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Subject: Re: [PATCH 4/7] perf: Check if HT is supported and enabled
From: Peter Zijlstra <a.p.zijlstra@chello.nl>
To: Lin Ming <ming.m.lin@intel.com>
Cc: Stephane Eranian <eranian@google.com>, "Luck, Tony" <tony.luck@intel.com>,
        "H. Peter Anvin" <hpa@zytor.com>, Ingo Molnar <mingo@elte.hu>,
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Date: Wed, 05 Jan 2011 11:08:41 +0100
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On Wed, 2011-01-05 at 13:45 +0800, Lin Ming wrote:
> static void intel_pmu_cpu_starting(int cpu)
> {
>         struct cpu_hw_events *cpuc = &per_cpu(cpu_hw_events, cpu);
>         struct intel_percore *pc;
>         int core_id = topology_core_id(cpu);
>         int i;
> 
>         init_debug_store_on_cpu(cpu);
>         /*
>          * Deal with CPUs that don't clear their LBRs on power-up.
>          */
>         intel_pmu_lbr_reset();
> 
>         if (!ht_enabled(cpu))
>                 return;
> 
>         cpuc->per_core = kzalloc_node(sizeof(struct intel_percore),
>                                       GFP_KERNEL, cpu_to_node(cpu));

#define CPU_STARTING            0x000A /* CPU (unsigned)v soon running.
                                        * Called on the new cpu, just before
                                        * enabling interrupts. Must not sleep,
                                        * must not fail */


>         if (!cpuc->per_core)
>                 return;
> 
>         raw_spin_lock_init(&cpuc->per_core->lock);
>         pc = cpuc->per_core;
> 
>         for_each_cpu(i, topology_thread_cpumask(cpu)) {
>                 cpuc = &per_cpu(cpu_hw_events, cpu);
> 
>                 cpuc->per_core = pc;
>                 cpuc->per_core->core_id = core_id;
>                 cpuc->per_core->refcnt++;
>         }
> } 

Anyway, I think something like:

static bool ht_capable(void)
{
	return boot_cpu_has(X86_FEATURE_HT) && smp_num_siblings > 1;
}

is sufficient, the whole hotplug issues makes relying on enumeration
results impractical.