linux-kernel - Re: Problem: scaling of /proc/stat on large systems

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Date:	Tue, 5 Oct 2010 17:19:07 +0900
From:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
To:	KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
Cc:	Jack Steiner <steiner@....com>, yinghai@...nel.org, mingo@...e.hu,
	akpm@...ux-foundation.org, linux-kernel@...r.kernel.org
Subject: Re: Problem: scaling of /proc/stat on large systems

On Tue, 5 Oct 2010 10:36:50 +0900
KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com> wrote:

> I guess this requres different approarch as per-cpu counter + threshould.
> like vmstat[] or lib/percpu_counter. 
> Maybe people don't like to access shared counter in IRQ.
> 
> But, this seems to call radixtree-lookup for the # of possible cpus.
> I guess impleimenting a call to calculate a sum of irqs in a radix-tree
> lookup will reduce overhead. If it's not enough, we'll have to make the
> counter not-precise. I'll write an another patch.
> 

How about this ? This is an add-on patch.
==
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>

In /proc/stat, the number of per-IRQ event is shown by making a sum
each irq's events on all cpus. But we can make use of kstat_irqs().

kstat_irqs() make a sum of IRQ events per cpu, if !CONFIG_GENERIC_HARDIRQ,
it's not a big cost. (Both of the number of cpus and irqs are small.)

If a system is very big, it does

	for_each_irq()
		for_each_cpu()
			- look up a radix tree
			- read desc->irq_stat[cpu]
This seems not efficient. This patch adds kstat_irqs() for CONFIG_GENRIC_HARDIRQ
and change the calculation as

	for_each_irq()
		look up radix tree
		for_each_cpu()
			- read desc->irq_stat[cpu]

and reduces cost.

Signged-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@...fujitsu.com>
---
 fs/proc/stat.c              |    9 ++-------
 include/linux/kernel_stat.h |    5 +++++
 kernel/irq/handle.c         |   16 ++++++++++++++++
 3 files changed, 23 insertions(+), 7 deletions(-)

Index: mmotm-0928/fs/proc/stat.c
===================================================================
--- mmotm-0928.orig/fs/proc/stat.c
+++ mmotm-0928/fs/proc/stat.c
@@ -108,13 +108,8 @@ static int show_stat(struct seq_file *p,
 	seq_printf(p, "intr %llu", (unsigned long long)sum);
 
 	/* sum again ? it could be updated? */
-	for_each_irq_nr(j) {
-		per_irq_sum = 0;
-		for_each_possible_cpu(i)
-			per_irq_sum += kstat_irqs_cpu(j, i);
-
-		seq_printf(p, " %u", per_irq_sum);
-	}
+	for_each_irq_nr(j)
+		seq_printf(p, " %u", kstat_irqs(j));
 
 	seq_printf(p,
 		"\nctxt %llu\n"
Index: mmotm-0928/include/linux/kernel_stat.h
===================================================================
--- mmotm-0928.orig/include/linux/kernel_stat.h
+++ mmotm-0928/include/linux/kernel_stat.h
@@ -62,6 +62,7 @@ static inline unsigned int kstat_irqs_cp
 {
        return kstat_cpu(cpu).irqs[irq];
 }
+
 #else
 #include <linux/irq.h>
 extern unsigned int kstat_irqs_cpu(unsigned int irq, int cpu);
@@ -86,6 +87,7 @@ static inline unsigned int kstat_softirq
 /*
  * Number of interrupts per specific IRQ source, since bootup
  */
+#ifndef CONFIG_GENERIC_HARDIRQS
 static inline unsigned int kstat_irqs(unsigned int irq)
 {
 	unsigned int sum = 0;
@@ -96,6 +98,9 @@ static inline unsigned int kstat_irqs(un
 
 	return sum;
 }
+#else
+extern unsigned int unsigned int kstat_irqs(unsigned int irq);
+#endif
 
 /*
  * Number of interrupts per cpu, since bootup
Index: mmotm-0928/kernel/irq/handle.c
===================================================================
--- mmotm-0928.orig/kernel/irq/handle.c
+++ mmotm-0928/kernel/irq/handle.c
@@ -553,3 +553,19 @@ unsigned int kstat_irqs_cpu(unsigned int
 }
 EXPORT_SYMBOL(kstat_irqs_cpu);
 
+#ifdef CONFIG_GENERIC_HARDIRQS
+unsigned int kstat_irqs(unsigned int irq)
+{
+	struct irq_desc *desc = irq_to_desc(irq);
+	int cpu;
+	int sum = 0;
+
+	if (!desc)
+		return 0;
+
+	for_each_possible_cpu(cpu)
+		sum += desc->kstat_irqs[cpu];
+	return sum;
+}
+EXPORT_SYMBOL(kstat_irqs);
+#endif

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