Subject: Re: CONFIG_XPS depends on L1_CACHE_BYTES being greater than sizeof(struct xps_map)

From: Alexander Duyck <[email-masked]>
Date: Fri, 23 Oct 2015 15:00:37 -0700

On 10/23/2015 02:08 PM, Helge Deller wrote:
> * Eric Dumazet <[email-masked]>:
>> On Fri, 2015-10-23 at 21:25 +0200, Helge Deller wrote:
>>
>>> Then, how about simply changing it to twice of L1_CACHE_BYTES ?
>>>
>>> #define XPS_MIN_MAP_ALLOC ((L1_CACHE_BYTES * 2 - sizeof(struct xps_map)) / sizeof(u16))
>>
>>
>> Seems good to me.
>
> Great!
>
> Can you then maybe give me an Acked-by or signed-off for the patch below?
> It further adds a compile-time check to avoid that XPS_MIN_MAP_ALLOC
> gets calculated to zero on any architecture - otherwise no queues would
> be allocated.
>
> In addition I would like to push it for v4.3 then through my parisc-tree
> (after keeping it in for-next for 1-2 days), together with the patch
> which reduces L1_CACHE_BYTES to 16 on parisc.
> Would that be OK too?
>
> Thanks!
> Helge
>
>
> [PATCH] net/xps: Increase initial number of xps queues
>
> Increase the number of initial allocated xps queues, so that the initial record
> allocates twice the size of L1_CACHE_BYTES bytes.
>
> This change is needed to copy with architectures where L1_CACHE_BYTES is
> defined to equal or less than 16 bytes.
>
> Signed-off-by: Helge Deller <[email-masked]>
>
> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
> index 2d15e38..d152788 100644
> --- a/include/linux/netdevice.h
> +++ b/include/linux/netdevice.h
> @@ -718,7 +718,7 @@ struct xps_map {
>   	u16 queues[0];
>   };
>   #define XPS_MAP_SIZE(_num) (sizeof(struct xps_map) + ((_num) * sizeof(u16)))
> -#define XPS_MIN_MAP_ALLOC ((L1_CACHE_BYTES - sizeof(struct xps_map))	\
> +#define XPS_MIN_MAP_ALLOC ((L1_CACHE_BYTES * 2 - sizeof(struct xps_map)) \
>       / sizeof(u16))
>
>   /*
> diff --git a/net/core/dev.c b/net/core/dev.c
> index 6bb6470..f6d6dd1 100644
> --- a/net/core/dev.c
> +++ b/net/core/dev.c
> @@ -1972,6 +1972,8 @@ static struct xps_map *expand_xps_map(struct xps_map *map,
>   	int alloc_len = XPS_MIN_MAP_ALLOC;
>   	int i, pos;
>
> +	BUILD_BUG_ON(XPS_MIN_MAP_ALLOC == 0);
> +
>   	for (pos = 0; map && pos < map->len; pos++) {
>   		if (map->queues[pos] != index)
>   			continue;
>
>

Rather then leaving a potential bug you could probably rewrite the macro 
so that it will give you at least 1.

All you need to do is something like the following
#define XPS_MIN_MAP_ALLOC \
	((L1_CACHE_ALIGN(offsetof(struct xps_map, queue[1])) - \
	  sizeof(struct xps_map)) / sizeof(u16))

That should give you at least an XPS_MIN_MAP_ALLOC of 1.

- Alex