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

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Date:	Fri, 9 May 2014 15:46:14 +0000
From:	David Laight <David.Laight@...LAB.COM>
To:	'Neil Horman' <nhorman@...driver.com>
CC:	"netdev@...r.kernel.org" <netdev@...r.kernel.org>,
	"davem@...emloft.net" <davem@...emloft.net>
Subject: RE: [RFC PATCH] net: Provide linear backoff mechanism for
 constrained resources at the driver

From: Neil Horman [mailto:nhorman@...driver.com]
...
> 2 Things:
> 
> 1) Need is really a strong term here.  The penalty for failing a dma mapping is
> to drop the frame.  Thats not unacceptible in many use cases.

Indeed, but dropping an ethernet frame will be recovered by the higher layers.
While not ideal, it is a 'last resort' action.
Note that I'm not suggesting that your deferred retry of the transmit isn't
a good idea, just that it is probably papering over the cracks.

> 2) It seems to me that global constraint here implies a static, well known
> number.  While its true we can interrogate an iommu, and compare its mapping
> size to the ring size of all the NICS/devices on a system to see if we're likely
> to exceed the iommu space available, we shouldn't do that.  If a given NIC
> doesn't produce much traffic, its ring sizes aren't relevant to the computation.

An idle NIC will be using a lot of iommu entries for its receive buffers.

> We're not trying to address a static allocation scheme here.  If a system boots,
> it implies that all the recive rings on all the devices were able to reserve the
> amount of space they needed in the iommu (as you note earlier, they populate
> their rings on init, effectively doing a iommu reservation).  The problem we're
> addressing is the periodic lack of space that arises from temporary exhaustion
> of iommu space under heavy I/O loads.  We won't know if that happens, until it
> happens, and we can't just allocate for the worst case, because then we're sure
> to run out of space as devices scale up.  Sharing is the way to do this whenever
> possible.

Do you have any data for which drivers have active iommu entries when an
allocate fails?

I can imagine systems where almost all the iommu entries are being used
for ethernet rx buffers, and everything else is fighting for the last
few entries.

	David



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