linux-kernel - Re: [PATCH] uio: Allow to take irq bottom-half into irq

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Date:   Wed, 6 Dec 2017 16:31:13 +0100
From:   Greg KH <gregkh@...uxfoundation.org>
To:     Andrey Zhizhikin <andrey.z@...il.com>
Cc:     linux-kernel@...r.kernel.org
Subject: Re: [PATCH] uio: Allow to take irq bottom-half into irq_handler with
 additional dt-binding

On Wed, Dec 06, 2017 at 03:55:40PM +0100, Andrey Zhizhikin wrote:
> Certain Kernel preemption models are using threaded interrupt handlers,
> which is in general quite beneficial. However, threaded handlers
> introducing additional scheduler overhead, when the bottom-half thread
> should be woken up and scheduled for execution. This can result is
> additional latency, which in certain cases is not desired.
> 
> UIO driver with Generic IRQ handler, that wraps a HW block might suffer
> a small degradation when it's bottom half is executed, since it needs
> its bottom half to be woken up by the scheduler every time INT is
> delivered. For high rate INT signals, this also bring additional
> undesired load on the scheduler itself.
> 
> Since the actual ACK is performed in the top-half, and bottom-half of
> the UIO driver with Generic IRQ handler is relatively slick (only flag
> is set based on the INT reception), it might be beneficial to move this
> bottom-half to the irq_handler itself, rather than to have a separate
> thread to service it.
> 
> This patch aims to address the task above, and in addition introduces
> a new dt-binding which could be configured on a per-node basis. That
> means developers utilizing the UIO driver could decide which UIO
> instance is critical in terms of interrupt processing, and move their
> corresponding bottom-halves to the irq_handler to fight additional
> scheduling latency.
> 
> New DT binding:
> - no-threaded-irq: when present, request_irq() is called with
>   IRQF_NO_THREAD flag set, effectively skipping threaded interrupt
>   handler and taking bottom-half into irq_handler
> 
> Signed-off-by: Andrey Zhizhikin <andrey.z@...il.com>

For new DT bindings, don't you have to add them to the in-kernel
documentation and get an ack from the DT maintainers?  Please do that
here.

ALso, how much does this really save in latency/delay by not allowing a
threaded irq?  What about systems that run all irqs in threaded mode?
Will that break something here?

thanks,

greg k-h