linux-kernel - Re: Threaded interrupt handling question in RT kernel

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Message-ID: <alpine.LFD.2.02.1108161729130.2807@ionos>
Date:	Tue, 16 Aug 2011 17:31:50 +0200 (CEST)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Jaccon Bastiaansen <jaccon.bastiaansen@...il.com>
cc:	Peter Zijlstra <peterz@...radead.org>,
	linux-rt-users <linux-rt-users@...r.kernel.org>,
	linux-kernel@...r.kernel.org
Subject: Re: Threaded interrupt handling question in RT kernel

On Tue, 16 Aug 2011, Jaccon Bastiaansen wrote:

> Hello Thomas, Peter
> 
> We have a question about threaded interrupt handling:
> 
> By default, when using threaded interrupt handlers, the IRQ is
> disabled in hard IRQ context and enabled again after all threaded
> interrupt handlers connected to that IRQ have run. In this way, high
> priority interrupt handlers can be delayed until the lowest priority
> interrupt handler thread has run. Therefore it seems that it's not
> useful to have a separate thread for each interrupt handler (what's
> the use of being able to set interrupt handler thread priorities when
> you still have to wait for the one with the lowest priority).

That's correct, but RT just makes use of the general facility which is
designed to have a separate thread for each device.
 
> So we think that we should use the request_threaded_irq() function.

You can do that for a specific driver, but we cannot do that in RT for
every driver in the kernel.

> The task of the handler that is executed in hard IRQ context is to
> check whether the device that it controls is generating an interrupt
> and if it does, deactivate the IRQ output of the device and wakeup the
> interrupt handler thread by returning IRQ_WAKE_THREAD. By deactivating
> the IRQ output, another device connected to the same IRQ can activate
> the IRQ again before the interrupt handler thread of the first device
> has run. This guarantees that a high priority threaded interrupt
> handler of a device on a shared IRQ can run before a low priority
> threaded interrupt handler of a device on the same IRQ has run. So
> when using threaded interrupt handlers for devices on a shared IRQ,
> make sure that all drivers have used request_threaded_irq().
> Otherwise, high priority threaded interrupt handlers can be delayed by
> low priority threaded interrupt handlers.
> 
> Is all this correct or do we miss something?

That's how it's designed to work.

Thanks,

	tglx
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