Questions about masked atomic

Questions about masked atomic

[liweihang]

Hi All,

I have two questions about masked atomic (Masked Compare and Swap & MFetchAdd):

1. The kernel now supports masked atomic, but the it does not support atomic
   operation. Is the masked atomic valid in kernel currently?
2. In the userspace, ofed does not have the corresponding opcode for the masked
   atomic (IB_WR_MASKED_ATOMIC_CMP_AND_SWP, IB_WR_MASKED_ATOMIC_FETCH_AND_ADD),
   and ibv_send_wr also has no related data segment for it. How to support it in
   userspace?

Thanks
Weihang

Re: Questions about masked atomic

[Leon Romanovsky]

On Mon, May 11, 2020 at 01:54:48PM +0000, liweihang wrote:
> Hi All,
>
> I have two questions about masked atomic (Masked Compare and Swap & MFetchAdd):
>
> 1. The kernel now supports masked atomic, but the it does not support atomic
>    operation. Is the masked atomic valid in kernel currently?

Yes, it is valid, but probably has a very little real value for the kernel ULPs.
I see code in the RDS that uses atomics, but it says nothing to me, because
upstream RDS and version in-real-use are completely different.

> 2. In the userspace, ofed does not have the corresponding opcode for the masked
>    atomic (IB_WR_MASKED_ATOMIC_CMP_AND_SWP, IB_WR_MASKED_ATOMIC_FETCH_AND_ADD),
>    and ibv_send_wr also has no related data segment for it. How to support it in
>    userspace?

ibv_send_wr is not extensible, so the real solution will need to extend ibv_wr_post() [1]
with specific and new post builders.

Thanks

[1] https://github.com/linux-rdma/rdma-core/blob/master/libibverbs/man/ibv_wr_post.3.md

>
> Thanks
> Weihang

Re: Questions about masked atomic

[liweihang]

On 2020/5/12 19:35, Leon Romanovsky wrote:
> On Mon, May 11, 2020 at 01:54:48PM +0000, liweihang wrote:
>> Hi All,
>>
>> I have two questions about masked atomic (Masked Compare and Swap & MFetchAdd):
>>
>> 1. The kernel now supports masked atomic, but the it does not support atomic
>>    operation. Is the masked atomic valid in kernel currently?
> 
> Yes, it is valid, but probably has a very little real value for the kernel ULPs.
> I see code in the RDS that uses atomics, but it says nothing to me, because
> upstream RDS and version in-real-use are completely different.
> 
>> 2. In the userspace, ofed does not have the corresponding opcode for the masked
>>    atomic (IB_WR_MASKED_ATOMIC_CMP_AND_SWP, IB_WR_MASKED_ATOMIC_FETCH_AND_ADD),
>>    and ibv_send_wr also has no related data segment for it. How to support it in
>>    userspace?
> 
> ibv_send_wr is not extensible, so the real solution will need to extend ibv_wr_post() [1]
> with specific and new post builders.
> 
> Thanks
> 
> [1] https://github.com/linux-rdma/rdma-core/blob/master/libibverbs/man/ibv_wr_post.3.md
> 

Hi Leon,

Thanks for your response. May I ask another question:

Why it's not encouraged to use atomic/extended atomic/masked atomic operations in kernel?
Jason said that there seems no kernel users of extended atomic, is there any other reasons?

Weihang

Re: Questions about masked atomic

[Leon Romanovsky]

On Fri, May 15, 2020 at 09:40:26AM +0000, liweihang wrote:
> On 2020/5/12 19:35, Leon Romanovsky wrote:
> > On Mon, May 11, 2020 at 01:54:48PM +0000, liweihang wrote:
> >> Hi All,
> >>
> >> I have two questions about masked atomic (Masked Compare and Swap & MFetchAdd):
> >>
> >> 1. The kernel now supports masked atomic, but the it does not support atomic
> >>    operation. Is the masked atomic valid in kernel currently?
> >
> > Yes, it is valid, but probably has a very little real value for the kernel ULPs.
> > I see code in the RDS that uses atomics, but it says nothing to me, because
> > upstream RDS and version in-real-use are completely different.
> >
> >> 2. In the userspace, ofed does not have the corresponding opcode for the masked
> >>    atomic (IB_WR_MASKED_ATOMIC_CMP_AND_SWP, IB_WR_MASKED_ATOMIC_FETCH_AND_ADD),
> >>    and ibv_send_wr also has no related data segment for it. How to support it in
> >>    userspace?
> >
> > ibv_send_wr is not extensible, so the real solution will need to extend ibv_wr_post() [1]
> > with specific and new post builders.
> >
> > Thanks
> >
> > [1] https://github.com/linux-rdma/rdma-core/blob/master/libibverbs/man/ibv_wr_post.3.md
> >
>
> Hi Leon,
>
> Thanks for your response. May I ask another question:
>
> Why it's not encouraged to use atomic/extended atomic/masked atomic operations in kernel?
> Jason said that there seems no kernel users of extended atomic, is there any other reasons?

I don't think that "it is not encouraged", the more accurate will be
"the IBTA atomics will give nothing to the kernel ULPs".

The atomic data is not necessary stored in the host memory, while ULPs
need it in the memory. It means that they anyway will need to do some
synchronization in the host and "cancel" any advantage of atomics if
they exist.

