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Date: Wed, 28 Oct 2015 01:34:18 +0100
From: Borislav Petkov <bp@suse.de>
To: Huang Rui <ray.huang@amd.com>
Cc: Guenter Roeck <linux@roeck-us.net>, Peter Zijlstra <peterz@infradead.org>,
        Jean Delvare <jdelvare@suse.de>, Andy Lutomirski <luto@amacapital.net>,
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        Fengguang Wu <fengguang.wu@intel.com>, Aaron Lu <aaron.lu@intel.com>,
        Tony Li <tony.li@amd.com>
Subject: Re: [PATCH v2 05/10] hwmon: (fam15h_power) Add compute unit
 accumulated power
Message-ID: <20151028003418.GA7802@nazgul.tnic>
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On Tue, Oct 27, 2015 at 10:53:40AM +0800, Huang Rui wrote:
> Preemption must be disabled when calling smp_call_function_many,
> get_cpu would did that. Will get_online_cpus have the same behavior
> like that?

Well, get_online_cpus() protects you against CPU hotplug operations in
general. If you want to protect yourself against CPUs going away only,
then I guess get_cpu()/put_cpu() is fine.

But since we're going to work with the masks as below, prohibiting any
changes to cpu_online_mask is probably the better/safe thing to do, i.e.

	get_online_cpus();
	preempt_disable();

	smp_call_function_many( ... );

	preempt_enable();
	put_online_cpus();

> That means "the value(cu_acc_power) of the compute unit", which does
> not represent the value of one CPU core.

No, I mean this: "This behavior can decrease IPI numbers between the
unit's."

I'm wondering whether it is really needed at all ...

> OK, how about below codes:
> 
> ---
> for (i = 0; i <= cores_per_cu / BITS_PER_LONG; i++) {
> 	offset = cores_per_cu % BITS_PER_LONG;
> 	if (i == cores_per_cu / BITS_PER_LONG) {
> 		cpumask_bits(src_mask)[i] = GENMASK(offset -1, 0);
> 		break;
> 	}
> 	cpumask_bits(src_mask)[i] = GENMASK(BITS_PER_LONG - 1, 0);
> }
> 
> for (i = 0; i < cu_num; i++) {
> 	cpumask_shift_left(dst, src_mask, cores_per_cu * i);
> 	cpumask_and(res, dst, cpu_online_mask);
> 	cpumask_set_cpu(cpumask_any(res), mask);
> }

I think you can make it even simpler:

	/* prepare CU temp mask */
	for (i = 0; i < cores_per_cu; i++)
		cpumask_set_cpu(i, tmp_mask);

	for (i = 0; i < cu_num; i++) {
		/* WARN_ON for empty CU masks */
		WARN_ON(!cpumask_and(res_mask, tmp_mask, cpu_online_mask));
		cpumask_set(cpumask_any(res_mask), call_mask);
		cpumask_shift_right(tmp_mask, tmp_mask, cores_per_cu);
	}

	smp_call_function_many(call_mask, .... );

Something like that...

-- 
Regards/Gruss,
    Boris.

SUSE Linux GmbH, GF: Felix Imendörffer, Jane Smithard, Graham Norton, HRB 21284 (AG Nürnberg)
--