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

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Date:	Tue, 29 Apr 2008 13:35:49 -0700
From:	Jeremy Fitzhardinge <jeremy@...p.org>
To:	Jens Axboe <jens.axboe@...cle.com>
CC:	linux-kernel@...r.kernel.org, peterz@...radead.org,
	npiggin@...e.de, linux-arch@...r.kernel.org, mingo@...e.hu,
	paulmck@...ux.vnet.ibm.com
Subject: Re: [PATCH 2/10] x86: convert to generic helpers for IPI function
 calls

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...)

    J
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