netdev - Re: [PATCH 2/2] lib: cpu_rmap: CPU affinity reverse-mapping

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Message-ID: <1294178690.3636.49.camel@bwh-desktop>
Date:	Tue, 04 Jan 2011 22:04:50 +0000
From:	Ben Hutchings <bhutchings@...arflare.com>
To:	Eric Dumazet <eric.dumazet@...il.com>
Cc:	Thomas Gleixner <tglx@...utronix.de>,
	David Miller <davem@...emloft.net>,
	Tom Herbert <therbert@...gle.com>,
	linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
	linux-net-drivers@...arflare.com
Subject: Re: [PATCH 2/2] lib: cpu_rmap: CPU affinity reverse-mapping

On Tue, 2011-01-04 at 22:45 +0100, Eric Dumazet wrote:
> Le mardi 04 janvier 2011 à 21:23 +0000, Ben Hutchings a écrit :
> > On Tue, 2011-01-04 at 22:17 +0100, Eric Dumazet wrote:
> > > Le mardi 04 janvier 2011 à 19:39 +0000, Ben Hutchings a écrit :
> > > > When initiating I/O on a multiqueue and multi-IRQ device, we may want
> > > > to select a queue for which the response will be handled on the same
> > > > or a nearby CPU.  This requires a reverse-map of IRQ affinity.  Add
> > > > library functions to support a generic reverse-mapping from CPUs to
> > > > objects with affinity and the specific case where the objects are
> > > > IRQs.
> > [...]
> > > > +/**
> > > > + * struct cpu_rmap - CPU affinity reverse-map
> > > > + * @near: For each CPU, the index and distance to the nearest object,
> > > > + *      based on affinity masks
> > > > + * @size: Number of objects to be reverse-mapped
> > > > + * @used: Number of objects added
> > > > + * @obj: Array of object pointers
> > > > + */
> > > > +struct cpu_rmap {
> > > > +	struct {
> > > > +		u16     index;
> > > > +		u16     dist;
> > > > +	} near[NR_CPUS];
> > > 
> > > This [NR_CPUS] is highly suspect.
> > > 
> > > Are you sure you cant use a per_cpu allocation here ?
> > 
> > I think that would be a waste of space in shared caches, as this is
> > read-mostly.
> 
> This is slow path, unless I dont understood the intent.

get_rps_cpu() will need to read from an arbitrary entry in cpu_rmap (not
the current CPU's entry) for each new flow and for each flow that went
idle for a while.  That's not fast path but it is part of the data path,
not the control path.

> Cache lines dont matter. I was not concerned about speed but memory
> needs.
> 
> NR_CPUS can be 4096 on some distros, that means a 32Kbyte allocation.
> 
> Really, you'll have to have very strong arguments to introduce an
> [NR_CPUS] array in the kernel today.

I could replace this with a pointer to an array of size
num_possible_cpus().  But I think per_cpu is wrong here.

Ben.

-- 
Ben Hutchings, Senior Software Engineer, Solarflare Communications
Not speaking for my employer; that's the marketing department's job.
They asked us to note that Solarflare product names are trademarked.

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