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

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Date:	Tue, 04 Jan 2011 21:23:36 +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 22:17 +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.

> > +	u16		size, used;
> > +	void		*obj[0];
> > +};
> > +#define CPU_RMAP_DIST_INF 0xffff
> > +
> 
> 
> > +
> > +/**
> > + * alloc_cpu_rmap - allocate CPU affinity reverse-map
> > + * @size: Number of objects to be mapped
> > + * @flags: Allocation flags e.g. %GFP_KERNEL
> > + */
> 
> I really doubt you need other than GFP_KERNEL. (Especially if you switch
> to per_cpu alloc ;) )
[...]

I agree, but this is consistent with ~all other allocation functions.

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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