From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: from linux-libre.fsfla.org ([208.118.235.54]:39942 "EHLO linux-libre.fsfla.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1753054Ab3BVBQH (ORCPT ); Thu, 21 Feb 2013 20:16:07 -0500 Received: from freie (home.lxoliva.fsfla.org [172.31.160.22]) by linux-libre.fsfla.org (8.14.3/8.14.3/Debian-9.1ubuntu1) with ESMTP id r1M1G3mK006372 for ; Fri, 22 Feb 2013 01:16:04 GMT From: Alexandre Oliva To: linux-btrfs@vger.kernel.org Subject: collapse concurrent forced allocations (was: Re: clear chunk_alloc flag on retryable failure) References: Date: Thu, 21 Feb 2013 22:15:49 -0300 In-Reply-To: (Alexandre Oliva's message of "Thu, 21 Feb 2013 18:15:14 -0300") Message-ID: MIME-Version: 1.0 Content-Type: multipart/mixed; boundary="=-=-=" Sender: linux-btrfs-owner@vger.kernel.org List-ID: --=-=-= On Feb 21, 2013, Alexandre Oliva 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? --=-=-= Content-Type: text/x-diff Content-Disposition: inline; filename=force-alloc-only-once.patch From: Alexandre Oliva 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 --- 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; } --=-=-= -- Alexandre Oliva, freedom fighter http://FSFLA.org/~lxoliva/ You must be the change you wish to see in the world. -- Gandhi Be Free! -- http://FSFLA.org/ FSF Latin America board member Free Software Evangelist Red Hat Brazil Compiler Engineer --=-=-=--