/proc/stat: fix scalability of irq sum of all cpu

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

[akpm@linux-foundation.org: unexport kstat_irqs, coding-style tweaks]
[akpm@linux-foundation.org: fix unused variable 'per_irq_sum']
Signed-off-by: KAMEZAWA Hiroyuki <kamezawa.hiroyu@jp.fujitsu.com>
Tested-by: Jack Steiner <steiner@sgi.com>
Acked-by: Jack Steiner <steiner@sgi.com>
Cc: Yinghai Lu <yinghai@kernel.org>
Cc: Ingo Molnar <mingo@elte.hu>
Cc: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>

1 files changed, 2 insertions, 8 deletions

diff --git a/fs/proc/stat.c b/fs/proc/stat.c
index b80c620565bf..e15a19c93bae 100644
--- a/fs/proc/stat.c
+++ b/fs/proc/stat.c
@@ -31,7 +31,6 @@ static int show_stat(struct seq_file *p, void *v)
         u64 sum_softirq = 0;
         unsigned int per_softirq_sums[NR_SOFTIRQS] = {0};
         struct timespec boottime;
-        unsigned int per_irq_sum;
 
         user = nice = system = idle = iowait =
                 irq = softirq = steal = cputime64_zero;
@@ -108,13 +107,8 @@ static int show_stat(struct seq_file *p, void *v)
         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"