Re: [PATCH v2] dmadev: fix structure alignment

[fengchengwen <fengchengwen@huawei.com>]

Hi Wenwu,

On 2024/3/15 15:44, Ma, WenwuX wrote:
> Hi Chengwen,
> 
>> -----Original Message-----
>> From: Ma, WenwuX
>> Sent: Friday, March 15, 2024 2:26 PM
>> To: fengchengwen <fengchengwen@huawei.com>; dev@dpdk.org
>> Cc: Jiale, SongX <songx.jiale@intel.com>; stable@dpdk.org
>> Subject: RE: [PATCH v2] dmadev: fix structure alignment
>>
>> Hi Chengwen,
>>
>>> -----Original Message-----
>>> From: fengchengwen <fengchengwen@huawei.com>
>>> Sent: Friday, March 15, 2024 2:06 PM
>>> To: Ma, WenwuX <wenwux.ma@intel.com>; dev@dpdk.org
>>> Cc: Jiale, SongX <songx.jiale@intel.com>; stable@dpdk.org
>>> Subject: Re: [PATCH v2] dmadev: fix structure alignment
>>>
>>> Hi Wenwu,
>>>
>>> On 2024/3/15 9:43, Wenwu Ma wrote:
>>>> The structure rte_dma_dev needs only 8 byte alignment.
>>>> This patch replaces __rte_cache_aligned of rte_dma_dev with
>>>> __rte_aligned(8).
>>>>
>>>> Fixes: b36970f2e13e ("dmadev: introduce DMA device library")
>>>> Cc: stable@dpdk.org
>>>>
>>>> Signed-off-by: Wenwu Ma <wenwux.ma@intel.com>
>>>> ---
>>>> v2:
>>>>  - Because of performance drop, adjust the code to
>>>>    no longer demand cache line alignment
>>>
>>> Which two versions observed performance drop? And which benchmark
>>> observed drop?
>>> Could you provide more information?
>>>
>>>>
>> V1 patch:
>> https://patches.dpdk.org/project/dpdk/patch/20240308053711.1260154-
>> 1-wenwux.ma@intel.com/
>>
>> To view detailed results, visit:
>> https://lab.dpdk.org/results/dashboard/patchsets/29472/
>>
>>>> ---
>>>>  lib/dmadev/rte_dmadev_pmd.h | 2 +-
>>>>  1 file changed, 1 insertion(+), 1 deletion(-)
>>>>
>>>> diff --git a/lib/dmadev/rte_dmadev_pmd.h
>>> b/lib/dmadev/rte_dmadev_pmd.h
>>>> index 58729088ff..b569bb3502 100644
>>>> --- a/lib/dmadev/rte_dmadev_pmd.h
>>>> +++ b/lib/dmadev/rte_dmadev_pmd.h
>>>> @@ -122,7 +122,7 @@ enum rte_dma_dev_state {
>>>>   * @internal
>>>>   * The generic data structure associated with each DMA device.
>>>>   */
>>>> -struct __rte_cache_aligned rte_dma_dev {
>>>> +struct __rte_aligned(8) rte_dma_dev {
>>>
>>> The DMA fast-path was implemented by struct rte_dma_fp_objs, which is
>>> not rte_dma_dev? So why is it a problem here?
>>>
>>> Thanks
>>>
>> The DMA device object is expected to align cache line, so clang will use
>> “vmovaps” assembly instruction,
>>
>> And the instruction demands 16 bytes alignment or will cause segment fault in
>> some environments.
>>
> Test case:
> 1. compile dpdk 
> rm -rf x86_64-native-linuxapp-clang
> CC=clang meson -Denable_kmods=True -Dlibdir=lib --default-library=static x86_64-native-linuxapp-clang
> ninja -C x86_64-native-linuxapp-clang -j 72 
> 2. start dpdk-test
> /root/dpdk/x86_64-native-linuxapp-clang/app/dpdk-test -l 0-39 --vdev=dma_skeleton -a 31:00.0 -a 31:00.1 -a 31:00.2 -a 31:00.3 （Note: If it cannot be reproduced, please try using a different core）
> 3. exit dpdk-test
> RTE>>quit
> Segmentation fault (core dumped)

I will try to reproduce, but still a question: does above test has already merged your patch [1] or the current main branch code has this problem?

[1] https://patches.dpdk.org/project/dpdk/patch/20240308053711.1260154-1-wenwux.ma@intel.com/

Thanks

> 
>>
>>>>  	/** Device info which supplied during device initialization. */
>>>>  	struct rte_device *device;
>>>>  	struct rte_dma_dev_data *data; /**< Pointer to shared device data.
>>>> */
>>>>