block: delete part_round_stats and switch to less precise counting

We want to convert to per-cpu in_flight counters.

The function part_round_stats needs the in_flight counter every jiffy, it
would be too costly to sum all the percpu variables every jiffy, so it
must be deleted. part_round_stats is used to calculate two counters -
time_in_queue and io_ticks.

time_in_queue can be calculated without part_round_stats, by adding the
duration of the I/O when the I/O ends (the value is almost as exact as the
previously calculated value, except that time for in-progress I/Os is not
counted).

io_ticks can be approximated by increasing the value when I/O is started
or ended and the jiffies value has changed. If the I/Os take less than a
jiffy, the value is as exact as the previously calculated value. If the
I/Os take more than a jiffy, io_ticks can drift behind the previously
calculated value.

Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>
Signed-off-by: Jens Axboe <axboe@kernel.dk>

1 files changed, 21 insertions, 3 deletions

diff --git a/block/bio.c b/block/bio.c
index 0aca870331c3..036e3f0cc736 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -1664,6 +1664,22 @@ defer:
 }
 EXPORT_SYMBOL_GPL(bio_check_pages_dirty);
 
+void update_io_ticks(struct hd_struct *part, unsigned long now)
+{
+        unsigned long stamp;
+again:
+        stamp = READ_ONCE(part->stamp);
+        if (unlikely(stamp != now)) {
+                if (likely(cmpxchg(&part->stamp, stamp, now) == stamp)) {
+                        __part_stat_add(part, io_ticks, 1);
+                }
+        }
+        if (part->partno) {
+                part = &part_to_disk(part)->part0;
+                goto again;
+        }
+}
+
 void generic_start_io_acct(struct request_queue *q, int op,
                            unsigned long sectors, struct hd_struct *part)
 {
@@ -1671,7 +1687,7 @@ void generic_start_io_acct(struct request_queue *q, int op,
 
         part_stat_lock();
 
-        part_round_stats(q, part);
+        update_io_ticks(part, jiffies);
         part_stat_inc(part, ios[sgrp]);
         part_stat_add(part, sectors[sgrp], sectors);
         part_inc_in_flight(q, part, op_is_write(op));
@@ -1683,13 +1699,15 @@ EXPORT_SYMBOL(generic_start_io_acct);
 void generic_end_io_acct(struct request_queue *q, int req_op,
                          struct hd_struct *part, unsigned long start_time)
 {
-        unsigned long duration = jiffies - start_time;
+        unsigned long now = jiffies;
+        unsigned long duration = now - start_time;
         const int sgrp = op_stat_group(req_op);
 
         part_stat_lock();
 
+        update_io_ticks(part, now);
         part_stat_add(part, nsecs[sgrp], jiffies_to_nsecs(duration));
-        part_round_stats(q, part);
+        part_stat_add(part, time_in_queue, duration);
         part_dec_in_flight(q, part, op_is_write(req_op));
 
         part_stat_unlock();