linux-kernel - Re: [PATCH 1/2] perf/x86/rapl: Move the pmu allocation out of CPU hotplug

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Message-ID: <87msjes56x.ffs@tglx>
Date: Tue, 08 Oct 2024 22:36:06 +0200
From: Thomas Gleixner <tglx@...utronix.de>
To: "Liang, Kan" <kan.liang@...ux.intel.com>, peterz@...radead.org,
 mingo@...hat.com, linux-kernel@...r.kernel.org
Cc: Oliver Sang <oliver.sang@...el.com>, Dhananjay Ugwekar
 <Dhananjay.Ugwekar@....com>
Subject: Re: [PATCH 1/2] perf/x86/rapl: Move the pmu allocation out of CPU
 hotplug

On Tue, Oct 08 2024 at 22:23, Thomas Gleixner wrote:
> On Tue, Oct 08 2024 at 16:10, Kan Liang wrote:
>> On 2024-10-08 12:33 p.m., Thomas Gleixner wrote:
>>> On Fri, Sep 13 2024 at 10:10, kan liang wrote:
>>>> +static void __init init_rapl_pmu(void)
>>>> +{
>>>> +	struct rapl_pmu *pmu;
>>>> +	s32 rapl_pmu_idx;
>>>> +	int cpu;
>>>> +
>>>> +	cpus_read_lock();
>>>> +
>>>> +	for_each_cpu(cpu, cpu_online_mask) {
>>> 
>>> How is that supposed to work, when not all CPUs are online when
>>> init_rapl_pmus() is invoked?
>>> 
>>
>> RAPL is a module. The module_init() is called during do_initcalls(),
>> which is after the smp_init(). The cpu_online_mask has been setup in the
>> smp_init().
>>
>> I also patched the kernel to double check. The cpu_online_mask indeed
>> shows all the online CPUs.
>>
>> [    7.021212] smp: Brought up 1 node, 48 CPUs
>> [    7.021212] smpboot: Total of 48 processors activated (211200.00
>> BogoMIPS)
>> ... ...
>> [   16.557323] RAPL PMU: rapl_pmu_init: cpu_online_mask 0xffffffffffff
>
>  1) Start your kernel with maxcpus=2 (not recommended, but ...)
>  2) Load the module
>  3) Online the rest of the CPUs from userspace
>
> If your machine has more than one die you might be surprised...

You can make this work because the new topology code allows you to
retrieve the possible number of cores/dies/packages even when they have
not been onlined yet. 

Thanks,

        tglx