Thanks

>
> Weihang

Re: Questions about masked atomic

[Tom Talpey]

On 5/17/2020 9:14 AM, Leon Romanovsky wrote:
> On Fri, May 15, 2020 at 09:40:26AM +0000, liweihang wrote:
>> On 2020/5/12 19:35, Leon Romanovsky wrote:
>>> On Mon, May 11, 2020 at 01:54:48PM +0000, liweihang wrote:
>>>> Hi All,
>>>>
>>>> I have two questions about masked atomic (Masked Compare and Swap & MFetchAdd):
>>>>
>>>> 1. The kernel now supports masked atomic, but the it does not support atomic
>>>>     operation. Is the masked atomic valid in kernel currently?
>>>
>>> Yes, it is valid, but probably has a very little real value for the kernel ULPs.
>>> I see code in the RDS that uses atomics, but it says nothing to me, because
>>> upstream RDS and version in-real-use are completely different.
>>>
>>>> 2. In the userspace, ofed does not have the corresponding opcode for the masked
>>>>     atomic (IB_WR_MASKED_ATOMIC_CMP_AND_SWP, IB_WR_MASKED_ATOMIC_FETCH_AND_ADD),
>>>>     and ibv_send_wr also has no related data segment for it. How to support it in
>>>>     userspace?
>>>
>>> ibv_send_wr is not extensible, so the real solution will need to extend ibv_wr_post() [1]
>>> with specific and new post builders.
>>>
>>> Thanks
>>>
>>> [1] https://github.com/linux-rdma/rdma-core/blob/master/libibverbs/man/ibv_wr_post.3.md
>>>
>>
>> Hi Leon,
>>
>> Thanks for your response. May I ask another question:
>>
>> Why it's not encouraged to use atomic/extended atomic/masked atomic operations in kernel?
>> Jason said that there seems no kernel users of extended atomic, is there any other reasons?
> 
> I don't think that "it is not encouraged", the more accurate will be
> "the IBTA atomics will give nothing to the kernel ULPs".
> 
> The atomic data is not necessary stored in the host memory, while ULPs
> need it in the memory. It means that they anyway will need to do some
> synchronization in the host and "cancel" any advantage of atomics if
> they exist.

Indeed, it is a common misconception by upper layer implementers that
the atomicity is available to the responder CPU. In fact, atomics work
only from the HCA that executes them, and the result is flushed to
memory, non-atomically, at some later time. These limitations greatly
reduce the motivation to use them at all, much less the exotic masked
ones.

I believe another reason they're not surfaced for kernel consumers is
that there aren't any. Primarily, the kernel consumers are storage, and
storage protocols stay far away from atomics.

Tom.

Re: Questions about masked atomic

[liweihang]

On 2020/5/18 2:58, Tom Talpey wrote:
> On 5/17/2020 9:14 AM, Leon Romanovsky wrote:
>> On Fri, May 15, 2020 at 09:40:26AM +0000, liweihang wrote:
>>> On 2020/5/12 19:35, Leon Romanovsky wrote:
>>>> On Mon, May 11, 2020 at 01:54:48PM +0000, liweihang wrote:
>>>>> Hi All,
>>>>>
>>>>> I have two questions about masked atomic (Masked Compare and Swap & MFetchAdd):
>>>>>
>>>>> 1. The kernel now supports masked atomic, but the it does not support atomic
>>>>>     operation. Is the masked atomic valid in kernel currently?
>>>>
>>>> Yes, it is valid, but probably has a very little real value for the kernel ULPs.
>>>> I see code in the RDS that uses atomics, but it says nothing to me, because
>>>> upstream RDS and version in-real-use are completely different.
>>>>
>>>>> 2. In the userspace, ofed does not have the corresponding opcode for the masked
>>>>>     atomic (IB_WR_MASKED_ATOMIC_CMP_AND_SWP, IB_WR_MASKED_ATOMIC_FETCH_AND_ADD),
>>>>>     and ibv_send_wr also has no related data segment for it. How to support it in
>>>>>     userspace?
>>>>
>>>> ibv_send_wr is not extensible, so the real solution will need to extend ibv_wr_post() [1]
>>>> with specific and new post builders.
>>>>
>>>> Thanks
>>>>
>>>> [1] https://github.com/linux-rdma/rdma-core/blob/master/libibverbs/man/ibv_wr_post.3.md
>>>>
>>>
>>> Hi Leon,
>>>
>>> Thanks for your response. May I ask another question:
>>>
>>> Why it's not encouraged to use atomic/extended atomic/masked atomic operations in kernel?
>>> Jason said that there seems no kernel users of extended atomic, is there any other reasons?
>>
>> I don't think that "it is not encouraged", the more accurate will be
>> "the IBTA atomics will give nothing to the kernel ULPs".
>>
>> The atomic data is not necessary stored in the host memory, while ULPs
>> need it in the memory. It means that they anyway will need to do some
>> synchronization in the host and "cancel" any advantage of atomics if
>> they exist.
> 
> Indeed, it is a common misconception by upper layer implementers that
> the atomicity is available to the responder CPU. In fact, atomics work
> only from the HCA that executes them, and the result is flushed to
> memory, non-atomically, at some later time. These limitations greatly
> reduce the motivation to use them at all, much less the exotic masked
> ones.
> 
> I believe another reason they're not surfaced for kernel consumers is
> that there aren't any. Primarily, the kernel consumers are storage, and
> storage protocols stay far away from atomics.
> 
> Tom.
> 

Hi Tom and Leon,

Thank you for the explanation, it helps me lot.

Weihang