netdev - Re: [PATCH 0/2] net: davinci_cpdma: reduce latency on -rt

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Message-ID: <20160727070302.GG5368@pengutronix.de>
Date:	Wed, 27 Jul 2016 09:03:02 +0200
From:	Uwe Kleine-König 
	<u.kleine-koenig@...gutronix.de>
To:	Grygorii Strashko <grygorii.strashko@...com>
Cc:	Mugunthan V N <mugunthanvnm@...com>, linux-omap@...r.kernel.org,
	netdev@...r.kernel.org, kernel@...gutronix.de
Subject: Re: [PATCH 0/2] net: davinci_cpdma: reduce latency on -rt

On Tue, Jul 26, 2016 at 05:36:49PM +0300, Grygorii Strashko wrote:
> On 07/26/2016 03:02 PM, Uwe Kleine-König wrote:
> >Hello,
> >
> >these patches are based on next-20160726. I didn't check yet how latency
> >improves by using these patches, but even if the improvment is small,
> >it's still a good idea to have them.
> 
> Sry, but how this will affect on -RT? This is not a raw locks, so
> they will be converted to rt-mutexes which are sleepable.
> Or I've missed smth?

They are still locks after all. On -rt I saw for the relevant
application:

  send package         |
    take lock          |
    write pckt to hw   |
                       | rcv irq
		       |   take lock
		       |     schedule
    drop lock	       | 
      schedule         |
                       |   get pckt from hw
		       |   drop lock

So reducing the time a lock is taken reduces the chances that the lock
is contended for another thread which results in extra context switches.

Best regards
Uwe

-- 
Pengutronix e.K.                           | Uwe Kleine-König            |
Industrial Linux Solutions                 | http://www.pengutronix.de/  |