On Wed, Sep 12, 2018 at 02:11:30PM +0200, Jan Kara wrote: > Hi! > > On Tue 11-09-18 17:10:55, Liu Bo wrote: > > With ext4's data=ordered mode and the underlying blk throttle setting, we > > can easily run to hang, > > > > 1. > > mount /dev/sdc /mnt -odata=ordered > > 2. > > mkdir /sys/fs/cgroup/unified/cg > > 3. > > echo "+io" > /sys/fs/cgroup/unified/cgroup.subtree_control > > 4. > > echo "`cat /sys/block/sdc/dev` wbps=$((1 << 20))" > /sys/fs/cgroup/unified/cg/io.max > > 5. > > echo $$ > /sys/fs/cgroup/unified/cg/cgroup.procs > > 6. > > // background dirtier > > xfs_io -f -c "pwrite 0 1G" $M/dummy & > > 7. > > echo $$ > /sys/fs/cgroup/unified/cgroup.procs > > 8. > > // issue synchronous IO > > for i in `seq 1 100`; > > do > > xfs_io -f -s -c "pwrite 0 4k" $M/foo > /dev/null > > done > > > > > > And the hang is like > > > > [jbd2-sdc] > > jbd2_journal_commit_transaction > > journal_submit_data_buffers > > # file 'dummy' has been written by writeback kthread > > journal_finish_inode_data_buffers > > # wait on page's writeback > > Yes, I guess you're speaking about the one Chris Mason mentioned [1]. Exactly. > Essentially it's a priority inversion where jbd2 thread gets blocked behind > writeback done on behalf of a heavily restricted process. It actually is > not related to dirty throttling or anything like that. And the solution for > this priority inversion is to use unwritten extents for writeback > unconditionally as I wrote in that thread. The core of this is implemented > and hidden behind dioread_nolock mount option but it needs some serious > polishing work and testing... Thank you so much for the details, so setting extent to unwritten and then converting it in endio does work and keeps the data=ordered semantic but I have to say the name, "dioread_nolock", is really confusing... thanks, -liubo > > [1] https://marc.info/?l=linux-fsdevel&m=151688776319077 > > Honza > -- > Jan Kara <jack@xxxxxxxx> > SUSE Labs, CR
