Re: Odd locking pattern introduced as part of "nowait aio support"

Ritesh Harjani
2019-09-11T10:31:28Z

Hi,

On 9/11/19 3:09 PM, Andres Freund wrote:
> Hi,
>
> On 2019-09-11 14:04:20 +1000, Dave Chinner wrote:
>> On Tue, Sep 10, 2019 at 03:33:27PM -0700, Andres Freund wrote:
>>> Hi,
>>>
>>> Especially with buffered io it's fairly easy to hit contention on the
>>> inode lock, during writes. With something like io_uring, it's even
>>> easier, because it currently (but see [1]) farms out buffered writes to
>>> workers, which then can easily contend on the inode lock, even if only
>>> one process submits writes. But I've seen it in plenty other cases too.
>>>
>>> Looking at the code I noticed that several parts of the "nowait aio
>>> support" (cf 728fbc0e10b7f3) series introduced code like:
>>>
>>> static ssize_t
>>> ext4_file_write_iter(struct kiocb *iocb, struct iov_iter *from)
>>> {
>>> ...
>>> if (!inode_trylock(inode)) {
>>> if (iocb->ki_flags & IOCB_NOWAIT)
>>> return -EAGAIN;
>>> inode_lock(inode);
>>> }
>>
>> The ext4 code is just buggy here - we don't support RWF_NOWAIT on
>> buffered write >
> But both buffered and non-buffered writes go through
> ext4_file_write_iter(). And there's a preceding check, at least these
> days, preventing IOCB_NOWAIT to apply to buffered writes:
>
> if (!o_direct && (iocb->ki_flags & IOCB_NOWAIT))
> return -EOPNOTSUPP;
>

-EOPNOTSUPP is now taken care in ext4 iomap patch series as well.

>
> I do really wish buffered NOWAIT was supported... The overhead of having
> to do async buffered writes through the workqueue in io_uring, even if
> an already existing page is targeted, is quite noticable. Even if it
> failed with EAGAIN as soon as the buffered write's target isn't in the
> page cache, it'd be worthwhile.
>
>
>>> isn't trylocking and then locking in a blocking fashion an inefficient
>>> pattern? I.e. I think this should be
>>>
>>> if (iocb->ki_flags & IOCB_NOWAIT) {
>>> if (!inode_trylock(inode))
>>> return -EAGAIN;
>>> }
>>> else
>>> inode_lock(inode);
>>
>> Yes, you are right.
>>
>> History: commit 91f9943e1c7b ("fs: support RWF_NOWAIT
>> for buffered reads") which introduced the first locking pattern
>> you describe in XFS.
>
> Seems, as part of the nowait work, the pattern was introduced in ext4,
> xfs and btrfs. And fixed in xfs.
>
>
>> That was followed soon after by:
>>
>> commit 942491c9e6d631c012f3c4ea8e7777b0b02edeab
>> Author: Christoph Hellwig <hch@lst.de>
>> Date: Mon Oct 23 18:31:50 2017 -0700
>>
>> xfs: fix AIM7 regression
>>
>> Apparently our current rwsem code doesn't like doing the trylock, then
>> lock for real scheme. So change our read/write methods to just do the
>> trylock for the RWF_NOWAIT case. This fixes a ~25% regression in
>> AIM7.
>>
>> Fixes: 91f9943e ("fs: support RWF_NOWAIT for buffered reads")
>> Reported-by: kernel test robot <xiaolong.ye@intel.com>
>> Signed-off-by: Christoph Hellwig <hch@lst.de>
>> Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
>> Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>
>>
>> Which changed all the trylock/eagain/lock patterns to the second
>> form you quote. None of the other filesystems had AIM7 regressions
>> reported against them, so nobody changed them....
>
> :(
>
>
>>> Obviously this isn't going to improve scalability to a very significant
>>> degree. But not unnecessarily doing two atomic ops on a contended lock
>>> can't hurt scalability either. Also, the current code just seems
>>> confusing.
>>>
>>> Am I missing something?
>>
>> Just that the sort of performance regression testing that uncovers
>> this sort of thing isn't widely done, and most filesystems are
>> concurrency limited in some way before they hit inode lock
>> scalability issues. Hence filesystem concurrency foccussed
>> benchmarks that could uncover it (like aim7) won't because the inode
>> locks don't end up stressed enough to make a difference to
>> benchmark performance.
>
> Ok. Goldwyn, do you want to write a patch, or do you want me to write
> one up?

I am anyways looking into ext4 performance issue of mixed parallel DIO
workload. This will require some new APIs for inode locking similar to
that of XFS.
In that I can take care of this symantics reported here by you (which is
taken care by XFS in above patch) for ext4.

Thanks
-ritesh

>
>
> Greetings,
>
> Andres Freund
>