[RFC PATCH v4 0/3] Softirq -rt Optimizations

[John Stultz]

Hey all,

This series is a set of patches that optimize scheduler decisions around
realtime tasks and softirqs.  This series is a rebased and reworked set
of changes that have been shipping on Android devices for a number of
years, originally created to resolve audio glitches seen on devices
caused by softirqs for network or storage drivers.

Long running softirqs cause issues because they aren’t currently taken
into account when a realtime task is woken up, but they will delay
realtime tasks from running if the realtime tasks are placed on a cpu
currently running a softirq.

This can easily be seen on some devices by running cyclictest* along
with some heavy background filesystems noise:

Without the patches:
T: 0 ( 7596) P:99 I:1000 C:  59980 Min:      7 Act:   13 Avg:   29 Max: 4107
T: 1 ( 7597) P:99 I:1500 C:  39990 Min:     14 Act:   26 Avg:   36 Max: 8994
T: 2 ( 7598) P:99 I:2000 C:  29995 Min:      7 Act:   22 Avg:   35 Max: 3616
T: 3 ( 7599) P:99 I:2500 C:  23915 Min:      7 Act:   25 Avg:   49 Max: 40273
T: 4 ( 7600) P:99 I:3000 C:  19995 Min:      8 Act:   22 Avg:   38 Max: 10510
T: 5 ( 7601) P:99 I:3500 C:  17135 Min:      7 Act:   26 Avg:   39 Max: 13194
T: 6 ( 7602) P:99 I:4000 C:  14990 Min:      7 Act:   26 Avg:   40 Max: 9470
T: 7 ( 7603) P:99 I:4500 C:  13318 Min:      8 Act:   29 Avg:   44 Max: 20101

Which you can visually see in the image here:
 https://github.com/johnstultz-work/misc/raw/main/images/2022-08-09-softirq-rt-big-latency.png

Which is from the perfetto trace captured here:
 https://ui.perfetto.dev/#!/?s=33661aec8ec82c2da0a59263f36f7d72b4a2f4e7a99b28b222bd12ad872f

The first patch adds a bit of generic infrastructure to get the per-cpu
softirq_pending flag.

The second patch in the series adds logic to account for when softirqs
are running, and then conditionally based on
CONFIG_RT_SOFTIRQ_OPTIMIZATION allows rt-task placement to be done in a
way that’s aware if a current softirq might be a long-running one, to
potentially place the rt task on another free core.

The third patch in the series adds logic in __do_softirq(), also under
CONFIG_RT_SOFTIRQ_OPTIMIZATION, to defer some of the potentially long
running softirqs to ksoftirqd if a -rt task is currently running on the
cpu. This patch also includes a folded down fix that stubbs out
ksoftirqd_running() based on CONFIG_RT_SOFTIRQ_OPTIMIZATION, since in
changing to more frequently defer long running softirqs, the logic using
ksoftirqd_running will end up being too conservative and needlessly
delay shorter-running softirqs.

With these patches we see dramatic improvements in the worst case
latencies in the cyclictest* + filesystem noise test above:

With the patches
T: 0 ( 7527) P:99 I:1000 C:  59998 Min:      6 Act:   29 Avg:   35 Max: 1734
T: 1 ( 7528) P:99 I:1500 C:  40000 Min:      7 Act:   39 Avg:   35 Max: 1181
T: 2 ( 7529) P:99 I:2000 C:  30000 Min:      7 Act:   25 Avg:   25 Max: 444
T: 3 ( 7530) P:99 I:2500 C:  24000 Min:      7 Act:   34 Avg:   36 Max: 1729
T: 4 ( 7531) P:99 I:3000 C:  20000 Min:      7 Act:   36 Avg:   25 Max: 406
T: 5 ( 7532) P:99 I:3500 C:  17143 Min:      7 Act:   38 Avg:   34 Max: 1264
T: 6 ( 7533) P:99 I:4000 C:  15000 Min:      7 Act:   27 Avg:   33 Max: 2351
T: 7 ( 7534) P:99 I:4500 C:  13334 Min:      7 Act:   41 Avg:   29 Max: 2285

Since these patches have been carried along for years, and have at times
badly collided with upstream, I wanted to submit them for some initial
review, discussion and feedback so we could hopefully eventually find a
reasonable solution that might land upstream.


* Unfortunately cyclictest had a bug that causes it to always affine
threads to cpus preventing them from being migrated. So you’ll need
to update to the latest version (which includes a fix) to reproduce.

Let me know what you think!

thanks
-john


Chages in v4:
* Drop TASKLET_SOFTIRQ from LONG_SOFTIRQS (suggested by Qais)
* Depend on !PREEMPT_RT (Suggested by Qais)
* Larger simplification of logic (suggested by Qais)
* Rework LONG_SOFTIRQS to use BIT() macros
* Rename task_may_preempt() to cpu_busy_with_softirqs()
* Fix commit message to accurately note long-running softirqs
  (suggested by Qais)
* Switch to using rt_task(current) (suggested by Qais)


Connor O'Brien (1):
  sched: Avoid placing RT threads on cores handling long softirqs

John Stultz (1):
  softirq: Add generic accessor to percpu softirq_pending data

Pavankumar Kondeti (1):
  softirq: defer softirq processing to ksoftirqd if CPU is busy with RT

 arch/s390/include/asm/hardirq.h |  6 ++++
 include/linux/interrupt.h       | 17 +++++++++
 init/Kconfig                    | 10 ++++++
 kernel/sched/rt.c               | 61 ++++++++++++++++++++++++++++-----
 kernel/softirq.c                | 38 ++++++++++++++++++--
 5 files changed, 121 insertions(+), 11 deletions(-)

-- 
2.38.0.rc1.362.ged0d419d3c-goog