Re: aio/dio write vs. file_update_time

[Brian Foster <bfoster@redhat.com>]

On Tue, Jan 23, 2018 at 07:25:05PM +0200, Avi Kivity wrote:
> 
> 
> On 01/23/2018 06:31 PM, Brian Foster wrote:
> > On Tue, Jan 23, 2018 at 06:10:51PM +0200, Avi Kivity wrote:
> > > I'm seeing the following lock contention in io_submit() (unfortunately,
> > > older kernel again)
> > > 
> > > 
> > >   0xffffffff816ab231 : __schedule+0x531/0x9b0 [kernel]
> > >   0xffffffff816ab6d9 : schedule+0x29/0x70 [kernel]
> > >   0xffffffff816acfc5 : rwsem_down_write_failed+0x225/0x3a0 [kernel]
> > >   0xffffffff81333ca7 : call_rwsem_down_write_failed+0x17/0x30 [kernel]
> > >   0xffff8819bc3f3bf8 : 0xffff8819bc3f3bf8
> > >   0xffffffff816aa8bd : down_write+0x2d/0x3d [kernel]
> > >   0xffffffffc00ca1d1 : xfs_ilock+0xc1/0x120 [xfs]
> > >   0xffffffffc00c7c8d : xfs_vn_update_time+0xcd/0x150 [xfs]
> > >   0xffffffff8121eda5 : update_time+0x25/0xd0 [kernel]
> > >   0xffffffff8121eef0 : file_update_time+0xa0/0xf0 [kernel]
> > >   0xffffffffc00be3a5 : xfs_file_aio_write_checks+0x185/0x1f0 [xfs]
> > >   0xffffffffc00be6c9 : xfs_file_dio_aio_write+0xd9/0x390 [xfs]
> > >   0xffffffffc00bed42 : xfs_file_aio_write+0x102/0x1b0 [xfs]
> > >   0xffffffffc00bec40 : xfs_file_aio_write+0x0/0x1b0 [xfs]
> > >   0xffffffff81255ff8 : do_io_submit+0x3b8/0x870 [kernel]
> > > 
> > > 
> > > There is only one thread issuing those writes, and nobody is reading the
> > > file. Who could possibly be contending on this lock?
> > > 
> > That looks like XFS_ILOCK_EXCL, which is a low level lock and thus not
> > necessarily restricted to user-driven operations. One possible example
> > of a background user is xfsaild, which acquires XFS_ILOCK_SHARED (and
> > thus locks out exclusive waiters) via xfs_inode_item_push() in order to
> > flush the dirty inode to disk (i.e., metadata writeback).
> > 
> > I'm not exactly sure that's what is going on in your particular case,
> > but I think tracepoints are your friend here. ;) E.g., 'trace-cmd record
> > -e xfs:xfs_ilock' for the ilock, perhaps others for more context if
> > necessary..
> 
> Here's what trace-cmd reported. I'm tracing xfs_ilock, xfs_iunlock, and
> sched_switch:
> 
>       syscall-14-12006 [007] 108264.972883: xfs_ilock: dev 9:0 ino
> 0x10008c9a flags ILOCK_SHARED caller xfs_file_fsync
> <snip>
>       reactor-14-11979 [007] 108264.973292: xfs_iunlock: dev 9:0 ino
> 0xa04d64c1 flags IOLOCK_SHARED caller xfs_file_dio_aio_read
>       reactor-14-11979 [007] 108264.973293: xfs_ilock: dev 9:0 ino
> 0x10008c9a flags IOLOCK_SHARED caller xfs_file_dio_aio_write
>       reactor-14-11979 [007] 108264.973296: xfs_ilock: dev 9:0 ino
> 0x10008c9a flags ILOCK_EXCL caller xfs_vn_update_time
>       reactor-14-11979 [007] 108264.973300: sched_switch: reactor-14:11979
> [120] D ==> kworker/7:1H:1350 [100]
> <snip>
>       syscall-14-12006 [007] 108265.015795: xfs_iunlock: dev 9:0 ino
> 0x10008c9a flags ILOCK_SHARED caller xfs_file_fsync
> 
> 
> Is IOLOCK_SHARED mutually exclusive with ILOCK_EXCL? I'm guessing not.
> 

Nope, they are separate locks.

Brian

> 
> Looks like fsync clashed with io_submit() here, which wasn't supposed to
> happen in my code, they ought to be mutually exclusive.
> 
> 
> > > I'm seeing 200ms stalls, so my guess is a log flush is involved.
> > > 
> > > 
> > > Is this lock contention covered by RWF_NOWAIT?
> > > 
> > I don't think so. It looked to me that RWF_NOWAIT basically just skips
> > allocations..
> > 
> > Brian
> > 
> > > --
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