Re: [RFC 0/2] ext4: Improve locking sequence in DIO write path

Jan Kara
2019-09-24T15:10:14Z

Hi Joseph!

On Wed 18-09-19 14:35:15, Joseph Qi wrote:
> On 19/9/17 18:32, Ritesh Harjani wrote:
> > Hello,
> >
> > This patch series is based on the upstream discussion with Jan
> > & Joseph @ [1].
> > It is based on top of Matthew's v3 ext4 iomap patch series [2]
> >
> > Patch-1: Adds the ext4_ilock/unlock APIs and also replaces all
> > inode_lock/unlock instances from fs/ext4/*
> >
> > For now I already accounted for trylock/lock issue symantics
> > (which was discussed here [3]) in the same patch,
> > since the this whole patch was around inode_lock/unlock API,
> > so I thought it will be best to address that issue in the same patch.
> > However, kindly let me know if otherwise.
> >
> > Patch-2: Commit msg of this patch describes in detail about
> > what it is doing.
> > In brief - we try to first take the shared lock (instead of exclusive
> > lock), unless it is a unaligned_io or extend_io. Then in
> > ext4_dio_write_checks(), if we start with shared lock, we see
> > if we can really continue with shared lock or not. If not, then
> > we release the shared lock then acquire exclusive lock
> > and restart ext4_dio_write_checks().
> >
> >
> > Tested against few xfstests (with dioread_nolock mount option),
> > those ran fine (ext4 & generic).
> >
> > I tried testing performance numbers on my VM (since I could not get
> > hold of any real h/w based test device). I could test the fact
> > that earlier we were trying to do downgrade_write() lock, but with
> > this patch, that path is now avoided for fio test case
> > (as reported by Joseph in [4]).
> > But for the actual results, I am not sure if VM machine testing could
> > really give the reliable perf numbers which we want to take a look at.
> > Though I do observe some form of perf improvements, but I could not
> > get any reliable numbers (not even with the same list of with/without
> > patches with which Joseph posted his numbers [1]).
> >
> >
> > @Joseph,
> > Would it be possible for you to give your test case a run with this
> > patches? That will be really helpful.
> >
> > Branch for this is hosted at below tree.
> >
> > https://github.com/riteshharjani/linux/tree/ext4-ilock-RFC
> >
> I've tested your branch, the result is:
> mounting with dioread_nolock, it behaves the same like reverting
> parallel dio reads + dioread_nolock;
> while mounting without dioread_nolock, no improvement, or even worse.
> Please refer the test data below.
>
> fio -name=parallel_dio_reads_test -filename=/mnt/nvme0n1/testfile
> -direct=1 -iodepth=1 -thread -rw=randrw -ioengine=psync -bs=$bs
> -size=20G -numjobs=8 -runtime=600 -group_reporting
>
> w/ = with parallel dio reads
> w/o = reverting parallel dio reads

This is with 16c54688592ce8 "ext4: Allow parallel DIO reads" reverted,
right?

> w/o+ = reverting parallel dio reads + dioread_nolock
> ilock = ext4-ilock-RFC
> ilock+ = ext4-ilock-RFC + dioread_nolock
>
> bs=4k:
> --------------------------------------------------------------
> | READ | WRITE |
> --------------------------------------------------------------
> w/ | 30898KB/s,7724,555.00us | 30875KB/s,7718,479.70us |
> --------------------------------------------------------------
> w/o | 117915KB/s,29478,248.18us | 117854KB/s,29463,21.91us |
> --------------------------------------------------------------

I'm really surprised by the numbers here. They would mean that when DIO
read takes i_rwsem exclusive lock instead of shared, it is a win for your
workload... Argh, now checking code in fs/direct-io.c I think I can see the
difference. The trick in do_blockdev_direct_IO() is:

if (iov_iter_rw(iter) == READ && (dio->flags & DIO_LOCKING))
inode_unlock(dio->inode);
if (dio->is_async && retval == 0 && dio->result &&
(iov_iter_rw(iter) == READ || dio->result == count))
retval = -EIOCBQUEUED;
else
dio_await_completion(dio);

So actually only direct IO read submission is protected by i_rwsem with
DIO_LOCKING. Actual waiting for sync DIO read happens with i_rwsem dropped.

After some thought I think the best solution for this is to just finally
finish the conversion of ext4 so that dioread_nolock is the only DIO path.
With i_rwsem held in shared mode even for "unlocked" DIO, it should be
actually relatively simple and most of the dances with unwritten extents
shouldn't be needed anymore.

Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR