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

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Message-ID: <87ploas5rb.ffs@tglx>
Date: Tue, 08 Oct 2024 22:23:52 +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 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...

Thanks,

        tglx