Re: [PATCH V4 5/5] riscv: atomic: Optimize LRSC-pairs atomic ops with .aqrl annotation

[Dan Lustig <dlustig@nvidia.com>]

On 7/13/2022 7:47 PM, Guo Ren wrote:
> On Thu, Jul 7, 2022 at 8:04 AM Boqun Feng <boqun.feng@gmail.com> wrote:
>>
>> On Sat, Jun 25, 2022 at 01:29:50PM +0800, Guo Ren wrote:
>>> On Fri, Jun 24, 2022 at 1:09 AM Dan Lustig <dlustig@nvidia.com> wrote:
>>>>
>>>> On 6/22/2022 11:31 PM, Boqun Feng wrote:
>>>>> Hi,
>>>>>
>>>>> On Tue, Jun 14, 2022 at 01:03:47PM +0200, Andrea Parri wrote:
>>>>> [...]
>>>>>>> 5ce6c1f3535f ("riscv/atomic: Strengthen implementations with fences")
>>>>>>> is about fixup wrong spinlock/unlock implementation and not relate to
>>>>>>> this patch.
>>>>>>
>>>>>> No.  The commit in question is evidence of the fact that the changes
>>>>>> you are presenting here (as an optimization) were buggy/incorrect at
>>>>>> the time in which that commit was worked out.
>>>>>>
>>>>>>
>>>>>>> Actually, sc.w.aqrl is very strong and the same with:
>>>>>>> fence rw, rw
>>>>>>> sc.w
>>>>>>> fence rw,rw
>>>>>>>
>>>>>>> So "which do not give full-ordering with .aqrl" is not writen in
>>>>>>> RISC-V ISA and we could use sc.w/d.aqrl with LKMM.
>>>>>>>
>>>>>>>>
>>>>>>>>>> describes the issue more specifically, that's when we added these
>>>>>>>>>> fences.  There have certainly been complains that these fences are too
>>>>>>>>>> heavyweight for the HW to go fast, but IIUC it's the best option we have
>>>>>>>>> Yeah, it would reduce the performance on D1 and our next-generation
>>>>>>>>> processor has optimized fence performance a lot.
>>>>>>>>
>>>>>>>> Definately a bummer that the fences make the HW go slow, but I don't
>>>>>>>> really see any other way to go about this.  If you think these mappings
>>>>>>>> are valid for LKMM and RVWMO then we should figure this out, but trying
>>>>>>>> to drop fences to make HW go faster in ways that violate the memory
>>>>>>>> model is going to lead to insanity.
>>>>>>> Actually, this patch is okay with the ISA spec, and Dan also thought
>>>>>>> it was valid.
>>>>>>>
>>>>>>> ref: https://lore.kernel.org/lkml/41e01514-74ca-84f2-f5cc-2645c444fd8e@nvidia.com/raw
>>>>>>
>>>>>> "Thoughts" on this regard have _changed_.  Please compare that quote
>>>>>> with, e.g.
>>>>>>
>>>>>>   https://lore.kernel.org/linux-riscv/ddd5ca34-805b-60c4-bf2a-d6a9d95d89e7@nvidia.com/
>>>>>>
>>>>>> So here's a suggestion:
>>>>>>
>>>>>> Reviewers of your patches have asked:  How come that code we used to
>>>>>> consider as buggy is now considered "an optimization" (correct)?
>>>>>>
>>>>>> Denying the evidence or going around it is not making their job (and
>>>>>> this upstreaming) easier, so why don't you address it?  Take time to
>>>>>> review previous works and discussions in this area, understand them,
>>>>>> and integrate such knowledge in future submissions.
>>>>>>
>>>>>
>>>>> I agree with Andrea.
>>>>>
>>>>> And I actually took a look into this, and I think I find some
>>>>> explanation. There are two versions of RISV memory model here:
>>>>>
>>>>> Model 2017: released at Dec 1, 2017 as a draft
>>>>>
>>>>>       https://groups.google.com/a/groups.riscv.org/g/isa-dev/c/hKywNHBkAXM/m/QzUtxEWLBQAJ
>>>>>
>>>>> Model 2018: released at May 2, 2018
>>>>>
>>>>>       https://groups.google.com/a/groups.riscv.org/g/isa-dev/c/xW03vmfmPuA/m/bMPk3UCWAgAJ
>>>>>
>>>>> Noted that previous conversation about commit 5ce6c1f3535f happened at
>>>>> March 2018. So the timeline is roughly:
>>>>>
>>>>>       Model 2017 -> commit 5ce6c1f3535f -> Model 2018
>>>>>
>>>>> And in the email thread of Model 2018, the commit related to model
>>>>> changes also got mentioned:
>>>>>
>>>>>       https://github.com/riscv/riscv-isa-manual/commit/b875fe417948635ed68b9644ffdf718cb343a81a
>>>>>
>>>>> in that commit, we can see the changes related to sc.aqrl are:
>>>>>
>>>>>        to have occurred between the LR and a successful SC.  The LR/SC
>>>>>        sequence can be given acquire semantics by setting the {\em aq} bit on
>>>>>       -the SC instruction.  The LR/SC sequence can be given release semantics
>>>>>       -by setting the {\em rl} bit on the LR instruction.  Setting both {\em
>>>>>       -  aq} and {\em rl} bits on the LR instruction, and setting the {\em
>>>>>       -  aq} bit on the SC instruction makes the LR/SC sequence sequentially
>>>>>       -consistent with respect to other sequentially consistent atomic
>>>>>       -operations.
>>>>>       +the LR instruction.  The LR/SC sequence can be given release semantics
>>>>>       +by setting the {\em rl} bit on the SC instruction.  Setting the {\em
>>>>>       +  aq} bit on the LR instruction, and setting both the {\em aq} and the {\em
>>>>>       +  rl} bit on the SC instruction makes the LR/SC sequence sequentially
>>>>>       +consistent, meaning that it cannot be reordered with earlier or
>>>>>       +later memory operations from the same hart.
>>>>>
>>>>> note that Model 2018 explicitly says that "ld.aq+sc.aqrl" is ordered
>>>>> against "earlier or later memory operations from the same hart", and
>>>>> this statement was not in Model 2017.
>>>>>
>>>>> So my understanding of the story is that at some point between March and
>>>>> May 2018, RISV memory model folks decided to add this rule, which does
>>>>> look more consistent with other parts of the model and is useful.
>>>>>
>>>>> And this is why (and when) "ld.aq+sc.aqrl" can be used as a fully-ordered
>>>>> barrier ;-)
>>>>>
>>>>> Now if my understanding is correct, to move forward, it's better that 1)
>>>>> this patch gets resend with the above information (better rewording a
>>>>> bit), and 2) gets an Acked-by from Dan to confirm this is a correct
>>>>> history ;-)
>>>>
>>>> I'm a bit lost as to why digging into RISC-V mailing list history is
>>>> relevant here...what's relevant is what was ratified in the RVWMO
>>>> chapter of the RISC-V spec, and whether the code you're proposing
>>>> is the most optimized code that is correct wrt RVWMO.
>>>>
>>>> Is your claim that the code you're proposing to fix was based on a
>>>> pre-RVWMO RISC-V memory model definition, and you're updating it to
>>>> be more RVWMO-compliant?
>>> Could "lr + beq + sc.aqrl" provides a conditional RCsc here with
>>> current spec? I only found "lr.aq + sc.aqrl" despcriton which is
>>> un-conditional RCsc.
>>>
>>
>> /me put the temporary RISCV memory model hat on and pretend to be a
>> RISCV memory expert.
>>
>> I think the answer is yes, it's actually quite straightforwards given
>> that RISCV treats PPO (Preserved Program Order) as part of GMO (Global
>> Memory Order), considering the following (A and B are memory accesses):
>>
>>         A
>>         ..
>>         sc.aqrl // M
>>         ..
>>         B
>>
>> , A has a ->ppo ordering to M since "sc.aqrl" is a RELEASE, and M has
>> a ->ppo ordeing to B since "sc.aqrl" is an AQUIRE, so
>>
>>         A ->ppo M ->ppo B
> That also means M must fence.rl + sc + fence.aq. But in the release
> consistency model, "rl + aq" is not legal and has no guarantee at all.
> 
> So sc.aqrl should be clarified in spec, but I only found "lr.aq +
> sc.aqrl" description, see the patch commit log.

