[dm-devel] dm: dm-mpath: Provide high-resolution timer to HST with bio-mpath

[Mike Snitzer]

On Wed, Apr 27 2022 at 12:57P -0400,
Gabriel Krisman Bertazi <krisman at collabora.com> wrote:

> 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 they 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.  I don't
> think anyone is using HST with BIO mpath, but it'd be neat to get it
> sorted out.
> 
> 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 at collabora.com>
> Acked-by: Gabriel Krisman Bertazi <krisman at collabora.com>

Overall your code was OK, but I nudged it a bit further to be
inkeeping with how 'features' flags have been implemented elsewhere
(e.g. dm_target_type's features) -- by using a healer to test the
flag, etc.

I also tweaked some other small implementation details.  Please see:
https://git.kernel.org/pub/scm/linux/kernel/git/device-mapper/linux-dm.git/commit/?h=dm-5.19&id=c06dfd124d46df9c482fbd1319b5fe19bcb1a110