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

Dave Chinner
2013-12-09T02:37:28Z

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....
>
> 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.

Cheers,

Dave.
--
Dave Chinner
david@fromorbit.com