lkml.org 
[lkml]   [2019]   [Sep]   [25]   [last100]   RSS Feed
Views: [wrap][no wrap]   [headers]  [forward] 
 
Messages in this thread
Patch in this message
/
Date
From
Subject[PATCH 2/3 for-5.4/block] iocost: improve nr_lagging handling
Some IOs may span multiple periods.  As latencies are collected on
completion, the inbetween periods won't register them and may
incorrectly decide to increase vrate. nr_lagging tracks these IOs to
avoid those situations. Currently, whenever there are IOs which are
spanning from the previous period, busy_level is reset to 0 if
negative thus suppressing vrate increase.

This has the following two problems.

* When latency target percentiles aren't set, vrate adjustment should
only be governed by queue depth depletion; however, the current code
keeps nr_lagging active which pulls in latency results and can keep
down vrate unexpectedly.

* When lagging condition is detected, it resets the entire negative
busy_level. This turned out to be way too aggressive on some
devices which sometimes experience extended latencies on a small
subset of commands. In addition, a lagging IO will be accounted as
latency target miss on completion anyway and resetting busy_level
amplifies its impact unnecessarily.

This patch fixes the above two problems by disabling nr_lagging
counting when latency target percentiles aren't set and blocking vrate
increases when there are lagging IOs while leaving busy_level as-is.

Signed-off-by: Tejun Heo <tj@kernel.org>
---
block/blk-iocost.c | 19 +++++++++++--------
1 file changed, 11 insertions(+), 8 deletions(-)

--- a/block/blk-iocost.c
+++ b/block/blk-iocost.c
@@ -1407,7 +1407,8 @@ static void ioc_timer_fn(struct timer_li
* comparing vdone against period start. If lagging behind
* IOs from past periods, don't increase vrate.
*/
- if (!atomic_read(&iocg_to_blkg(iocg)->use_delay) &&
+ if ((ppm_rthr != MILLION || ppm_wthr != MILLION) &&
+ !atomic_read(&iocg_to_blkg(iocg)->use_delay) &&
time_after64(vtime, vdone) &&
time_after64(vtime, now.vnow -
MAX_LAGGING_PERIODS * period_vtime) &&
@@ -1537,21 +1538,23 @@ skip_surplus_transfers:
missed_ppm[WRITE] > ppm_wthr) {
ioc->busy_level = max(ioc->busy_level, 0);
ioc->busy_level++;
- } else if (nr_lagging) {
- ioc->busy_level = max(ioc->busy_level, 0);
- } else if (nr_shortages && !nr_surpluses &&
- rq_wait_pct <= RQ_WAIT_BUSY_PCT * UNBUSY_THR_PCT / 100 &&
+ } else if (rq_wait_pct <= RQ_WAIT_BUSY_PCT * UNBUSY_THR_PCT / 100 &&
missed_ppm[READ] <= ppm_rthr * UNBUSY_THR_PCT / 100 &&
missed_ppm[WRITE] <= ppm_wthr * UNBUSY_THR_PCT / 100) {
- ioc->busy_level = min(ioc->busy_level, 0);
- ioc->busy_level--;
+ /* take action iff there is contention */
+ if (nr_shortages && !nr_lagging) {
+ ioc->busy_level = min(ioc->busy_level, 0);
+ /* redistribute surpluses first */
+ if (!nr_surpluses)
+ ioc->busy_level--;
+ }
} else {
ioc->busy_level = 0;
}

ioc->busy_level = clamp(ioc->busy_level, -1000, 1000);

- if (ioc->busy_level) {
+ if (ioc->busy_level > 0 || (ioc->busy_level < 0 && !nr_lagging)) {
u64 vrate = atomic64_read(&ioc->vtime_rate);
u64 vrate_min = ioc->vrate_min, vrate_max = ioc->vrate_max;

\
 
 \ /
  Last update: 2019-09-26 01:03    [W:0.060 / U:24.620 seconds]
©2003-2020 Jasper Spaans|hosted at Digital Ocean and TransIP|Read the blog|Advertise on this site