Re: [dpdk-dev] [PATCH] ring: relax alignment constraint on ring structure

["Burakov, Anatoly" <anatoly.burakov@intel.com>]

On 25-May-18 11:59 AM, Burakov, Anatoly wrote:
> On 03-Apr-18 2:26 PM, Olivier Matz wrote:
>> The initial objective of
>> commit d9f0d3a1ffd4 ("ring: remove split cacheline build setting")
>> was to add an empty cache line betwee, the producer and consumer
>> data (on platform with cache line size = 64B), preventing from
>> having them on adjacent cache lines.
>>
>> Following discussion on the mailing list, it appears that this
>> also imposes an alignment constraint that is not required.
>>
>> This patch removes the extra alignment constraint and adds the
>> empty cache lines using padding fields in the structure. The
>> size of rte_ring structure and the offset of the fields remain
>> the same on platforms with cache line size = 64B:
>>
>>    rte_ring = 384
>>    rte_ring.name = 0
>>    rte_ring.flags = 32
>>    rte_ring.memzone = 40
>>    rte_ring.size = 48
>>    rte_ring.mask = 52
>>    rte_ring.prod = 128
>>    rte_ring.cons = 256
>>
>> But it has an impact on platform where cache line size is 128B:
>>
>>    rte_ring = 384        -> 768
>>    rte_ring.name = 0
>>    rte_ring.flags = 32
>>    rte_ring.memzone = 40
>>    rte_ring.size = 48
>>    rte_ring.mask = 52
>>    rte_ring.prod = 128   -> 256
>>    rte_ring.cons = 256   -> 512
>>
>> Link: http://dpdk.org/dev/patchwork/patch/25039/
>> Suggested-by: Konstantin Ananyev <konstantin.ananyev@intel.com>
>> Signed-off-by: Olivier Matz <olivier.matz@6wind.com>
>> ---
> 
> This patch causes eal_flags_autotest to hang on FreeBSD. Root cause at 
> this time is unknown, but this is a weird one - the test seems to hang 
> while doing read() in bsd/eal_thread.c:59. Reverting this patch on top 
> of rc5 results in eal_flags_autotest passing.
> 

Further investigation shows that for some reason, if Enter is pressed 
while the test is seemingly "hung", it continues and passes. Or rather, 
it hangs on one test, and if Enter is pressed, it finishes that test and 
hangs on another, after which pressing Enter again will result in test 
succeeding. Weird...

-- 
Thanks,
Anatoly