[PATCH V6 0/6] SCHED_DEADLINE server infrastructure
From: Daniel Bristot de Oliveira
Date: Fri Apr 05 2024 - 13:28:33 EST
This is v6 of Peter's SCHED_DEADLINE server infrastructure
implementation [1].
SCHED_DEADLINE servers can help fixing starvation issues of low priority
tasks (e.g., SCHED_OTHER) when higher priority tasks monopolize CPU
cycles. Today we have RT Throttling; DEADLINE servers should be able to
replace and improve that.
In the v1 there was discussion raised about the consequence of using
deadline based servers on the fixed-priority workloads. For a demonstration
here is the baseline of timerlat scheduling latency as-is, with kernel
build background workload:
# rtla timerlat top -u -d 10m
--------------------- %< ------------------------
Timer Latency
0 01:42:24 | IRQ Timer Latency (us) | Thread Timer Latency (us) | Ret user Timer Latency (us)
CPU COUNT | cur min avg max | cur min avg max | cur min avg max
0 #6143559 | 0 0 0 92 | 2 1 3 98 | 4 1 5 100
1 #6143559 | 1 0 0 97 | 7 1 5 101 | 9 1 7 103
2 #6143559 | 0 0 0 88 | 3 1 5 95 | 5 1 7 99
3 #6143559 | 0 0 0 90 | 6 1 5 103 | 10 1 7 126
4 #6143558 | 1 0 0 81 | 7 1 4 86 | 9 1 7 90
5 #6143558 | 0 0 0 74 | 3 1 5 79 | 4 1 7 83
6 #6143558 | 0 0 0 83 | 2 1 5 89 | 3 0 7 108
7 #6143558 | 0 0 0 85 | 3 1 4 126 | 5 1 6 137
--------------------- >% ------------------------
And this is the same tests with DL server activating without any delay:
--------------------- %< ------------------------
0 00:10:01 | IRQ Timer Latency (us) | Thread Timer Latency (us) | Ret user Timer Latency (us)
CPU COUNT | cur min avg max | cur min avg max | cur min avg max
0 #579147 | 0 0 0 54 | 2 1 52 61095 | 2 2 56 61102
1 #578766 | 0 0 0 83 | 2 1 49 55824 | 3 2 53 55831
2 #578559 | 0 0 1 59 | 2 1 50 55760 | 3 2 54 55770
3 #578318 | 0 0 0 76 | 2 1 49 55751 | 3 2 54 55760
4 #578611 | 0 0 0 64 | 2 1 49 55811 | 3 2 53 55820
5 #578347 | 0 0 1 40 | 2 1 50 56121 | 3 2 55 56133
6 #578938 | 0 0 1 75 | 2 1 49 55755 | 3 2 53 55764
7 #578631 | 0 0 1 36 | 3 1 51 55528 | 4 2 55 55541
--------------------- >% ------------------------
The problem with DL server only implementation is that FIFO tasks might
suffer preemption from NORMAL even when spare CPU cycles are available.
In fact, fair deadline server is enqueued right away when NORMAL tasks
wake up and they are first scheduled by the server, thus potentially
preempting a well behaving FIFO task. This is of course not ideal.
We had discussions about it, and one of the possibilities would be
using a different scheduling algorithm for this. But IMHO that is
an overkill.
Juri and I discussed this and though about delaying the server
activation for the (period - runtime), thus enabling the server
only if the fair scheduler is about to starve.
The patch 2 adds the possibility to defer the server start to the
(absolute deadline - runtime) point in time. This is achieved by
starting the dl server throttled, with a next replenishing time
set to activate the server at (absolute deadline - runtime).
The server is enqueued with the runtime replenished. As the fair
scheduler runs without boost, its runtime is consumed. If the
fair server has its runtime before the runtime - deadline time,
the a new period is set, and the timer armed for the new
deadline.
The patch 3 add a per_rq interface for the knobs:
fair_server_runtime (950 ms)
fair_server_period (1s)
fair_server_defer (enabled)
With defer enabled on CPUs [0:3], the results get better, having a
behavior similar to the one we have with the rt throttling.
