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Subject[PATCH v2 2/2] try to fix /proc/stat scalability of irq sum of all cpu
From: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.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() do the same calculation, 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 and CONFIG_GENERIC_HARDIRQ, 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]

This reduces cost.

A test on (4096cpusp, 256 nodes, 4592 irqs) host (by Jack Steiner)

%time cat /proc/stat > /dev/null

Before Patch: 2.459 sec
After Patch : .561 sec

Changelog:
- rebased onto mmotm-1020 (kernel/irq/handle.c is modified)

Tested-by: Jack Steiner <steiner@sgi.com>
Acked-by: Jack Steiner <steiner@sgi.com>
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
---
fs/proc/stat.c | 9 ++-------
include/linux/kernel_stat.h | 4 ++++
kernel/irq/irqdesc.c | 16 ++++++++++++++++
3 files changed, 22 insertions(+), 7 deletions(-)

Index: mmotm-1020/fs/proc/stat.c
===================================================================
--- mmotm-1020.orig/fs/proc/stat.c
+++ mmotm-1020/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-1020/include/linux/kernel_stat.h
===================================================================
--- mmotm-1020.orig/include/linux/kernel_stat.h
+++ mmotm-1020/include/linux/kernel_stat.h
@@ -86,6 +86,7 @@ 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 +97,9 @@ static inline unsigned int kstat_irqs(un

return sum;
}
+#else
+extern unsigned int kstat_irqs(unsigned int irq);
+#endif

/*
* Number of interrupts per cpu, since bootup
Index: mmotm-1020/kernel/irq/irqdesc.c
===================================================================
--- mmotm-1020.orig/kernel/irq/irqdesc.c
+++ mmotm-1020/kernel/irq/irqdesc.c
@@ -393,3 +393,19 @@ unsigned int kstat_irqs_cpu(unsigned int
struct irq_desc *desc = irq_to_desc(irq);
return desc ? desc->kstat_irqs[cpu] : 0;
}
+
+#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_GPL(kstat_irqs);
+#endif /*CONFIG_GENERIC_HARDIRQS*/


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