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]. 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... [1] https://marc.info/?l=linux-fsdevel&m=151688776319077 Honza -- Jan Kara <jack@xxxxxxxx> SUSE Labs, CR