The spec doesn't try to enumerate every possible synchronization
instruction sequence.  That's why the RVWMO rules are in place.

> Could we treat sc.aqrl as a whole in ISA? Because in micro-arch, we
> must separate it into pieces for implementation.
> 
> That is what the RVWMO should give out.

What exactly would you like the spec to say about this?  RVWMO and the
RISC-V spec in general describe the required architecturally observable
behavior.  They're not implementation guides. 

Generally speaking, I would expect splitting an sc.aqrl into a
".rl; sc; .aq" pattern to be OK.  That wouldn't introduce new observable
behaviors compared to treating the sc.aqrl as a single indivisible
operation, would it?

Dan

>> And since RISCV describes that PPO is part of GMO:
>>
>> """
>> The subset of program order that must be respected by the global memory
>> order is known as preserved program order.
>> """
>>
>> also in the herd model:
>>
>>         (* Main model axiom *)
>>         acyclic co | rfe | fr | ppo as Model
> If the herd7 model has defined that, I think it should be legal. Good catch.
> 
> 
>>
>> , therefore the ordering between A and B is GMO and GMO should be
>> respected by all harts.
>>
>> Regards,
>> Boqun
>>
>>>>
>>>> Dan
>>>>
>>>>> Regards,
>>>>> Boqun
>>>>>
>>>>>>   Andrea
>>>>>>
>>>>>>
>>>>> [...]
>>>
>>>
>>>
>>> --
>>> Best Regards
>>>  Guo Ren
>>>
>>> ML: https://lore.kernel.org/linux-csky/
> 
> 
>