On Mon, August 05, 2013 at 16:18 (+0200), Liu Bo wrote:
> On Mon, Aug 05, 2013 at 02:34:30PM +0200, Jan Schmidt wrote:
>> Nice try hiding this one in a dedup patch set, but I finally found it :-)
>
> Ahhhh, I didn't mean to ;-)
>
>>
>> On Wed, July 31, 2013 at 17:37 (+0200), Liu Bo wrote:
>>> So we don't need to do qgroups accounting trick without enabling quota.
>>> This reduces my tester's costing time from ~28s to ~23s.
>>>
>>> Signed-off-by: Liu Bo <bo.li....@oracle.com>
>>> ---
>>> fs/btrfs/extent-tree.c | 6 ++++++
>>> fs/btrfs/qgroup.c | 6 ++++++
>>> 2 files changed, 12 insertions(+), 0 deletions(-)
>>>
>>> diff --git a/fs/btrfs/extent-tree.c b/fs/btrfs/extent-tree.c
>>> index 10a5c72..c6612f5 100644
>>> --- a/fs/btrfs/extent-tree.c
>>> +++ b/fs/btrfs/extent-tree.c
>>> @@ -2524,6 +2524,12 @@ int btrfs_delayed_refs_qgroup_accounting(struct
>>> btrfs_trans_handle *trans,
>>> struct qgroup_update *qgroup_update;
>>> int ret = 0;
>>>
>>> + if (!trans->root->fs_info->quota_enabled) {
>>> + if (trans->delayed_ref_elem.seq)
>>> + btrfs_put_tree_mod_seq(fs_info,
>>> &trans->delayed_ref_elem);
>>> + return 0;
>>> + }
>>> +
>>> if (list_empty(&trans->qgroup_ref_list) !=
>>> !trans->delayed_ref_elem.seq) {
>>> /* list without seq or seq without list */
>>> diff --git a/fs/btrfs/qgroup.c b/fs/btrfs/qgroup.c
>>> index 1280eff..f3e82aa 100644
>>> --- a/fs/btrfs/qgroup.c
>>> +++ b/fs/btrfs/qgroup.c
>>> @@ -1200,6 +1200,9 @@ int btrfs_qgroup_record_ref(struct btrfs_trans_handle
>>> *trans,
>>> {
>>> struct qgroup_update *u;
>>>
>>> + if (!trans->root->fs_info->quota_enabled)
>>> + return 0;
>>> +
>>> BUG_ON(!trans->delayed_ref_elem.seq);
>>> u = kmalloc(sizeof(*u), GFP_NOFS);
>>> if (!u)
>>> @@ -1850,6 +1853,9 @@ out:
>>>
>>> void assert_qgroups_uptodate(struct btrfs_trans_handle *trans)
>>> {
>>> + if (!trans->root->fs_info->quota_enabled)
>>> + return;
>>> +
>>> if (list_empty(&trans->qgroup_ref_list) && !trans->delayed_ref_elem.seq)
>>> return;
>>> pr_err("btrfs: qgroups not uptodate in trans handle %p: list is%s
>>> empty, seq is %#x.%x\n",
>>>
>>
>> The second hunk looks sensible at first sight. However, hunk 1 and 3 don't.
>> They
>> assert consistency of qgroup state in well defined places. The fact that you
>> need to disable those checks shows that skipping addition to the list in the
>> second hunk cannot be right, or at least not sufficient.
>
> I agree, only hunk 2 is necessary.
>
>>
>> We've got the list of qgroup operations trans->qgroup_ref_list and we've got
>> the
>> qgroup's delayed ref blocker, trans->delayed_ref_elem. If you stop adding to
>> the
>> list (hunk 2) which seems reasonable when quota is disabled, then you also
>> must
>> ensure you're not acquiring the delayed ref blocker element, which should
>> give
>> another performance boost.
>
> WHY a 'must' here?
Because otherwise you are going to hit the BUG_ONs you avoided with hunk 1 and
3.
>>
>> need_ref_seq may be the right place for this change. It just feels a bit too
>> obvious. The critical cases obviously are quota enable and quota disable. I
>> just
>> don't recall why it wasn't that way from the very beginning of qgroups, I
>> might
>> be missing something fundamental here.
>
> Yeah I thought about 'need_ref_seq', but the point is that delayed ref blocker
> not only serves qgroups accounting, but also features based on backref
> walking, such as scrub, snapshot-aware defragment.
I think you're confusing trans->delayed_ref_elem with other callers of
btrfs_get_tree_mod_seq() and btrfs_put_tree_mod_seq(). trans->delayed_ref_elem
is only used in qgroup context, as far as my grep reaches. There are other
callers of btrfs_get_tree_mod_seq() that can put their blocker element on the
stack, such as iterate_extent_inodes().
But I still might be missing something.
Thanks,
-Jan
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