linux-kernel - Re: [PATCH] Fix data loss in cdc-acm

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Date:	Wed, 05 Aug 2015 13:36:35 -0400
From:	Peter Hurley <peter@...leysoftware.com>
To:	Sven Brauch <mail@...nbrauch.de>
CC:	Oliver Neukum <oneukum@...e.com>, Johan Hovold <johan@...nel.org>,
	Linux Kernel Mailing List <linux-kernel@...r.kernel.org>,
	One Thousand Gnomes <gnomes@...rguk.ukuu.org.uk>,
	Toby Gray <toby.gray@...lvnc.com>, linux-usb@...r.kernel.org,
	linux-serial@...r.kernel.org
Subject: Re: [PATCH] Fix data loss in cdc-acm

On 07/22/2015 06:53 PM, Sven Brauch wrote:
> Hi,
> 
> On 23/07/15 00:12, Peter Hurley wrote:
>> The premature unthrottle actually leads to the data loss but the throttling
>> with a mere 2K left is _way too late_.
> Ok, yes, I think so too.
> 
>> 10ms is a _really_ long time for a cpu not to attend to a kworker.

I haven't forgotten about this problem and I still plan to look into why
the long delay, particularly focusing on the scheduling latency from
tty_flip_buffer_push() => flush_to_ldisc().

Regards,
Peter Hurley

>> Which raises 2 questions:
>> 1. What are the termios settings of the tty receiving input? Is it 'raw'
>>    mode or typical terminal mode (icanon, echo, etc.) or something else?
> In my test code, I open the tty like
>   fd = open("/dev/ttyACM0", O_RDWR | O_NOCTTY | O_NONBLOCK);
> I don't make any other changes to the default settings. To be honest,
> I'm not sure in which mode it is operating then (I was assuming raw, but
> I might be wrong?).
> 
>> 2. Are there RT threads that are hogging cpu time?
> I can't see any, I think the only thing which occasionally goes to RT is
> pulseaudio (but during at least some of the tests I wasn't even playing
> audio, so that sounds very unplausible to me). I also cannot see a
> correlation between failure rate and CPU load.
> 
> Best regards,
> Sven
> 

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