On 06/28/2017 03:54 PM, Jens Axboe wrote:

On 06/28/2017 03:12 PM, Brian King wrote:

quoted

-static inline int part_in_flight(struct hd_struct *part)
+static inline unsigned long part_in_flight(struct hd_struct *part)
 {
-	return atomic_read(&part->in_flight[0]) + atomic_read(&part->in_flight[1]);
+	return part_stat_read(part, in_flight[0]) + part_stat_read(part, in_flight[1]);

One obvious improvement would be to not do this twice, but only have to
loop once. Instead of making this an array, make it a structure with a
read and write count.

It still doesn't really fix the issue of someone running on a kernel
with a ton of possible CPUs configured. But it does reduce the overhead
by 50%.

Or something as simple as this:

#define part_stat_read_double(part, field1, field2)			\
({									\
	typeof((part)->dkstats->field1) res = 0;			\
	unsigned int _cpu;						\
	for_each_possible_cpu(_cpu) {					\
		res += per_cpu_ptr((part)->dkstats, _cpu)->field1;	\
		res += per_cpu_ptr((part)->dkstats, _cpu)->field2;	\
	}								\
	res;								\
})

static inline unsigned long part_in_flight(struct hd_struct *part)
{
	return part_stat_read_double(part, in_flight[0], in_flight[1]);
}

-- 
Jens Axboe