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

From: Helge Deller <[email-masked]>
Date: Sat, 24 Oct 2015 00:17:36 +0200

On 24.10.2015 00:00, Alexander Duyck wrote:
> 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.

Yes, good idea!
What makes me wonder though (because I have no idea about the XPS code/layer):
How likely is it, that more than 1 (e.g. minimum "X") queues are needed? 
E.g. if a typical system needs at least 3 queues, then doesn't it make sense to allocate
at least 3 initially by using queue[3] in your proposed patch above ?
What would "X" be then?

Helge