On Wed, Apr 30, 2008 at 01:35:42PM +0200, Jens Axboe wrote: > On Tue, Apr 29 2008, Jeremy Fitzhardinge wrote: > > Jens Axboe wrote: > > >-int xen_smp_call_function_mask(cpumask_t mask, void (*func)(void *), > > >- void *info, int wait) > > > > > [...] > > >- /* Send a message to other CPUs and wait for them to respond */ > > >- xen_send_IPI_mask(mask, XEN_CALL_FUNCTION_VECTOR); > > >- > > >- /* Make sure other vcpus get a chance to run if they need to. */ > > >- yield = false; > > >- for_each_cpu_mask(cpu, mask) > > >- if (xen_vcpu_stolen(cpu)) > > >- yield = true; > > >- > > >- if (yield) > > >- HYPERVISOR_sched_op(SCHEDOP_yield, 0); > > > > > > > I added this to deal with the case where you're sending an IPI to > > another VCPU which isn't currently running on a real cpu. In this case > > you could end up spinning while the other VCPU is waiting for a real CPU > > to run on. (Basically the same problem that spinlocks have in a virtual > > environment.) > > > > However, this is at best a partial solution to the problem, and I never > > benchmarked if it really makes a difference. Since any other virtual > > environment would have the same problem, its best if we can solve it > > generically. (Of course a synchronous single-target cross-cpu call is a > > simple cross-cpu rpc, which could be implemented very efficiently in the > > host/hypervisor by simply doing a vcpu context switch...) > > So, what would your advice be? Seems safe enough to ignore for now and > attack it if it becomes a real problem. How about an arch-specific function/macro invoked in the spin loop? The generic implementation would do nothing, but things like Xen could implement as above. Thanx, Paul -- 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