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SubjectRe: [PATCH 2/10] x86: convert to generic helpers for IPI function calls
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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