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>

5 files changed, 25 insertions, 68 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();
diff --git a/block/blk-core.c b/block/blk-core.c
index 734b768c9d9d..268d2b8e9843 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -584,62 +584,6 @@ struct request *blk_get_request(struct request_queue *q, unsigned int op,
 }
 EXPORT_SYMBOL(blk_get_request);
 
-static void part_round_stats_single(struct request_queue *q,
-                                    struct hd_struct *part, unsigned long now,
-                                    unsigned int inflight)
-{
-        if (inflight) {
-                __part_stat_add(part, time_in_queue,
-                                inflight * (now - part->stamp));
-                __part_stat_add(part, io_ticks, (now - part->stamp));
-        }
-        part->stamp = now;
-}
-
-/**
- * part_round_stats() - Round off the performance stats on a struct disk_stats.
- * @q: target block queue
- * @part: target partition
- *
- * The average IO queue length and utilisation statistics are maintained
- * by observing the current state of the queue length and the amount of
- * time it has been in this state for.
- *
- * Normally, that accounting is done on IO completion, but that can result
- * in more than a second's worth of IO being accounted for within any one
- * second, leading to >100% utilisation.  To deal with that, we call this
- * function to do a round-off before returning the results when reading
- * /proc/diskstats.  This accounts immediately for all queue usage up to
- * the current jiffies and restarts the counters again.
- */
-void part_round_stats(struct request_queue *q, struct hd_struct *part)
-{
-        struct hd_struct *part2 = NULL;
-        unsigned long now = jiffies;
-        unsigned int inflight[2];
-        int stats = 0;
-
-        if (part->stamp != now)
-                stats |= 1;
-
-        if (part->partno) {
-                part2 = &part_to_disk(part)->part0;
-                if (part2->stamp != now)
-                        stats |= 2;
-        }
-
-        if (!stats)
-                return;
-
-        part_in_flight(q, part, inflight);
-
-        if (stats & 2)
-                part_round_stats_single(q, part2, now, inflight[1]);
-        if (stats & 1)
-                part_round_stats_single(q, part, now, inflight[0]);
-}
-EXPORT_SYMBOL_GPL(part_round_stats);
-
 void blk_put_request(struct request *req)
 {
         blk_mq_free_request(req);
@@ -1383,9 +1327,10 @@ void blk_account_io_done(struct request *req, u64 now)
                 part_stat_lock();
                 part = req->part;
 
+                update_io_ticks(part, jiffies);
                 part_stat_inc(part, ios[sgrp]);
                 part_stat_add(part, nsecs[sgrp], now - req->start_time_ns);
-                part_round_stats(req->q, part);
+                part_stat_add(part, time_in_queue, nsecs_to_jiffies64(now - req->start_time_ns));
                 part_dec_in_flight(req->q, part, rq_data_dir(req));
 
                 hd_struct_put(part);
@@ -1420,11 +1365,12 @@ void blk_account_io_start(struct request *rq, bool new_io)
                         part = &rq->rq_disk->part0;
                         hd_struct_get(part);
                 }
-                part_round_stats(rq->q, part);
                 part_inc_in_flight(rq->q, part, rw);
                 rq->part = part;
         }
 
+        update_io_ticks(part, jiffies);
+
         part_stat_unlock();
 }
 
diff --git a/block/blk-merge.c b/block/blk-merge.c
index a120d59b9705..9da5629d0887 100644
--- a/block/blk-merge.c
+++ b/block/blk-merge.c
@@ -689,7 +689,6 @@ static void blk_account_io_merge(struct request *req)
                 part_stat_lock();
                 part = req->part;
 
-                part_round_stats(req->q, part);
                 part_dec_in_flight(req->q, part, rq_data_dir(req));
 
                 hd_struct_put(part);
diff --git a/block/genhd.c b/block/genhd.c
index 2fe00cf32b93..cdf174d7d329 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -1337,9 +1337,6 @@ static int diskstats_show(struct seq_file *seqf, void *v)
 
         disk_part_iter_init(&piter, gp, DISK_PITER_INCL_EMPTY_PART0);
         while ((hd = disk_part_iter_next(&piter))) {
-                part_stat_lock();
-                part_round_stats(gp->queue, hd);
-                part_stat_unlock();
                 part_in_flight(gp->queue, hd, inflight);
                 seq_printf(seqf, "%4d %7d %s "
                            "%lu %lu %lu %u "
diff --git a/block/partition-generic.c b/block/partition-generic.c
index 7e663cfb1487..42d6138ac876 100644
--- a/block/partition-generic.c
+++ b/block/partition-generic.c
@@ -122,9 +122,6 @@ ssize_t part_stat_show(struct device *dev,
         struct request_queue *q = part_to_disk(p)->queue;
         unsigned int inflight[2];
 
-        part_stat_lock();
-        part_round_stats(q, p);
-        part_stat_unlock();
         part_in_flight(q, p, inflight);
         return sprintf(buf,
                 "%8lu %8lu %8llu %8u "