dm mpath: provide high-resolution timer to HST for bio-based

The precision loss of reading IO start_time with jiffies_to_nsecs
instead of using a high resolution timer degrades HST path prediction
for BIO-based mpath on high load workloads.

Below, I show the utilization percentage of a 10 disk multipath with
asymmetrical disk access cost, while being exercised by a randwrite FIO
benchmark with high submission queue depth (depth=64).  It is possible
to see that the HST path selection degrades heavily for high-iops in
BIO-mpath, underutilizing the slower paths way beyond expected.  This
seems to be caused by the start_time truncation, which makes some IO to
seem much slower than it actually is.  In this scenario ST outperforms
HST for bio-mpath, but not for mq-mpath, which already uses ktime_get_ns().

The third column shows utilization with this patch applied.  It is easy
to see that now HST prediction is much closer to the ideal distribution
(calculated considering the real cost of each path).

|     |   ST | HST (orig) | HST(ktime) | Best |
| sdd | 0.17 |       0.20 |       0.17 | 0.18 |
| sde | 0.17 |       0.20 |       0.17 | 0.18 |
| sdf | 0.17 |       0.20 |       0.17 | 0.18 |
| sdg | 0.06 |       0.00 |       0.06 | 0.04 |
| sdh | 0.03 |       0.00 |       0.03 | 0.02 |
| sdi | 0.03 |       0.00 |       0.03 | 0.02 |
| sdj | 0.02 |       0.00 |       0.01 | 0.01 |
| sdk | 0.02 |       0.00 |       0.01 | 0.01 |
| sdl | 0.17 |       0.20 |       0.17 | 0.18 |
| sdm | 0.17 |       0.20 |       0.17 | 0.18 |

This issue was originally discussed [1] when we first merged HST, and
this patch was left as a low hanging fruit to be solved later.

Regarding the implementation, as suggested by Mike in that mail thread,
in order to avoid the overhead of ktime_get_ns for other selectors, this
patch adds a flag for the selector code to request the high-resolution
timer.

I tested this using the same benchmark used in the original HST submission.

Full test and benchmark scripts are available here:

  https://people.collabora.com/~krisman/HST-BIO-MPATH/

[1] https://lore.kernel.org/lkml/85tv0am9de.fsf@collabora.com/T/

Signed-off-by: Gabriel Krisman Bertazi <krisman@collabora.com>
[snitzer: cleaned up various implementation details]
Signed-off-by: Mike Snitzer <snitzer@kernel.org>

1 files changed, 15 insertions, 0 deletions

diff --git a/drivers/md/dm-path-selector.h b/drivers/md/dm-path-selector.h
index c47bc0e20275..83cac2b04b66 100644
--- a/drivers/md/dm-path-selector.h
+++ b/drivers/md/dm-path-selector.h
@@ -26,11 +26,26 @@ struct path_selector {
 	void *context;
 };
 
+/*
+ * If a path selector uses this flag, a high resolution timer is used
+ * (via ktime_get_ns) to account for IO start time in BIO-based mpath.
+ * This improves performance of some path selectors (i.e. HST), in
+ * exchange for slightly higher overhead when submitting the BIO.
+ * The extra cost is usually offset by improved path selection for
+ * some benchmarks.
+ *
+ * This has no effect for request-based mpath, since it already uses a
+ * higher precision timer by default.
+ */
+#define DM_PS_USE_HR_TIMER		0x00000001
+#define dm_ps_use_hr_timer(type)	((type)->features & DM_PS_USE_HR_TIMER)
+
 /* Information about a path selector type */
 struct path_selector_type {
 	char *name;
 	struct module *module;
 
+	unsigned int features;
 	unsigned int table_args;
 	unsigned int info_args;