netdev - Re: CONFIG_XPS depends on L1_CACHE_BYTES being greater than sizeof(struct xps

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Date:	Fri, 23 Oct 2015 15:16:17 -0700
From:	Tom Herbert <tom@...bertland.com>
To:	Helge Deller <deller@....de>
Cc:	Tom Herbert <therbert@...gle.com>,
	"David S. Miller" <davem@...emloft.net>,
	Linux Kernel Network Developers <netdev@...r.kernel.org>,
	linux-parisc List <linux-parisc@...r.kernel.org>,
	James Bottomley <James.Bottomley@...senpartnership.com>,
	John David Anglin <dave.anglin@...l.net>
Subject: Re: CONFIG_XPS depends on L1_CACHE_BYTES being greater than
 sizeof(struct xps_map)

On Fri, Oct 23, 2015 at 12:21 PM, Helge Deller <deller@....de> wrote:
> On 22.10.2015 23:37, Tom Herbert wrote:
>> On Thu, Oct 22, 2015 at 1:00 PM, Helge Deller <deller@....de> wrote:
>>> Hi Tom & David,
>>>
>>> I've queued-up a patch for the parisc architecture which reduces L1_CACHE_BYTES from 32 to 16:
>>>   https://patchwork.kernel.org/patch/7399291/
>>>
>>> But this change will break the kernel build like this:
>>>
>>> In file included from net/core/dev.c:92:0:
>>> net/core/dev.c: In function ‘expand_xps_map’:
>>> include/linux/netdevice.h:721:27: warning: overflow in implicit constant conversion [-Woverflow]
>>>    #define XPS_MIN_MAP_ALLOC ((L1_CACHE_BYTES - sizeof(struct xps_map)) \
>>> net/core/dev.c:1972:18: note: in expansion of macro ‘XPS_MIN_MAP_ALLOC’
>>>    int alloc_len = XPS_MIN_MAP_ALLOC;
>>>
>>> Do you see an easy way to fix this ?
>>>
>> How about
>>
>>  #define XPS_MIN_MAP_ALLOC ((L1_CACHE_BYTES - ((sizeof(struct xps_map)
>> % L1_CACHE_BYTES)) \
>
> The full line would then be:
> #define XPS_MIN_MAP_ALLOC ((L1_CACHE_BYTES - (sizeof(struct xps_map) % L1_CACHE_BYTES)) / sizeof(u16))
>
> The only problem I see with this is, that XPS_MIN_MAP_ALLOC might become zero.
> In that case the call to kzalloc_node() in expand_xps_map() doesn't allocate any memory for the queues.
>
I believe this wouldn't ever be zero (add 1 in to avoid result is 1 /
2 possibility):

#define XPS_MIN_MAP_ALLOC ((L1_CACHE_BYTES + 1 - (sizeof(struct
xps_map) % L1_CACHE_BYTES)) / sizeof(u16))

Tom

> Helge
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