dm stats: fix too short end duration_ns when using precise_timestamps

dm_stats_account_io()'s STAT_PRECISE_TIMESTAMPS support doesn't handle
the fact that with commit b879f915 ("dm: properly fix redundant
bio-based IO accounting") io->start_time _may_ be in the past (meaning
the start_io_acct() was deferred until later).

Add a new dm_stats_recalc_precise_timestamps() helper that will
set/clear a new 'precise_timestamps' flag in the dm_stats struct based
on whether any configured stats enable STAT_PRECISE_TIMESTAMPS.
And update DM core's alloc_io() to use dm_stats_record_start() to set
stats_aux.duration_ns if stats->precise_timestamps is true.

Also, remove unused 'last_sector' and 'last_rw' members from the
dm_stats struct.

Fixes: b879f915 ("dm: properly fix redundant bio-based IO accounting")
Cc: stable@vger.kernel.org
Co-developed-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mikulas Patocka <mpatocka@redhat.com>
Signed-off-by: Mike Snitzer <snitzer@redhat.com>

drivers/md/dm-stats.c

@@ -195,6 +195,7 @@ void dm_stats_init(struct dm_stats *stats)
 
 	mutex_init(&stats->mutex);
 	INIT_LIST_HEAD(&stats->list);
+	stats->precise_timestamps = false;
 	stats->last = alloc_percpu(struct dm_stats_last_position);
 	for_each_possible_cpu(cpu) {
 		last = per_cpu_ptr(stats->last, cpu);
@@ -231,6 +232,22 @@ void dm_stats_cleanup(struct dm_stats *stats)
 	mutex_destroy(&stats->mutex);
 }
 
+static void dm_stats_recalc_precise_timestamps(struct dm_stats *stats)
+{
+	struct list_head *l;
+	struct dm_stat *tmp_s;
+	bool precise_timestamps = false;
+
+	list_for_each(l, &stats->list) {
+		tmp_s = container_of(l, struct dm_stat, list_entry);
+		if (tmp_s->stat_flags & STAT_PRECISE_TIMESTAMPS) {
+			precise_timestamps = true;
+			break;
+		}
+	}
+	stats->precise_timestamps = precise_timestamps;
+}
+
 static int dm_stats_create(struct dm_stats *stats, sector_t start, sector_t end,
 			   sector_t step, unsigned stat_flags,
 			   unsigned n_histogram_entries,
@@ -376,6 +393,9 @@ static int dm_stats_create(struct dm_stats *stats, sector_t start, sector_t end,
 	}
 	ret_id = s->id;
 	list_add_tail_rcu(&s->list_entry, l);
+
+	dm_stats_recalc_precise_timestamps(stats);
+
 	mutex_unlock(&stats->mutex);
 
 	resume_callback(md);
@@ -418,6 +438,9 @@ static int dm_stats_delete(struct dm_stats *stats, int id)
 	}
 
 	list_del_rcu(&s->list_entry);
+
+	dm_stats_recalc_precise_timestamps(stats);
+
 	mutex_unlock(&stats->mutex);
 
 	/*
@@ -656,9 +679,8 @@ void dm_stats_account_io(struct dm_stats *stats, unsigned long bi_rw,
 	got_precise_time = false;
 	list_for_each_entry_rcu(s, &stats->list, list_entry) {
 		if (s->stat_flags & STAT_PRECISE_TIMESTAMPS && !got_precise_time) {
-			if (!end)
-				stats_aux->duration_ns = ktime_to_ns(ktime_get());
-			else
+			/* start (!end) duration_ns is set by DM core's alloc_io() */
+			if (end)
 				stats_aux->duration_ns = ktime_to_ns(ktime_get()) - stats_aux->duration_ns;
 			got_precise_time = true;
 		}

drivers/md/dm-stats.h

@@ -13,8 +13,7 @@ struct dm_stats {
 	struct mutex mutex;
 	struct list_head list;	/* list of struct dm_stat */
 	struct dm_stats_last_position __percpu *last;
-	sector_t last_sector;
-	unsigned last_rw;
+	bool precise_timestamps;
 };
 
 struct dm_stats_aux {
@@ -40,4 +39,10 @@ static inline bool dm_stats_used(struct dm_stats *st)
 	return !list_empty(&st->list);
 }
 
+static inline void dm_stats_record_start(struct dm_stats *stats, struct dm_stats_aux *aux)
+{
+	if (unlikely(stats->precise_timestamps))
+		aux->duration_ns = ktime_to_ns(ktime_get());
+}
+
 #endif

drivers/md/dm.c

@@ -552,6 +552,8 @@ static struct dm_io *alloc_io(struct mapped_device *md, struct bio *bio)
 
 	io->start_time = jiffies;
 
+	dm_stats_record_start(&md->stats, &io->stats_aux);
+
 	return io;
 }