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


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