netdev - Re: [RFC PATCH] net: Provide linear backoff mechanism for constrained resources at the driver

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Date:	Wed, 21 May 2014 11:05:28 -0700
From:	Cong Wang <cwang@...pensource.com>
To:	Neil Horman <nhorman@...driver.com>
Cc:	netdev <netdev@...r.kernel.org>, David Miller <davem@...emloft.net>
Subject: Re: [RFC PATCH] net: Provide linear backoff mechanism for constrained
 resources at the driver

On Thu, May 8, 2014 at 12:29 PM, Neil Horman <nhorman@...driver.com> wrote:
> What about something like this?  Its not even compile tested, but let me know
> what you think of the idea.  The reasoning behind it is that transient resources
> like dma address ranges in the iommu or swiotlb have the following attributes
>
> 1) they are quickly allocated and freed
>
> 2) Usually handled by simply trying again at some later point in time
>
> 3) Likely to fail again if tried again immediately.
>
> 4) Not condusive to interlocked signaling mechanisms as the time it takes to
> find and signal a waiting tx queue that resources are now available may take
> longer than needed, and may still result in failure, as competing allocators
> may race in and claim said resources during the signaling period.
>
> As such, what if we try something more simple like a linear backoff? In this
> example we add a TX return code that indicates that the driver is not busy, but
> unable to transmit due to resource constraints.  This signals the qdisc layer to
> skip trying to drain this transmit queue for a short period of time, with
> subsequent simmilar errors causing increased backoff.  Once the condition is
> cleared, the backoff delay is removed and operation returns to normal.

Loos like this is a more general issue which should be solved for every
spinlock:

http://thread.gmane.org/gmane.linux.kernel/1437186
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