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>