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 11:37:14 -0700
From:	Suresh Siddha <suresh.b.siddha@...el.com>
To:	Linus Torvalds <torvalds@...ux-foundation.org>
Cc:	Ingo Molnar <mingo@...nel.org>,
	Alexander Gordeev <agordeev@...hat.com>,
	Arjan van de Ven <arjan@...radead.org>,
	linux-kernel@...r.kernel.org, x86@...nel.org,
	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

On Mon, 2012-05-21 at 11:18 -0700, Linus Torvalds wrote:
> On Mon, May 21, 2012 at 11:07 AM, Suresh Siddha
> <suresh.b.siddha@...el.com> wrote:
> >
> > All the cluster members of a given x2apic cluster belong to the same
> > package. These x2apic cluster id's are setup by the HW and not by the
> > SW. And only one cluster (with one or multiple members of that cluster
> > set) can be specified in the interrupt destination field of the routing
> > table entry.
> 
> Ok, then the main question ends up being if there are enough cache or
> power domains within a cluster to still worry about it.

There are 16 members with in a x2apic cluster. With two HT siblings,
that will still leave 8-cores.

> 
> For example, you say "package", but that can sometimes mean multiple
> dies, or even just split caches that are big enough to matter
> (although I can't think of any such right now on the x86 side - Core2
> Duo had huge L2's, but they were shared, not split).

Most likely multiple dies or split caches will have different
cluster-id's. I don't know of any upcoming implementations that will
have such an implementation supporting x2apic, but will keep an eye.

> 
> > Power aware interrupt routing in IVB does this. And the policy of
> > whether you want the interrupt to be routed to the busy core (to save
> > power) or an idle core (for minimizing the interruptions on the busy
> > core) can be selected by the SW (using IA32_ENERGY_PERF_BIAS MSR).
> 
> Sounds like we definitely would want to support this at least in the
> IVB timeframe then.
> 
> But I do agree with Ingo that it would be really good to actually see
> numbers (and no, I don't mean "look here, now the irq's are nicely
> spread out", but power and/or performance numbers showing that it
> actually helps something).

I agree. This is the reason why I held up posting these patches before.
I can come up with micro-benchmarks that can show some difference but
the key is to find good workload/benchmark that can show measurable
difference. Any suggestions?

thanks,
suresh

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