[PATCH] proc/stat: Separate out individual irq counts into /proc/stat_irqs

[Waiman Long <longman@redhat.com>]

It was found that reading /proc/stat could be time consuming on
systems with a lot of irqs. For example, reading /proc/stat in a
certain 2-socket Skylake server took about 4.6ms because it had over
5k irqs. In that particular case, the majority of the CPU cycles for
reading /proc/stat was spent in the kstat_irqs() function.  Therefore,
application performance can be impacted if the application reads
/proc/stat rather frequently.

The "intr" line within /proc/stat contains a sum total of all the irqs
that have happened followed by a list of irq counts for each individual
irq number. In many cases, the first number is good enough. The
individual irq counts may not provide that much more information.

In order to avoid this kind of performance issue, all these individual
irq counts are now separated into a new /proc/stat_irqs file. The
sum total irq count will stay in /proc/stat and be duplicated in
/proc/stat_irqs. Applications that need to look up individual irq counts
will now have to look into /proc/stat_irqs instead of /proc/stat.

Signed-off-by: Waiman Long <longman@redhat.com>
---
 fs/proc/stat.c | 48 +++++++++++++++++++++++++++++++++++++++++-------
 1 file changed, 41 insertions(+), 7 deletions(-)

diff --git a/fs/proc/stat.c b/fs/proc/stat.c
index 59749df..79e3c03 100644
--- a/fs/proc/stat.c
+++ b/fs/proc/stat.c
@@ -155,11 +155,6 @@ static int show_stat(struct seq_file *p, void *v)
 		seq_putc(p, '\n');
 	}
 	seq_put_decimal_ull(p, "intr ", (unsigned long long)sum);
-
-	/* sum again ? it could be updated? */
-	for_each_irq_nr(j)
-		seq_put_decimal_ull(p, " ", kstat_irqs_usr(j));
-
 	seq_printf(p,
 		"\nctxt %llu\n"
 		"btime %llu\n"
@@ -181,15 +176,46 @@ static int show_stat(struct seq_file *p, void *v)
 	return 0;
 }
 
+/*
+ * Showing individual irq counts can be expensive if there are a lot of
+ * irqs. So it is done in a separate procfs file to reduce performance
+ * overhead of reading other statistical counts.
+ */
+static int show_stat_irqs(struct seq_file *p, void *v)
+{
+	int i, j;
+	u64 sum = 0;
+
+	for_each_possible_cpu(i) {
+		sum += kstat_cpu_irqs_sum(i);
+		sum += arch_irq_stat_cpu(i);
+	}
+	sum += arch_irq_stat();
+
+	seq_put_decimal_ull(p, "intr ", (unsigned long long)sum);
+
+	for_each_irq_nr(j)
+		seq_put_decimal_ull(p, " ", kstat_irqs_usr(j));
+
+	seq_putc(p, '\n');
+
+	return 0;
+}
+
 static int stat_open(struct inode *inode, struct file *file)
 {
 	size_t size = 1024 + 128 * num_online_cpus();
 
-	/* minimum size to display an interrupt count : 2 bytes */
-	size += 2 * nr_irqs;
 	return single_open_size(file, show_stat, NULL, size);
 }
 
+static int stat_irqs_open(struct inode *inode, struct file *file)
+{
+	size_t size = 1024 + 16 * nr_irqs;
+
+	return single_open_size(file, show_stat_irqs, NULL, size);
+}
+
 static const struct file_operations proc_stat_operations = {
 	.open		= stat_open,
 	.read		= seq_read,
@@ -197,9 +223,17 @@ static int stat_open(struct inode *inode, struct file *file)
 	.release	= single_release,
 };
 
+static const struct file_operations proc_stat_irqs_operations = {
+	.open		= stat_irqs_open,
+	.read		= seq_read,
+	.llseek		= seq_lseek,
+	.release	= single_release,
+};
+
 static int __init proc_stat_init(void)
 {
 	proc_create("stat", 0, NULL, &proc_stat_operations);
+	proc_create("stat_irqs", 0, NULL, &proc_stat_irqs_operations);
 	return 0;
 }
 fs_initcall(proc_stat_init);
-- 
1.8.3.1