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..

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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