[PATCH][RFC] Implement arch primitives for busywait loops

[Nicholas Piggin]

On Tue, 20 Sep 2016 14:35:45 +0200
Christian Borntraeger <borntraeger at de.ibm.com> wrote:

> On 09/20/2016 02:27 PM, Nicholas Piggin wrote:
> > On Tue, 20 Sep 2016 13:19:30 +0200
> > Christian Borntraeger <borntraeger at de.ibm.com> wrote:
> >   
> >> On 09/16/2016 10:57 AM, Nicholas Piggin wrote:  
> >>> Implementing busy wait loops with cpu_relax() in callers poses
> >>> some difficulties for powerpc.
> >>>
> >>> First, we want to put our SMT thread into a low priority mode for the
> >>> duration of the loop, but then return to normal priority after exiting
> >>> the loop.  Dependong on the CPU design, 'HMT_low() ; HMT_medium();' as
> >>> cpu_relax() does may have HMT_medium take effect before HMT_low made
> >>> any (or much) difference.
> >>>
> >>> Second, it can be beneficial for some implementations to spin on the
> >>> exit condition with a statically predicted-not-taken branch (i.e.,
> >>> always predict the loop will exit).
> >>>
> >>> This is a quick RFC with a couple of users converted to see what
> >>> people think. I don't use a C branch with hints, because we don't want
> >>> the compiler moving the loop body out of line, which makes it a bit
> >>> messy unfortunately. If there's a better way to do it, I'm all ears.
> >>>
> >>> I would not propose to switch all callers immediately, just some
> >>> core synchronisation primitives.    
> >> Just a FYA,
> >>
> >> On s390 we have a private version of cpu_relax that yields the cpu
> >> time slice back to the hypervisor via a hypercall.  
> > 
> > The powerpc guest also wants to yield to hypervisor in some busywait
> > situations.
> >   
> >> As this turned out
> >> to be problematic in some cases there is also now a cpu_relax_lowlatency.
> >>
> >> Now, this seems still problematic as there are too many places still 
> >> using cpu_relax instead of cpu_relax_lowlatency. So my plan is to do 
> >> a change of that, make cpu_relax just be a barrier and add a new 
> >> cpu_relax_yield that gives up the time slice. (so that s390 cpu_relax
> >> is just like any other cpu_relax)
> >>
> >> As far as I can tell the only place where I want to change cpu_relax
> >> to cpu_relax_lowlatency after that change is the stop machine run 
> >> code, so I hope to have no conflicts with your changes.  
> > 
> > I don't think there should be any conflicts, but it would be good to
> > make sure busy wait primitives can be usable by s390. So I can add
> > _yield variants that can do the right thing for s390.  
> 
> I was distracted by "more important work" (TM) but I will put you on
> CC when ready.
> > 
> > I need to think more about virtualization, so I'm glad you commented.
> > Powerpc would like to be told when a busywait loop knows the CPU it is
> > waiting for. So perhaps also a _yield_to_cpu variant as well.  
> 
> Yes, we also have 2 hypercalls: one that yields somehow and one that yields
> to a specific CPU. The latter is strongly preferred.

Okay, sounds good. I'll send out some updated patches soon too, so I'll
cc you on those. It would be good to come up with some basic guidelines
for when to use each variant too.

Thanks,
Nick