linux-kernel - Re: [PATCH v2 05/10] hwmon: (fam15h_power) Add compute unit accumulated power

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Message-ID: <20151028003418.GA7802@nazgul.tnic>
Date:	Wed, 28 Oct 2015 01:34:18 +0100
From:	Borislav Petkov <bp@...e.de>
To:	Huang Rui <ray.huang@....com>
Cc:	Guenter Roeck <linux@...ck-us.net>,
	Peter Zijlstra <peterz@...radead.org>,
	Jean Delvare <jdelvare@...e.de>,
	Andy Lutomirski <luto@...capital.net>,
	Andreas Herrmann <herrmann.der.user@...il.com>,
	Thomas Gleixner <tglx@...utronix.de>,
	Ingo Molnar <mingo@...nel.org>,
	"Rafael J. Wysocki" <rjw@...ysocki.net>,
	Len Brown <lenb@...nel.org>,
	John Stultz <john.stultz@...aro.org>,
	Frédéric Weisbecker <fweisbec@...il.com>,
	lm-sensors@...sensors.org, linux-kernel@...r.kernel.org,
	x86@...nel.org,
	Andreas Herrmann <herrmann.der.user@...glemail.com>,
	Aravind Gopalakrishnan <Aravind.Gopalakrishnan@....com>,
	Fengguang Wu <fengguang.wu@...el.com>,
	Aaron Lu <aaron.lu@...el.com>, Tony Li <tony.li@....com>
Subject: Re: [PATCH v2 05/10] hwmon: (fam15h_power) Add compute unit
 accumulated power

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)
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