linux-kernel - Re: [PATCH v4 2/2] powercap/rapl: reduce ipi calls

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Date:	Fri, 19 Feb 2016 22:50:54 +0100 (CET)
From:	Thomas Gleixner <tglx@...utronix.de>
To:	Jacob Pan <jacob.jun.pan@...ux.intel.com>
cc:	LKML <linux-kernel@...r.kernel.org>,
	Rafael Wysocki <rafael.j.wysocki@...el.com>,
	Linux PM <linux-pm@...r.kernel.org>,
	Srinivas Pandruvada <srinivas.pandruvada@...ux.intel.com>,
	Peter Zijlstra <peterz@...radead.org>,
	"David S. Miller" <davem@...emloft.net>,
	Andrew Morton <akpm@...ux-foundation.org>,
	Rusty Russell <rusty@...tcorp.com.au>
Subject: Re: [PATCH v4 2/2] powercap/rapl: reduce ipi calls

On Fri, 19 Feb 2016, Jacob Pan wrote:
> @@ -1380,6 +1375,7 @@ static int rapl_detect_topology(void)
>  	int i;
>  	int phy_package_id;
>  	struct rapl_package *new_package, *rp;
> +	int lead_cpu;
>  
>  	for_each_online_cpu(i) {
>  		phy_package_id = topology_physical_package_id(i);
> @@ -1392,7 +1388,11 @@ static int rapl_detect_topology(void)
>  			/* add the new package to the list */
>  			new_package->id = phy_package_id;
>  			new_package->nr_cpus = 1;
> -
> +			/* find the first active cpu of the package */
> +			lead_cpu = cpumask_any_and(topology_core_cpumask(i),
> +						cpumask_of(i));

Yuck. Why any_and? The result is i, simply because i is online otherwise you
would not be there. 

> +			if (lead_cpu < nr_cpu_ids)
> +				new_package->lead_cpu = lead_cpu;

So the above is identical to 

			new_package->lead_cpu = lead_cpu;

Hmm?

> @@ -1500,6 +1503,15 @@ static int rapl_cpu_callback(struct notifier_block *nfb,
>  			break;
>  		if (--rp->nr_cpus == 0)
>  			rapl_remove_package(rp);
> +		else if (cpu == rp->lead_cpu) {
> +			/* choose another active cpu in the package */
> +			lead_cpu = cpumask_any_but(topology_core_cpumask(cpu), cpu);

This part is correct.

Thanks,

	tglx