Re: AW: Slow I/O on USB media after commit f664a3cc17b7d0a2bc3b3ab96181e1029b0ec0e6

[Ming Lei]

On Tue, Dec 10, 2019 at 08:35:43AM +0100, Andrea Vai wrote:
> Il giorno mar, 03/12/2019 alle 10.23 +0800, Ming Lei ha scritto:
> > On Fri, Nov 29, 2019 at 03:41:01PM +0100, Andrea Vai wrote:
> > > Il giorno ven, 29/11/2019 alle 10.35 +0800, Ming Lei ha scritto:
> > > > On Fri, Nov 29, 2019 at 08:57:34AM +0800, Ming Lei wrote:
> > > > 
> > > > > [...]
> > > > 
> > > > > Andrea, can you collect the following log when running the
> > test
> > > > > on current new(bad) kernel?
> > > > > 
> > > > > 	/usr/share/bcc/tools/stackcount  -K
> > blk_mq_make_request
> > > > 
> > > > Instead, please run the following trace, given insert may be
> > > > called from other paths, such as flush plug:
> > > > 
> > > > 	/usr/share/bcc/tools/stackcount -K t:block:block_rq_insert
> > > 
> > > Attached, for new (patched) bad kernel.
> > > 
> > > Produced by: start the trace script (with the pendrive already
> > > plugged), wait some seconds, run the test (1 trial, 1 GB), wait
> > for
> > > the test to finish, stop the trace.
> > > 
> > > The copy took ~1700 seconds.
> > 
> > See the two path[1][2] of inserting request, and path[1] is
> > triggered
> > 4358 times, and the path[2] is triggered 5763 times.
> > 
> > The path[2] is expected behaviour. Not sure path [1] is correct,
> > given
> > ext4_release_file() is supposed to be called when this inode is
> > released. That means the file is closed 4358 times during 1GB file
> > copying to usb storage.
> > 
> > Cc filesystem list.
> > 
> > 
> > [1] insert requests when returning to user mode from syscall
> > 
> >   b'blk_mq_sched_request_inserted'
> >   b'blk_mq_sched_request_inserted'
> >   b'dd_insert_requests'
> >   b'blk_mq_sched_insert_requests'
> >   b'blk_mq_flush_plug_list'
> >   b'blk_flush_plug_list'
> >   b'io_schedule_prepare'
> >   b'io_schedule'
> >   b'rq_qos_wait'
> >   b'wbt_wait'
> >   b'__rq_qos_throttle'
> >   b'blk_mq_make_request'
> >   b'generic_make_request'
> >   b'submit_bio'
> >   b'ext4_io_submit'
> >   b'ext4_writepages'
> >   b'do_writepages'
> >   b'__filemap_fdatawrite_range'
> >   b'ext4_release_file'
> >   b'__fput'
> >   b'task_work_run'
> >   b'exit_to_usermode_loop'
> >   b'do_syscall_64'
> >   b'entry_SYSCALL_64_after_hwframe'
> >     4358
> > 
> > [2] insert requests from writeback wq context
> > 
> >   b'blk_mq_sched_request_inserted'
> >   b'blk_mq_sched_request_inserted'
> >   b'dd_insert_requests'
> >   b'blk_mq_sched_insert_requests'
> >   b'blk_mq_flush_plug_list'
> >   b'blk_flush_plug_list'
> >   b'io_schedule_prepare'
> >   b'io_schedule'
> >   b'rq_qos_wait'
> >   b'wbt_wait'
> >   b'__rq_qos_throttle'
> >   b'blk_mq_make_request'
> >   b'generic_make_request'
> >   b'submit_bio'
> >   b'ext4_io_submit'
> >   b'ext4_bio_write_page'
> >   b'mpage_submit_page'
> >   b'mpage_process_page_bufs'
> >   b'mpage_prepare_extent_to_map'
> >   b'ext4_writepages'
> >   b'do_writepages'
> >   b'__writeback_single_inode'
> >   b'writeback_sb_inodes'
> >   b'__writeback_inodes_wb'
> >   b'wb_writeback'
> >   b'wb_workfn'
> >   b'process_one_work'
> >   b'worker_thread'
> >   b'kthread'
> >   b'ret_from_fork'
> >     5763
> > 
> > Thanks,
> > Ming
> > 
> 
> Is there any update on this? Sorry if I am making noise, but I would
> like to help to improve the kernel (or fix it) if I can help.
> Otherwise, please let me know how to consider this case,

IMO, the extra write path from exit_to_usermode_loop() isn't expected,
that should be the reason why write IO order is changed, then performance
drops on your USB storage.

We need our fs/ext4 experts to take a look.

Or can you reproduce the issue on xfs or btrfs?

Thanks,
Ming