On Tue, 20 Sep 2016 13:19:30 +0200 Christian Borntraeger <borntraeger@xxxxxxxxxx> 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 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. Something that will work with mutex_spin_on_owner and similar would be nice too. As far as I can tell, powerpc may want to yield to hypervisor when the owner's vcpu is scheduled off in that case too. Thanks, Nick -- To unsubscribe from this list: send the line "unsubscribe linux-arch" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
