Re: [PATCH v2 2/3] xfs: fix infinite loop by detaching the group/project hints from user dquot

Jeff Liu
2013-12-09T03:27:11Z

On 12/09 2013 10:36 AM, Dave Chinner wrote:
> On Mon, Dec 09, 2013 at 12:26:42PM +1100, Dave Chinner wrote:
>> On Sat, Dec 07, 2013 at 01:51:24PM +0800, Jeff Liu wrote:
>>> Hi Ben,
>>>
>> ....
>>>>> void
>>>>> xfs_qm_dqpurge_all()
>>>>> {
>>>>> xfs_qm_dquot_walk(mp, XFS_DQ_USER, xfs_qm_dqpurge_hints, NULL);
>>>>>
>>>>> if (flags & XFS_QMOPT_UQUOTA)
>>>>> xfs_qm_dquot_walk(mp, XFS_DQ_USER, xfs_qm_dqpurge, NULL);
>>>>> if (flags & XFS_QMOPT_GQUOTA)
>>>>> xfs_qm_dquot_walk(mp, XFS_DQ_GROUP, xfs_qm_dqpurge, NULL);
>>>>> if (flags & XFS_QMOPT_PQUOTA)
>>>>> xfs_qm_dquot_walk(mp, XFS_DQ_PROJ, xfs_qm_dqpurge, NULL);
>>>>> }
>>>>>
>>>>> Above code is what I can figured out as per your suggestions for now, but it
>>>>> would introduce overheads for walking through user dquots to release hints
>>>>> separately if we want to turn user quota off.
>>>>>
>>>>> Any thoughts?
>>>>
>>>> I was gonna pull in the single walk version, but now I realize that it is still
>>>> under discussion. I'm happy with either implementation, with maybe a slight
>>>> preference for a single user quota walk. Can you and Christoph come to an
>>>> agreement?
>>> For now, I can not figure out a more optimized solution. Well, I just realized
>>> I don't need to initialize both gdqp and pdqp to NULL at xfs_qm_dqpurge_hints()
>>> since they will be evaluated by dqp pointers dereference anyway. As a minor fix,
>>> the revised version was shown as follows.
>>>
>>> Christoph, as I mentioned previously, keeping a separate walk to release the user
>>> dquots would also have overloads in some cases, would you happy to have this fix
>>> although it is not most optimized?
>>
>> I'm happy either way it is done - I'd prefer we fix the problem than
>> bikeshed over an extra radix tree walk or not given for most people
>> the overhead won't be significant.
>>
>>> From: Jie Liu <jeff.liu@oracle.com>
>>>
>>> xfs_quota(8) will hang up if trying to turn group/project quota off
>>> before the user quota is off, this could be 100% reproduced by:
>> .....
>>
>> So from the perspective, I'm happy to consider the updated
>> patch as:
>>
>> Reviewed-by: Dave Chinner <dchinner@redhat.com>
>>
>> However, I question the need for the hints at all now. The hints
>> were necessary back when the quota manager had global lists and
>> hashes, and the lookups were expensive. Hence there was a
>> significant win to caching the group dquot on the user dquot as it
>> avoided a significant amount of code, locks and dirty cachelines.
>>
>> Now, it's just a radix tree lookup under only a single lock and the
>> process dirties far fewer cachelines (none in the radix tree at all)
>> and so should be substantially faster than the old code. And with
>> the dquots being attached and cached on inodes in the first place, I
>> don't see much advantage to keeping hints on the user dquot. THis is
>> especially true for project quotas where a user might be accessing
>> files in different projects all the time and so thrashing the
>> project quota hint on the user dquot....
Ah, that accounts for it! Yesterday, I even thought to add an udquot
member to struct xfs_dquot in order to avoid walk though user quota
while turning off others, i.e,

diff --git a/fs/xfs/xfs_dquot.h b/fs/xfs/xfs_dquot.h
index d22ed00..0037c7e 100644
--- a/fs/xfs/xfs_dquot.h
+++ b/fs/xfs/xfs_dquot.h
@@ -52,6 +52,13 @@ typedef struct xfs_dquot {
int q_bufoffset; /* off of dq in buffer (# dquots) */
xfs_fileoff_t q_fileoffset; /* offset in quotas file */

