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

[Guo Ren <guoren@kernel.org>]

On Wed, Jul 13, 2022 at 9:38 PM Dan Lustig <dlustig@nvidia.com> wrote:
>
> On 7/6/2022 8:03 PM, Boqun Feng 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
> >
> > 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
> >
> > , therefore the ordering between A and B is GMO and GMO should be
> > respected by all harts.
> >
> > Regards,
> > Boqun
>
> I agree with Boqun's reasoning, at least for the case where there
> is no branch.
>
> But to confirm, was the original question about also having a branch,
> I assume to the instruction immediately after the sc?  If so, then
> yes, that would make the .aqrl effect conditional.

>> 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.
In ISA spec, I found:
"Setting the aq bit on the LR instruction, and setting both the aq and
the 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."
No "lr + bnez + sc.aqrl" or "lr.aq + bnez + sc.aqrl" example description.

>> Could "lr + beq + sc.aqrl" provides a conditional RCsc here with current spec?
So, the above is legal for the RVWMO & LKMM?


>
> Dan
>
> >
> >>>
> >>> Dan
> >>>
> >>>> Regards,
> >>>> Boqun
> >>>>
> >>>>>   Andrea
> >>>>>
> >>>>>
> >>>> [...]
> >>
> >>
> >>
> >> --
> >> Best Regards
> >>  Guo Ren
> >>
> >> ML: https://lore.kernel.org/linux-csky/



-- 
Best Regards
 Guo Ren