linux-kernel - Re: [PATCH 2/3] x86: x2apic/cluster: Make use of lowest priority delivery mode

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Date:	Mon, 21 May 2012 10:22:40 +0200
From:	Ingo Molnar <mingo@...nel.org>
To:	Alexander Gordeev <agordeev@...hat.com>
Cc:	linux-kernel@...r.kernel.org, x86@...nel.org,
	Suresh Siddha <suresh.b.siddha@...el.com>,
	Cyrill Gorcunov <gorcunov@...nvz.org>,
	Yinghai Lu <yinghai@...nel.org>
Subject: Re: [PATCH 2/3] x86: x2apic/cluster: Make use of lowest priority
 delivery mode


* Alexander Gordeev <agordeev@...hat.com> wrote:

> Currently x2APIC in logical destination mode delivers 
> interrupts to a single CPU, no matter how many CPUs were 
> specified in the destination cpumask.
> 
> This fix enables delivery of interrupts to multiple CPUs by 
> bit-ORing Logical IDs of destination CPUs that have matching 
> Cluster ID.
> 
> Because only one cluster could be specified in a message 
> destination address, the destination cpumask is tried for a 
> cluster that contains maximum number of CPUs matching this 
> cpumask. The CPUs in this cluster are selected to receive the 
> interrupts while all other CPUs (in the cpumask) are ignored.

I'm wondering how you tested this. AFAICS current irqbalanced 
will create masks but on x2apic only the first CPU is targeted 
by the kernel.

So, in theory, prior the patch you should be seeing irqs go to 
only one CPU, while after the patch they are spread out amongst 
the CPU. If it's using LowestPrio delivery then we depend on the 
hardware doing this for us - how does this work out in practice, 
are the target CPUs round-robin-ed, with a new CPU for every new 
IRQ delivered?

Thanks,

	Ingo
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