linux-kernel - Re: [PATCH 4/7] perf: Check if HT is supported and enabled

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Message-ID: <AANLkTima_wh1Nd2E-zQAcymmux1O0e1GzVEdnhLEMOo5@mail.gmail.com>
Date:	Tue, 4 Jan 2011 14:52:36 +0100
From:	Stephane Eranian <eranian@...gle.com>
To:	Peter Zijlstra <a.p.zijlstra@...llo.nl>
Cc:	"H. Peter Anvin" <hpa@...or.com>, Lin Ming <ming.m.lin@...el.com>,
	Ingo Molnar <mingo@...e.hu>, Andi Kleen <andi@...stfloor.org>,
	lkml <linux-kernel@...r.kernel.org>
Subject: Re: [PATCH 4/7] perf: Check if HT is supported and enabled

On Tue, Jan 4, 2011 at 2:44 PM, Peter Zijlstra <a.p.zijlstra@...llo.nl> wrote:
> On Tue, 2011-01-04 at 14:38 +0100, Stephane Eranian wrote:
>> My solution at the time (2.6.30) was to do:
>>         ht_enabled = cpumask_weight(__get_cpu_var(cpu_sibling_map)) > 1;
>
> Won't that report a machine a HT disabled when you offline a sibling?

I think you're right. I was not dealing with hotplug CPU.

> Which kinda defeats the purpose of our usage here, since we need to know
> it before either sibling comes online.

Then, it seems the only hope is to peek at a MSR that reports the BIOS setting.
But I don't know which one it is.

Couldn't you simply over-provision, and then when the CPU is online, use my
ht_enabled statement to figure out whether or not you need to handle the sharing
issue?
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