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

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Message-ID: <20191202070135.GA4634@brian.unipv.it>
Date:   Mon, 2 Dec 2019 08:01:35 +0100
From:   Andrea Vai <andrea.vai@...pv.it>
To:     Bernd Schubert <bs_lists@...ef.fastmail.fm>
Cc:     Ming Lei <ming.lei@...hat.com>,
        "Schmid, Carsten" <Carsten_Schmid@...tor.com>,
        Finn Thain <fthain@...egraphics.com.au>,
        Damien Le Moal <Damien.LeMoal@....com>,
        Alan Stern <stern@...land.harvard.edu>,
        Jens Axboe <axboe@...nel.dk>,
        Johannes Thumshirn <jthumshirn@...e.de>,
        USB list <linux-usb@...r.kernel.org>,
        SCSI development list <linux-scsi@...r.kernel.org>,
        Himanshu Madhani <himanshu.madhani@...ium.com>,
        Hannes Reinecke <hare@...e.com>,
        Omar Sandoval <osandov@...com>,
        "Martin K. Petersen" <martin.petersen@...cle.com>,
        Greg KH <gregkh@...uxfoundation.org>,
        Hans Holmberg <Hans.Holmberg@....com>,
        Kernel development list <linux-kernel@...r.kernel.org>
Subject: Re: AW: Slow I/O on USB media after commit
 f664a3cc17b7d0a2bc3b3ab96181e1029b0ec0e6

On 29/11/19 12:44:53, Bernd Schubert wrote:
> >> Trace attached. 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 73 seconds, roughly as already seen before with the fast
> >> old kernel.
> > 
> > This trace shows a good write IO order because the writeback IOs are
> > queued to block layer serially from the 'cp' task and writeback wq.
> > 
> > However, writeback IO order is changed in current linus tree because
> > the IOs are queued to block layer concurrently from the 'cp' task
> > and writeback wq. It might be related with killing queue_congestion
> > by blk-mq.
> 
> What about using direct-io to ensure order is guaranteed? Pity that 'cp'
> doesn't seem to have an option for it. But dd should do the trick.
> Andrea, can you replace cp with a dd command (on the slow kernel)?
> 
> dd if=<path-to-src-file> of=<path-to-copy-on-flash-device> bs=1M
> oflag=direct

On the "new bad patched" kernel, this command take 68 seconds to complete (mean on 100 trials, with a narrow standard deviation), so perfectly
aligned with the cp command on the old fast good kernel.

Thanks, and bye
Andrea