linux-kernel - RE: [PATCH] x86: Add check for number of available vectors before CPU down [v6]

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Date:	Tue, 7 Jan 2014 17:54:22 +0000
From:	"Luck, Tony" <tony.luck@...el.com>
To:	Prarit Bhargava <prarit@...hat.com>,
	"linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>
CC:	Andi Kleen <ak@...ux.intel.com>,
	Michel Lespinasse <walken@...gle.com>,
	Seiji Aguchi <seiji.aguchi@....com>,
	"Zhang, Yang Z" <yang.z.zhang@...el.com>,
	"Gortmaker, Paul (Wind River)" <paul.gortmaker@...driver.com>,
	"Morgan, Janet" <janet.morgan@...el.com>,
	Ruiv Wang <ruiv.wang@...il.com>,
	"H. Peter Anvin" <hpa@...ux.intel.com>,
	"x86@...nel.org" <x86@...nel.org>
Subject: RE: [PATCH] x86: Add check for number of available vectors before
 CPU down [v6]

+	for (vector = FIRST_EXTERNAL_VECTOR; vector < NR_VECTORS; vector++) {
+		irq = __this_cpu_read(vector_irq[vector]);
+		if (irq >= 0) {
+			desc = irq_to_desc(irq);
+			data = irq_desc_get_irq_data(desc);
+			cpumask_copy(&affinity_new, data->affinity);
+			cpu_clear(this_cpu, affinity_new);
+			/*
+			 * The check below determines if this irq requires
+			 * an empty vector_irq[irq] entry on an online
+			 * cpu.
+			 *
+			 * The code only counts active non-percpu irqs, and
+			 * those irqs that are not linked to on an online cpu.
+			 * The first test is trivial, the second is not.
+			 *
+			 * The second test takes into account the
+			 * account that a single irq may be mapped to multiple
+			 * cpu's vector_irq[] (for example IOAPIC cluster
+			 * mode).  In this case we have two
+			 * possibilities:
+			 *
+			 * 1) the resulting affinity mask is empty; that is
+			 * this the down'd cpu is the last cpu in the irq's
+			 * affinity mask, and
Code says "||" - so I think comment should say "or".
+			 *
+			 * 2) the resulting affinity mask is no longer
+			 * a subset of the online cpus but the affinity
+			 * mask is not zero; that is the down'd cpu is the
+			 * last online cpu in a user set affinity mask.
+			 *
+			 * In both possibilities, we need to remap the irq
+			 * to a new vector_irq[].
+			 *
+			 */
+			if (irq_has_action(irq) && !irqd_is_per_cpu(data) &&
+			    (cpumask_empty(&affinity_new) ||
+			     !cpumask_subset(&affinity_new, &online_new)))
+				this_count++;
+		}

That's an impressive 6:1 ratio of lines-of-comment to lines-of-code!

Perhaps it would be less scary if the test were broken up into the easy/obvious part
and the one that has taken all these revisions to work out?  E.g.

			/* no need to count inactive or per-cpu irqs */
			if (!irq_has_action(irq) || irqd_is_per_cpu(data))
				continue;

			/*
			  * We need to look for a new home for this irq if:
				... paste in 1), 2) from above here ... (but s/and/or/ to match code)
			 */
			if (cpumask_empty(&affinity_new) ||
			    !cpumask_subset(&affinity_new, &online_new))
				this_count++;

-Tony
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