+ union {
+ struct xfs_dquot *q_udquot;
+ struct {
+ struct xfs_dquot *q_pdquot;
+ struct xfs_dquot *q_gdquot;
+ } gp_hints;
+ } hints;
struct xfs_dquot*q_gdquot; /* group dquot, hint only */
struct xfs_dquot*q_pdquot; /* project dquot, hint only */
xfs_disk_dquot_t q_core; /* actual usage & quotas */

In this way, I can attach the q_udquot to group/project dquots while
attaching them to the user's. Thus I don't need to walk through user
dquots to fetch the hints but to fetch them via:
gdquot->hints.q_udquot.g_pdquot/g_gdquot and then decrease the reference
count, but that need more code changes and add complexities.

>>
>> Hence I wonder if removing the dquot hint caching altogether would
>> result in smaller, simpler, faster code. And, in reality, if the
>> radix tree lock is a contention point on lookup after removing the
>> hints, then we can fix that quite easily by switching to RCU-based
>> lockless lookups like we do for the inode cache....
>
> Actually, scalability couldn't get any worse by removing the hints.
> If I run a concurrent workload with quota enabled, the global dquot
> locks (be it user, quota or project) completely serialises the
> workload. This result if from u/g/p all enabled, run by a single
> user in a single group and a project ID of zero:
>
> ./fs_mark -D 10000 -S0 -n 100000 -s 0 -L 32 -d /mnt/scratch/0 -d /mnt/scratch/1 -d /mnt/scratch/2 -d /mnt/scratch/3 -d /mnt/scratch/4 -d /mnt/scratch/5 -d /mnt/scratch/6 -d /mnt/scratch/7 -d /mnt/scratch/8 -d /mnt/scratch/9 -d /mnt/scratch/10 -d /mnt/scratch/11 -d /mnt/scratch/12 -d /mnt/scratch/13 -d /mnt/scratch/14 -d /mnt/scratch/15
> # Version 3.3, 16 thread(s) starting at Mon Dec 9 12:53:46 2013
> # Sync method: NO SYNC: Test does not issue sync() or fsync() calls.
> # Directories: Time based hash between directories across 10000 subdirectories with 180 seconds per subdirectory.
> # File names: 40 bytes long, (16 initial bytes of time stamp with 24 random bytes at end of name)
> # Files info: size 0 bytes, written with an IO size of 16384 bytes per write
> # App overhead is time in microseconds spent in the test not doing file writing related system calls.
>
> FSUse% Count Size Files/sec App Overhead
> 0 1600000 0 17666.5 15377143
> 0 3200000 0 17018.6 15922906
> 0 4800000 0 17373.5 16149660
> 0 6400000 0 16564.9 17234139
> ....
>
> Without quota enabled, that workload runs at >250,000 files/sec.
>
> Serialisation is completely on the dquot locks - so I don't see
> anything right now that hints are going to buy us in terms of
> improving concurrency or scalability, so I think we probably can
> just get rid of them.
>
> FWIW, getting rid of the hints and converting the dquot reference
> counter to an atomic actually improves performance a bit:
>
> FSUse% Count Size Files/sec App Overhead
> 0 1600000 0 17559.3 15606077
> 0 3200000 0 18738.9 14026009
> 0 4800000 0 18960.0 14381162
> 0 6400000 0 19026.5 14422024
> 0 8000000 0 18456.6 15369059
>
> Sure, 10% improvement is 10%, but concurrency still sucks. At least
> it narrows down the cause - the transactional modifications are the
> serialisation issue.
Admire!! I'm still in considering of remove the hints but you have already
shown the measuring results. :)
Would you like to fix it in this way directly or make it as a increased
improvement once my current fix got merged? Both fine to me.

Thanks,
-Jeff