Messages in this thread | | | Date | Wed, 30 Apr 2008 13:35:42 +0200 | From | Jens Axboe <> | Subject | Re: [PATCH 2/10] x86: convert to generic helpers for IPI function calls |
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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.
-- Jens Axboe
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