Re: sched/deadline: Use revised wakeup rule for dl_server
From: Andreas Ziegler
Date: Fri May 08 2026 - 08:09:49 EST
Hi Christian,
On 2026-05-08 09:20, Christian Loehle wrote:
On 5/8/26 09:09, Andreas Ziegler wrote:
Linux kernel version: 6.12
CONFIG_PREEMPT_RT (w/ PREEMPT_RT patch applied)
Architecture: aarch64
Platform: Raspberry Pi 4
Hi everyone,
Commit d66792919d4f (sched/deadline: Use revised wakeup rule for dl_server) [1] introduced a marked degradation in scheduling latency for real-time tasks in the presence of heavy I/O load.
--- a/kernel/sched/deadline.c
+++ b/kernel/sched/deadline.c
@@ -1079,7 +1079,7 @@ static void update_dl_entity(struct sched_dl_entity *dl_se)
if (dl_time_before(dl_se->deadline, rq_clock(rq)) ||
dl_entity_overflow(dl_se, rq_clock(rq))) {
- if (unlikely(!dl_is_implicit(dl_se) &&
+ if (unlikely((!dl_is_implicit(dl_se) || dl_se->dl_defer) &&
!dl_time_before(dl_se->deadline, rq_clock(rq)) &&
!is_dl_boosted(dl_se))) {
update_dl_revised_wakeup(dl_se, rq);
This was observed using a modified version of Con Kolivas' interactivity benchmark [2]; kernel bisection eventually pointed to the above mentioned commit.
Benchmark results before d66792919d4f:
--- Benchmarking simulated cpu of Audio real time in the presence of simulated ---
Load Latency +/- SD median max [100n] Desired CPU Deadlines met [%]
None 76.6 +/- 8.3654 76 166
Video 78.5 +/- 3.9433 78 107
X 76.4 +/- 8.123 75 157
Burn 72.0 +/- 6.4733 71 127
Write 255.3 +/- 26.627 252 331
Read 226.6 +/- 12.38 227 262
Ring 84.2 +/- 6.6207 83 125
Compile 225.3 +/- 23.949 222 328
136.8 +/- 78.462 331
Benchmark results after d66792919d4f:
--- Benchmarking simulated cpu of Audio real time in the presence of simulated ---
Load Latency +/- SD median max [100n] Desired CPU Deadlines met [%]
None 68.4 +/- 9.7864 67 169
Video 74.4 +/- 3.724 74 97
X 72.0 +/- 6.5681 71 129
Burn 66.9 +/- 5.9059 66 117
Write 9576.9 +/- 67639 250500418 98.1 98.1
Read 209.3 +/- 11.018 209 267
Ring 80.5 +/- 8.0993 78 125
Compile 239.0 +/- 29.447 234 372
1298.4 +/- 24118 500418
Reverting this commit obviously solves the issue for me. I have no idea why this issue appears exclusively with heavy write loads in the background.
Is this a scheduler issue, or rather something in the background?
Hi Andreas,
You're using cpufreq schedutil for your tests I'm assuming?
Is there a difference in cpufreq behavior (avg cpufreq or OPP residencies?)
Does the regression also happen on powersave/performance governor?
Actually this is a very stripped-down system. The 'performance' cpufreq governor is the only one compiled in, the processor cores run on a fixed frequency. CONFIG_PM_OPP is not set.
Removing the frequency constraint and using 'powersave' governor lets the latency values rise generally, but the anomaly under write loads persists. The cpu frequency does not change, but remains stuck on the lowest level.
--- Benchmarking simulated cpu of Audio real time in the presence of simulated ---
Load Latency +/- SD median max [100n] Desired CPU Deadlines met [%]
None 238.7 +/- 31.416 229 405
Video 228.6 +/- 13.668 226 291
X 247.8 +/- 29.196 239 425
Burn 222.6 +/- 30.631 215 348
Write 1214.8 +/- 20397 369500411 99.8 99.8
Read 393.9 +/- 21.375 394 476
Ring 250.3 +/- 27.59 241 365
Compile 411.2 +/- 23.41 411 474
401.0 +/- 7218.2 500411
Same with 'schedutil' governor; the cpu frequency adjusts with the load.
--- Benchmarking simulated cpu of Audio real time in the presence of simulated ---
Load Latency +/- SD median max [100n] Desired CPU Deadlines met [%]
None 200.9 +/- 57.332 208 431
Video 136.2 +/- 23.784 136 250
X 172.3 +/- 59.286 174 404
Burn 104.1 +/- 22.847 97 247
Write 5337.5 +/- 49960 286500394 99 99
Read 300.5 +/- 18.65 301 359
Ring 119.8 +/- 15.8 115 196
Compile 282.7 +/- 25.056 280 469
831.7 +/- 17746 500394
Kind regards,
Andreas