block: use nanosecond resolution for iostat

Commit Message

Omar Sandoval Sept. 21, 2018, 11:44 p.m. UTC

From: Omar Sandoval <[e-mail]>

Klaus Kusche reported that the I/O busy time in /proc/diskstats was not
updating properly on 4.18. This is because we started using ktime to
track elapsed time, and we convert nanoseconds to jiffies when we update
the partition counter. However, this gets rounded down, so any I/Os that
take less than a jiffy are not accounted for. Previously in this case,
the value of jiffies would sometimes increment while we were doing I/O,
so at least some I/Os were accounted for.

Let's convert the stats to use nanoseconds internally. We still report
milliseconds as before, now more accurately than ever. The value is
still truncated to 32 bits for backwards compatibility.

Fixes: 522a777566f5 ("block: consolidate struct request timestamp fields")
Cc: [e-mail]
Reported-by: Klaus Kusche <[e-mail]>
Signed-off-by: Omar Sandoval <[e-mail]>
---
 block/bio.c               | 2 +-
 block/blk-core.c          | 4 +---
 block/genhd.c             | 6 +++---
 block/partition-generic.c | 6 +++---
 include/linux/genhd.h     | 5 ++++-
 5 files changed, 12 insertions(+), 11 deletions(-)

Comments

Jens Axboe Sept. 22, 2018, 2:27 a.m. UTC | #1

On 9/21/18 5:44 PM, Omar Sandoval wrote:
> From: Omar Sandoval <[e-mail]>
> 
> Klaus Kusche reported that the I/O busy time in /proc/diskstats was not
> updating properly on 4.18. This is because we started using ktime to
> track elapsed time, and we convert nanoseconds to jiffies when we update
> the partition counter. However, this gets rounded down, so any I/Os that
> take less than a jiffy are not accounted for. Previously in this case,
> the value of jiffies would sometimes increment while we were doing I/O,
> so at least some I/Os were accounted for.
> 
> Let's convert the stats to use nanoseconds internally. We still report
> milliseconds as before, now more accurately than ever. The value is
> still truncated to 32 bits for backwards compatibility.

Thanks Omar, applied for 4.19.

Omar Sandoval Sept. 24, 2018, 5:31 p.m. UTC | #2

On Fri, Sep 21, 2018 at 08:27:17PM -0600, Jens Axboe wrote:
> On 9/21/18 5:44 PM, Omar Sandoval wrote:
> > From: Omar Sandoval <[e-mail]>
> > 
> > Klaus Kusche reported that the I/O busy time in /proc/diskstats was not
> > updating properly on 4.18. This is because we started using ktime to
> > track elapsed time, and we convert nanoseconds to jiffies when we update
> > the partition counter. However, this gets rounded down, so any I/Os that
> > take less than a jiffy are not accounted for. Previously in this case,
> > the value of jiffies would sometimes increment while we were doing I/O,
> > so at least some I/Os were accounted for.
> > 
> > Let's convert the stats to use nanoseconds internally. We still report
> > milliseconds as before, now more accurately than ever. The value is
> > still truncated to 32 bits for backwards compatibility.
> 
> Thanks Omar, applied for 4.19.

Thanks, Jens. I also just pushed a regression test to blktests.

Patch

diff --git a/block/bio.c b/block/bio.c
index 8c680a776171..0093bed81c0e 100644
--- a/block/bio.c
+++ b/block/bio.c
@@ -1684,7 +1684,7 @@  void generic_end_io_acct(struct request_queue *q, int req_op,
 	const int sgrp = op_stat_group(req_op);
 	int cpu = part_stat_lock();
 
-	part_stat_add(cpu, part, ticks[sgrp], duration);
+	part_stat_add(cpu, part, nsecs[sgrp], jiffies_to_nsecs(duration));
 	part_round_stats(q, cpu, part);
 	part_dec_in_flight(q, part, op_is_write(req_op));
 
diff --git a/block/blk-core.c b/block/blk-core.c
index 4dbc93f43b38..cff0a60ee200 100644
--- a/block/blk-core.c
+++ b/block/blk-core.c
@@ -2733,17 +2733,15 @@  void blk_account_io_done(struct request *req, u64 now)
 	 * containing request is enough.
 	 */
 	if (blk_do_io_stat(req) && !(req->rq_flags & RQF_FLUSH_SEQ)) {
-		unsigned long duration;
 		const int sgrp = op_stat_group(req_op(req));
 		struct hd_struct *part;
 		int cpu;
 
