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Date:   Wed, 21 Sep 2022 01:25:47 +0000
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Message-ID: <20220921012550.3288570-1-jstultz@google.com>
Subject: [RFC][PATCH v3 0/3] Softirq -rt Optimizations
From:   John Stultz <jstultz@google.com>
To:     LKML <linux-kernel@vger.kernel.org>
Cc:     John Stultz <jstultz@google.com>, John Dias <joaodias@google.com>,
        "Connor O'Brien" <connoro@google.com>,
        Rick Yiu <rickyiu@google.com>, John Kacur <jkacur@redhat.com>,
        Qais Yousef <qais.yousef@arm.com>,
        Chris Redpath <chris.redpath@arm.com>,
        Abhijeet Dharmapurikar <adharmap@quicinc.com>,
        Peter Zijlstra <peterz@infradead.org>,
        Ingo Molnar <mingo@redhat.com>,
        Juri Lelli <juri.lelli@redhat.com>,
        Vincent Guittot <vincent.guittot@linaro.org>,
        Dietmar Eggemann <dietmar.eggemann@arm.com>,
        Steven Rostedt <rostedt@goodmis.org>,
        Thomas Gleixner <tglx@linutronix.de>,
        Heiko Carstens <hca@linux.ibm.com>,
        Vasily Gorbik <gor@linux.ibm.com>, kernel-team@android.com
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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=E2=80=99t currently ta=
ken
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: 410=
7
T: 1 ( 7597) P:99 I:1500 C:  39990 Min:     14 Act:   26 Avg:   36 Max: 899=
4
T: 2 ( 7598) P:99 I:2000 C:  29995 Min:      7 Act:   22 Avg:   35 Max: 361=
6
T: 3 ( 7599) P:99 I:2500 C:  23915 Min:      7 Act:   25 Avg:   49 Max: 402=
73
T: 4 ( 7600) P:99 I:3000 C:  19995 Min:      8 Act:   22 Avg:   38 Max: 105=
10
T: 5 ( 7601) P:99 I:3500 C:  17135 Min:      7 Act:   26 Avg:   39 Max: 131=
94
T: 6 ( 7602) P:99 I:4000 C:  14990 Min:      7 Act:   26 Avg:   40 Max: 947=
0
T: 7 ( 7603) P:99 I:4500 C:  13318 Min:      8 Act:   29 Avg:   44 Max: 201=
01

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=3D33661aec8ec82c2da0a59263f36f7d72b4a2f4e7a9=
9b28b222bd12ad872f

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=E2=80=99s 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: 173=
4
T: 1 ( 7528) P:99 I:1500 C:  40000 Min:      7 Act:   39 Avg:   35 Max: 118=
1
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: 172=
9
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: 126=
4
T: 6 ( 7533) P:99 I:4000 C:  15000 Min:      7 Act:   27 Avg:   33 Max: 235=
1
T: 7 ( 7534) P:99 I:4500 C:  13334 Min:      7 Act:   41 Avg:   29 Max: 228=
5

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=E2=80=99ll need
to update to the latest version (which includes a fix) to reproduce.

Let me know what you think!

thanks
-john

Changes in v3:
* Added generic __cpu_softirq_pending() accessor to avoid s390 build
  trouble

Cc: John Dias <joaodias@google.com>
Cc: Connor O'Brien <connoro@google.com>
Cc: Rick Yiu <rickyiu@google.com>
Cc: John Kacur <jkacur@redhat.com>
Cc: Qais Yousef <qais.yousef@arm.com>
Cc: Chris Redpath <chris.redpath@arm.com>
Cc: Abhijeet Dharmapurikar <adharmap@quicinc.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Juri Lelli <juri.lelli@redhat.com>
Cc: Vincent Guittot <vincent.guittot@linaro.org>
Cc: Dietmar Eggemann <dietmar.eggemann@arm.com>
Cc: Steven Rostedt <rostedt@goodmis.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: Heiko Carstens <hca@linux.ibm.com>
Cc: Vasily Gorbik <gor@linux.ibm.com>
Cc: kernel-team@android.com


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       | 18 ++++++++++
 include/linux/sched.h           | 10 ++++++
 init/Kconfig                    | 10 ++++++
 kernel/sched/cpupri.c           | 13 +++++++
 kernel/sched/rt.c               | 64 ++++++++++++++++++++++++++++-----
 kernel/softirq.c                | 34 ++++++++++++++++--
 7 files changed, 144 insertions(+), 11 deletions(-)

--=20
2.37.3.968.ga6b4b080e4-goog