linux-kernel - Re: [PATCH 5/6] perf: Optimise topology iteration

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Date:	Mon, 21 Feb 2011 11:29:24 +0800
From:	Lin Ming <ming.m.lin@...el.com>
To:	Andi Kleen <andi@...stfloor.org>
Cc:	Peter Zijlstra <a.p.zijlstra@...llo.nl>,
	Ingo Molnar <mingo@...e.hu>,
	Stephane Eranian <eranian@...gle.com>,
	linux-kernel <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 5/6] perf: Optimise topology iteration

On Mon, 2011-02-21 at 05:15 +0800, Andi Kleen wrote:
> On Mon, Feb 21, 2011 at 12:57:39AM +0800, Lin Ming wrote:
> > Currently we iterate the full machine looking for a matching core_id/nb
> > for the percore and the amd northbridge stuff , using a smaller topology
> > mask makes sense.
> 
> This is still wrong for CPU hotplug. The CPU "owning" the per core
> does not necessarily need to be online anymore.

This is remain issue for hotplug case, no matter we use
for_each_online_cpu or topology_thread_cpumask.

> Please drop this patch.

Re-look at the code, I think for_each_online_cpu is wrong for percore,
we should use topology_thread_cpumask instead.

for_each_online_cpu(i) {
	struct intel_percore *pc = per_cpu(cpu_hw_events, i).per_core;

	if (pc && pc->core_id == core_id) {       
		kfree(cpuc->per_core);
		cpuc->per_core = pc;              
		break;
	}
}

Assume 2 sockets,

//socket 0
cpu 0: core_id 0
cpu 1: core_id 0

//socket 1
cpu 2: core_id 0
cpu 3: core_id 0

If for_each_online_cpu is used, apparently 4 logical cpus will share the
same percore. This is wrong.

If topology_thread_cpumask is used, then cpu0 and cpu1 share one percore
and cpu2 and cpu3 share another percore. This is what we want.

Lin Ming

> 
> -Andi


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