linux-kernel - Re: linux-next: workqueues tree build failure

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Date:	Thu, 26 Nov 2009 10:24:30 +0100
From:	Takashi Iwai <tiwai@...e.de>
To:	Tejun Heo <tj@...nel.org>
Cc:	Peter Ujfalusi <peter.ujfalusi@...ia.com>,
	Stephen Rothwell <sfr@...b.auug.org.au>,
	"linux-next@...r.kernel.org" <linux-next@...r.kernel.org>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
	Mark Brown <broonie@...nsource.wolfsonmicro.com>
Subject: Re: linux-next: workqueues tree build failure

At Thu, 26 Nov 2009 18:12:26 +0900,
Tejun Heo wrote:
> 
> Hello,
> 
> 11/26/2009 05:16 PM, Peter Ujfalusi wrote:
> >> Takashi, RT workqueue is going away.  Do you really need it?
> > 
> > What can be used instead of RT workqueue?
> > The tlv320dac33 needs RT workqueue because I need to send the I2C
> > command with minimum delay to the codec. If this can not be done
> > (the workqueue is delayed), and the codec does not receive the
> > command in time, it will literally die.  What are the options to
> > replace the RT workqueue?
> 
> The problem with RT workqueue is that RT and queue don't really mix
> well.  To act in real time, it requires all the resource pre-allocated
> and dedicated to it making queueing or pooling meaningless.  The
> original workqueue code created dedicated pool of threads for each
> workqueue so it could be used for RT but new implementation uses
> shared worker pool, so it can't be used as an interface to dedicated
> threads.
> 
> I haven't read the code but,
> 
> * If you need to respond fast, wouldn't you be doing that from IRQ
>   handler or softirq?  Do you need task context?
> 
> * Or is it that it's not triggered by IRQ but once the transfer
>   started it can't be interrupted?  But in this case preempt_disable()
>   or local_irq_disable() should suffice.

The relevant code uses the workqueue as a sort of BH, just triggers
from the hard irq handler.  If any, we may use a threaded handler or
so...


thanks,

Takashi
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