-		duration = nsecs_to_jiffies(now - req->start_time_ns);
 		cpu = part_stat_lock();
 		part = req->part;
 
 		part_stat_inc(cpu, part, ios[sgrp]);
-		part_stat_add(cpu, part, ticks[sgrp], duration);
+		part_stat_add(cpu, part, nsecs[sgrp], now - req->start_time_ns);
 		part_round_stats(req->q, cpu, part);
 		part_dec_in_flight(req->q, part, rq_data_dir(req));
 
diff --git a/block/genhd.c b/block/genhd.c
index 8cc719a37b32..be5bab20b2ab 100644
--- a/block/genhd.c
+++ b/block/genhd.c
@@ -1343,18 +1343,18 @@  static int diskstats_show(struct seq_file *seqf, void *v)
 			   part_stat_read(hd, ios[STAT_READ]),
 			   part_stat_read(hd, merges[STAT_READ]),
 			   part_stat_read(hd, sectors[STAT_READ]),
-			   jiffies_to_msecs(part_stat_read(hd, ticks[STAT_READ])),
+			   (unsigned int)part_stat_read_msecs(hd, STAT_READ),
 			   part_stat_read(hd, ios[STAT_WRITE]),
 			   part_stat_read(hd, merges[STAT_WRITE]),
 			   part_stat_read(hd, sectors[STAT_WRITE]),
-			   jiffies_to_msecs(part_stat_read(hd, ticks[STAT_WRITE])),
+			   (unsigned int)part_stat_read_msecs(hd, STAT_WRITE),
 			   inflight[0],
 			   jiffies_to_msecs(part_stat_read(hd, io_ticks)),
 			   jiffies_to_msecs(part_stat_read(hd, time_in_queue)),
 			   part_stat_read(hd, ios[STAT_DISCARD]),
 			   part_stat_read(hd, merges[STAT_DISCARD]),
 			   part_stat_read(hd, sectors[STAT_DISCARD]),
-			   jiffies_to_msecs(part_stat_read(hd, ticks[STAT_DISCARD]))
+			   (unsigned int)part_stat_read_msecs(hd, STAT_DISCARD)
 			);
 	}
 	disk_part_iter_exit(&piter);
diff --git a/block/partition-generic.c b/block/partition-generic.c
index 5a8975a1201c..d3d14e81fb12 100644
--- a/block/partition-generic.c
+++ b/block/partition-generic.c
@@ -136,18 +136,18 @@  ssize_t part_stat_show(struct device *dev,
 		part_stat_read(p, ios[STAT_READ]),
 		part_stat_read(p, merges[STAT_READ]),
 		(unsigned long long)part_stat_read(p, sectors[STAT_READ]),
-		jiffies_to_msecs(part_stat_read(p, ticks[STAT_READ])),
+		(unsigned int)part_stat_read_msecs(p, STAT_READ),
 		part_stat_read(p, ios[STAT_WRITE]),
 		part_stat_read(p, merges[STAT_WRITE]),
 		(unsigned long long)part_stat_read(p, sectors[STAT_WRITE]),
-		jiffies_to_msecs(part_stat_read(p, ticks[STAT_WRITE])),
+		(unsigned int)part_stat_read_msecs(p, STAT_WRITE),
 		inflight[0],
 		jiffies_to_msecs(part_stat_read(p, io_ticks)),
 		jiffies_to_msecs(part_stat_read(p, time_in_queue)),
 		part_stat_read(p, ios[STAT_DISCARD]),
 		part_stat_read(p, merges[STAT_DISCARD]),
 		(unsigned long long)part_stat_read(p, sectors[STAT_DISCARD]),
-		jiffies_to_msecs(part_stat_read(p, ticks[STAT_DISCARD])));
+		(unsigned int)part_stat_read_msecs(p, STAT_DISCARD));
 }
 
 ssize_t part_inflight_show(struct device *dev, struct device_attribute *attr,
diff --git a/include/linux/genhd.h b/include/linux/genhd.h
index 57864422a2c8..25c08c6c7f99 100644
--- a/include/linux/genhd.h
+++ b/include/linux/genhd.h
@@ -83,10 +83,10 @@  struct partition {
 } __attribute__((packed));
 
 struct disk_stats {
+	u64 nsecs[NR_STAT_GROUPS];
 	unsigned long sectors[NR_STAT_GROUPS];
 	unsigned long ios[NR_STAT_GROUPS];
 	unsigned long merges[NR_STAT_GROUPS];
-	unsigned long ticks[NR_STAT_GROUPS];
 	unsigned long io_ticks;
 	unsigned long time_in_queue;
 };
@@ -354,6 +354,9 @@  static inline void free_part_stats(struct hd_struct *part)
 
 #endif /* CONFIG_SMP */
 
+#define part_stat_read_msecs(part, which)				\
+	div_u64(part_stat_read(part, nsecs[which]), NSEC_PER_MSEC)
+
 #define part_stat_read_accum(part, field)				\
 	(part_stat_read(part, field[STAT_READ]) +			\
 	 part_stat_read(part, field[STAT_WRITE]) +			\