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

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Date:   Fri, 29 Nov 2019 12:44:53 +0100
From:   Bernd Schubert <bs_lists@...ef.fastmail.fm>
To:     Ming Lei <ming.lei@...hat.com>, Andrea Vai <andrea.vai@...pv.it>
Cc:     "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

>> 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

 - Bernd