--------------------- %< ------------------------
Timer Latency
0 00:10:01 | IRQ Timer Latency (us) | Thread Timer Latency (us) | Ret user Timer Latency (us)
CPU COUNT | cur min avg max | cur min avg max | cur min avg max
0 #599979 | 0 0 0 64 | 4 1 4 67 | 6 1 5 69
1 #599979 | 0 0 1 17 | 6 1 5 50 | 10 2 7 71
2 #599984 | 1 0 1 22 | 4 1 5 78 | 5 2 7 107
3 #599986 | 0 0 1 72 | 7 1 5 79 | 10 2 7 82
4 #581580 | 1 0 1 37 | 6 1 38 52797 | 10 2 41 52805
5 #583270 | 1 0 1 41 | 9 1 36 52617 | 12 2 38 52623
6 #581240 | 0 0 1 25 | 7 1 39 52870 | 11 2 41 52876
7 #581208 | 0 0 1 69 | 6 1 39 52917 | 9 2 41 52923
--------------------- >% ------------------------
Here are some osnoise measurement, with osnoise threads running as FIFO:1 with
different setups (defer enabled):
- CPU 2 isolated
- CPU 3 isolated shared with a CFS busy loop task
- CPU 8 non-isolated
- CPU 9 non-isolated shared with a CFS busy loop task
--------------------- %< ------------------------
~# pgrep ktimer | while read pid; do chrt -p -f 2 $pid; done # for RT kernel
~# sysctl kernel.sched_rt_runtime_us=-1
~# tuna isolate -c 2
~# tuna isolate -c 3
~# taskset -c 3 ./f &
~# taskset -c 9 ./f &
~# osnoise -P f:1 -c 2,3,8,9 -T 1 -d 10m -H 1
Operating System Noise
duration: 0 00:10:00 | time is in us
CPU Period Runtime Noise % CPU Aval Max Noise Max Single HW NMI IRQ Softirq Thread
2 #599 599000000 178 99.99997 18 2 0 0 270 0 0
3 #598 598054434 31351553 94.75774 104442 104442 0 0 2837523 0 1794
8 #599 599000001 567456 99.90526 3260 2375 2 89 620490 0 13539
9 #598 598021196 31742537 94.69207 71707 53357 0 90 3411023 0 1762
--------------------- >% ------------------------
the system runs fine!
- no crashes (famous last words)
- FIFO property is kept
- per cpu interface because it is more flexible - and to detach this from
the throttling concept.
Global is broken, but it will > /dev/null.
The selftest mentioned in the sched/core patches is here:
https://lore.kernel.org/all/20240313012451.1693807-8-joel@xxxxxxxxxxxxxxxxx/
Thanks people at google for testing/suggesting:
- Suleiman Souhlal <suleiman@xxxxxxxxxx>
- Youssef Esmat <youssefesmat@xxxxxxxxxx>
- Joel Fernandes (Google) <joel@xxxxxxxxxxxxxxxxx>
- Vineeth Pillai <vineeth@xxxxxxxxxxxxxxx>
Changes from V5:
- Fixes DL server for core scheduling (patches 3 and 4)
(Joel/Vineeth/Suleiman)
- Add a function to attach the fair server bandwidth to the
new root domain when the rq changes root domain (Daniel)
- Postpone the replenishment timer of the server if the defer reservation
could consume runtime while waiting to boost (patch 2) (Daniel)
- Add the running state to defer mode avoid forcing the defer mechanism
if the server continues to be activated due to starvation (patch 2)
(Daniel)
- Consider idle time as time for the defer server to avoid penalty on RT
tasks (patch 2) (Daniel)
- Mark DL server as unthrottled before enqueue (patch 2)(Joel)
- Make start_dl_timer callers more robust (patch 2) (Joel)
- Do not restart the DL server on replenish from timer (patch 2)(Joel)
- Fix Reverse args to dl_time_before in replenish (patch 2) (Suleiman Souhlal)
- Removed the negative runtime optimization (patch 2)
- Start the dl server as disabled in patch 1, enabling it only after
removing the RT throttling, to avoid having two mechanism together
by default (patch 1) (Daniel).
- Added a check need resched after dl_server_start (patch 1) (Daniel)
- reset dl_server pointer at put_prev_task_balance (patch 1) (Joel)
- Do not include the already merged patches
- Rebased to 6.9-rc2
Changes from V4:
- Enable the server when nr fair tasks is > 0 (peter)
- Consume runtime if the zerolax server is not boosted (peterz)
- Adjust interface to deal with admission control (peterz)
- Rebased to 6.6
Changes from V3:
- Add the defer server (Daniel)
- Add an per rq interface (Daniel with peter's feedback)
- Add an option not defer the server
- Typos and 1-liner fixes (Valentin, Luca, Peter)
- Fair scheduler running on dl server do not account as RT task (Daniel)
- Changed the condition to enable the server (RT & fair tasks) (Daniel)
Changes from v2:
- Refactor/rephrase/typos changes
- Defferable server using throttling
- The server starts when RT && Fair tasks are enqueued
- Interface with runtime/period/defer option
Changes from v1:
- rebased on 6.4-rc1 tip/sched/core
Daniel Bristot de Oliveira (2):
sched/deadline: Deferrable dl server
sched/fair: Fair server interface
Joel Fernandes (Google) (2):
sched/core: Fix priority checking for DL server picks
sched/core: Fix picking of tasks for core scheduling with DL server
Peter Zijlstra (2):
sched/fair: Add trivial fair server
sched/rt: Remove default bandwidth control
include/linux/sched.h | 6 +-
kernel/sched/core.c | 56 +++--
kernel/sched/deadline.c | 452 ++++++++++++++++++++++++++++++++++------
kernel/sched/debug.c | 209 +++++++++++++++++++
kernel/sched/fair.c | 66 +++++-
kernel/sched/idle.c | 2 +
kernel/sched/rt.c | 242 ++++++++++-----------
kernel/sched/sched.h | 17 +-
kernel/sched/topology.c | 8 +
9 files changed, 843 insertions(+), 215 deletions(-)
--
2.44.0