > 1 file changed, 8 insertions(+) > > diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c > index 6ee89d5..66283f7 100644 > --- a/fs/btrfs/extent-tree.c > +++ b/fs/btrfs/extent-tree.c > @@ -3574,8 +3574,12 @@ static int do_chunk_alloc(struct btrfs_trans_handle *trans, > > again: > spin_lock(&space_info->lock); > + > + /* Bring force_alloc to force and tentatively consume it. */ > if (force < space_info->force_alloc) > force = space_info->force_alloc; > + space_info->force_alloc = CHUNK_ALLOC_NO_FORCE; > + > if (space_info->full) { > spin_unlock(&space_info->lock); > return 0; > @@ -3586,6 +3590,10 @@ again: > return 0; > } else if (space_info->chunk_alloc) { > wait_for_alloc = 1; > + /* Reset force_alloc so that it's consumed by the > + first thread that completes the allocation. */ > + space_info->force_alloc = force; > + force = CHUNK_ALLOC_NO_FORCE; So I understand what you are getting at, but I think you are doing it wrong. If we're calling with CHUNK_ALLOC_FORCE, but somebody has already started to allocate with CHUNK_ALLOC_NO_FORCE, we'll reset the space_info->force_alloc to our original caller's CHUNK_ALLOC_FORCE. So we only really care about making sure a chunk is actually allocated, instead of doing this flag shuffling we should just do if (space_info->chunk_alloc) { spin_unlock(&space_info->lock); wait_event(!space_info->chunk_alloc); return 0; } and that way we don't allocate more chunks than normal. Thanks, Josef -- To unsubscribe from this list: send the line "unsubscribe linux-btrfs" in the body of a message to majordomo@vger.kernel.org More majordomo info at http://vger.kernel.org/majordomo-info.html
On Feb 22, 2013, Josef Bacik <jbacik@fusionio.com> wrote: > So I understand what you are getting at, but I think you are doing it wrong. If > we're calling with CHUNK_ALLOC_FORCE, but somebody has already started to > allocate with CHUNK_ALLOC_NO_FORCE, we'll reset the space_info->force_alloc to > our original caller's CHUNK_ALLOC_FORCE. But that's ok, do_chunk_alloc will set space_info->force_alloc to CHUNK_ALLOC_NO_FORCE at the end, when it succeeds allocating, and then anyone else waiting on the mutex to try to allocate will load the NO_FORCE from space_info. > So we only really care about making sure a chunk is actually > allocated, instead of doing this flag shuffling we should just do > if (space_info->chunk_alloc) { > spin_unlock(&space_info->lock); > wait_event(!space_info->chunk_alloc); > return 0; > } Sorry, I don't follow.
On Feb 23, 2013, Alexandre Oliva <oliva@gnu.org> wrote: > On Feb 22, 2013, Josef Bacik <jbacik@fusionio.com> wrote: >> So I understand what you are getting at, but I think you are doing it wrong. If >> we're calling with CHUNK_ALLOC_FORCE, but somebody has already started to >> allocate with CHUNK_ALLOC_NO_FORCE, we'll reset the space_info->force_alloc to >> our original caller's CHUNK_ALLOC_FORCE. > But that's ok, do_chunk_alloc will set space_info->force_alloc to > CHUNK_ALLOC_NO_FORCE at the end, when it succeeds allocating, and then > anyone else waiting on the mutex to try to allocate will load the > NO_FORCE from space_info. >> So we only really care about making sure a chunk is actually >> allocated, instead of doing this flag shuffling we should just do >> if (space_info->chunk_alloc) { spin_unlock(&space_info->lock); >> wait_event(!space_info->chunk_alloc); return 0; I looked a bit further into it. I think I this would work if we had a wait_queue for space_info->chunk_alloc. We don't, so the mutex interface is probably the best we can do. OTOH, I found out we seem to get into an allocate spree when a large file is being quickly created, such as when creating a ceph journal or making a copy of a multi-GB file. I suppose btrfs is just trying to allocate contiguous space for the file, but unfortunately there doesn't seem to be a fallback for allocation failure: as soon as data allocation fails and space_info is set as full, the large write fails and the filesystem becomes full, without even trying to use non-contiguous storage. Isn't that a bug? I've also been trying to track down why, on a single-data filesystem, (compressed?) data reads that fail because of bad blocks also spike the CPU load and lock the file that failed to map in and the entire filesystem, so that the only way to recover is to force a reboot. Does this sound familiar to anyone?
From: Alexandre Oliva <oliva@gnu.org> btrfs: consume force_alloc in the first thread to chunk_alloc Even if multiple threads in do_chunk_alloc look at force_alloc and see a force flag, it suffices that one of them consumes the flag. Arrange for an incoming force argument to make to force_alloc in case of concurrent calls, so that it is used only by the first thread to get to allocation after the initial request. Signed-off-by: Alexandre Oliva <oliva@gnu.org> --- fs/btrfs/extent-tree.c | 8 ++++++++ 1 file changed, 8 insertions(+) diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c index 6ee89d5..66283f7 100644 --- a/fs/btrfs/extent-tree.c +++ b/fs/btrfs/extent-tree.c @@ -3574,8 +3574,12 @@ static int do_chunk_alloc(struct btrfs_trans_handle *trans, again: spin_lock(&space_info->lock); + + /* Bring force_alloc to force and tentatively consume it. */ if (force < space_info->force_alloc) force = space_info->force_alloc; + space_info->force_alloc = CHUNK_ALLOC_NO_FORCE; + if (space_info->full) { spin_unlock(&space_info->lock); return 0; @@ -3586,6 +3590,10 @@ again: return 0; } else if (space_info->chunk_alloc) { wait_for_alloc = 1; + /* Reset force_alloc so that it's consumed by the + first thread that completes the allocation. */ + space_info->force_alloc = force; + force = CHUNK_ALLOC_NO_FORCE; } else { space_info->chunk_alloc = 1; }