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

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

On 25-May-18 1:18 PM, Burakov, Anatoly wrote:
> 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...
> 

This patch is OK, the problem is in the test :) With this patch, it 
apparently blew up in just the right way, but really, the EAL flags 
autotest is wrong on FreeBSD because it launches primary processes with 
'--no-shconf' option, which corrupt main process's memory. For some 
reason this only began manifesting with this patch.

-- 
Thanks,
Anatoly