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

[Andrea Vai]

Il giorno gio, 12/12/2019 alle 05.33 +0800, Ming Lei ha scritto:
> On Wed, Dec 11, 2019 at 11:07:45AM -0500, Theodore Y. Ts'o wrote:
> > On Wed, Dec 11, 2019 at 12:00:58PM +0800, Ming Lei wrote:
> > > I didn't reproduce the issue in my test environment, and follows
> > > Andrea's test commands[1]:
> > > 
> > >   mount UUID=$uuid /mnt/pendrive 2>&1 |tee -a $logfile
> > >   SECONDS=0
> > >   cp $testfile /mnt/pendrive 2>&1 |tee -a $logfile
> > >   umount /mnt/pendrive 2>&1 |tee -a $logfile
> > > 
> > > The 'cp' command supposes to open/close the file just once,
> however
> > > ext4_release_file() & write pages is observed to run for 4358
> times
> > > when executing the above 'cp' test.
> > 
> > Why are we sure the ext4_release_file() / _fput() is coming from
> the
> > cp command, as opposed to something else that might be running on
> the
> > system under test?  _fput() is called by the kernel when the last
> 
> Please see the log:
> 
> https://lore.kernel.org/linux-scsi/3af3666920e7d46f8f0c6d88612f143ffabc743c.camel@xxxxxxxx/2-log_ming.zip
> 
> Which is collected by:
> 
> #!/bin/sh
> MAJ=$1
> MIN=$2
> MAJ=$(( $MAJ << 20 ))
> DEV=$(( $MAJ | $MIN ))
> 
> /usr/share/bcc/tools/trace -t -C \
>     't:block:block_rq_issue (args->dev == '$DEV') "%s %d %d", args-
> >rwbs, args->sector, args->nr_sector' \
>     't:block:block_rq_insert (args->dev == '$DEV') "%s %d %d", args-
> >rwbs, args->sector, args->nr_sector'
> 
> $MAJ:$MIN points to the USB storage disk.
> 
> From the above IO trace, there are two write paths, one is from cp,
> another is from writeback wq.
> 
> The stackcount trace[1] is consistent with the IO trace log since it
> only shows two IO paths, that is why I concluded that the write done
> via
> ext4_release_file() is from 'cp'.
> 
> [1] 
> https://lore.kernel.org/linux-scsi/320b315b9c87543d4fb919ecbdf841596c8fbcea.camel@xxxxxxxx/2-log_ming_20191129_150609.zip
> 
> > reference to a struct file is released.  (Specifically, if you
> have a
> > fd which is dup'ed, it's only when the last fd corresponding to
> the
> > struct file is closed, and the struct file is about to be
> released,
> > does the file system's f_ops->release function get called.)
> > 
> > So the first question I'd ask is whether there is anything else
> going
> > on the system, and whether the writes are happening to the USB
> thumb
> > drive, or to some other storage device.  And if there is something
> > else which is writing to the pendrive, maybe that's why no one
> else
> > has been able to reproduce the OP's complaint....
> 
> OK, we can ask Andrea to confirm that via the following trace, which
> will add pid/comm info in the stack trace:
> 
> /usr/share/bcc/tools/stackcount  blk_mq_sched_request_inserted
> 
> Andrea, could you collect the above log again when running new/bad
> kernel for confirming if the write done by ext4_release_file() is
> from
> the 'cp' process?

Yes, I will try to do it as soon as possible and let you know.
I will also try xfs or btrfs, as you suggested in another message.

Thanks, and bye
Andrea