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

[Jens Axboe]

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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