Re: [RFC PATCH] sched/fair: Choose the CPU where short task is running during wake up

From: Honglei Wang
Date: Thu Sep 29 2022 - 03:02:46 EST

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On 2022/9/29 13:25, Chen Yu wrote:

Hi Prateek,
On 2022-09-26 at 11:20:16 +0530, K Prateek Nayak wrote:

Hello Chenyu,

When testing the patch on a dual socket Zen3 system (3 x 64C/128T) we
noticed some regressions in some standard benchmark.

tl;dr

o Hackbench shows noticeable regression in most cases. Looking at schedstat
data, we see there is an increased number of affine wakeup and an increase
in the average wait time. As the LLC size on the Zen3 machine is only
16 CPUs, there is good chance the LLC was overloaded and it required
intervention from load balancer to distribute tasks optimally.

o There is a regression in Stream which is cause by piling up of more than
one Stream thread on the same LLC. This happens as a result of migration
in the wakeup path where the logic goes for an affine wakeup if the
waker is short lived task even if sync flag is not set and the previous
CPU might be idle.

Nice analysis and thanks for your testing.

I'll inline the results are detailed observation below:

On 9/15/2022 10:24 PM, Chen Yu wrote:

[Background]
At LPC 2022 Real-time and Scheduling Micro Conference we presented
the cross CPU wakeup issue. This patch is a text version of the
talk, and hopefully we can clarify the problem and appreciate for any
feedback.

[re-send due to the previous one did not reach LKML, sorry
for any inconvenience.]

[Problem Statement]
For a workload that is doing frequent context switches, the throughput
scales well until the number of instances reaches a peak point. After
that peak point, the throughput drops significantly if the number of
instances continues to increase.

The will-it-scale context_switch1 test case exposes the issue. The
test platform has 112 CPUs per LLC domain. The will-it-scale launches
1, 8, 16 ... 112 instances respectively. Each instance is composed
of 2 tasks, and each pair of tasks would do ping-pong scheduling via
pipe_read() and pipe_write(). No task is bound to any CPU.
We found that, once the number of instances is higher than
56(112 tasks in total, every CPU has 1 task), the throughput
drops accordingly if the instance number continues to increase:

^
throughput|
| X
| X X X
| X X X
| X X
| X X
| X
| X
| X
| X
|
+-----------------.------------------->
56
number of instances

[Symptom analysis]
Both perf profile and lockstat have shown that, the bottleneck
is the runqueue spinlock. Take perf profile for example:

nr_instance rq lock percentage
1 1.22%
8 1.17%
16 1.20%
24 1.22%
32 1.46%
40 1.61%
48 1.63%
56 1.65%
--------------------------
64 3.77% |
72 5.90% | increase
80 7.95% |
88 9.98% v
96 11.81%
104 13.54%
112 15.13%

And the rq lock bottleneck is composed of two paths(perf profile):

(path1):
raw_spin_rq_lock_nested.constprop.0;
try_to_wake_up;
default_wake_function;
autoremove_wake_function;
__wake_up_common;
__wake_up_common_lock;
__wake_up_sync_key;
pipe_write;
new_sync_write;
vfs_write;
ksys_write;
__x64_sys_write;
do_syscall_64;
entry_SYSCALL_64_after_hwframe;write

(path2):
raw_spin_rq_lock_nested.constprop.0;
__sched_text_start;
schedule_idle;
do_idle;
cpu_startup_entry;
start_secondary;
secondary_startup_64_no_verify

The idle percentage is around 30% when there are 112 instances:
%Cpu0 : 2.7 us, 66.7 sy, 0.0 ni, 30.7 id

As a comparison, if we set CPU affinity to these workloads,
which stops them from migrating among CPUs, the idle percentage
drops to nearly 0%, and the throughput increases by about 300%.
This indicates that there is room for optimization.

A possible scenario to describe the lock contention:
task A tries to wakeup task B on CPU1, then task A grabs the
runqueue lock of CPU1. If CPU1 is about to quit idle, it needs
to grab its own lock which has been taken by someone else. Then
CPU1 takes more time to quit which hurts the performance.

TTWU_QUEUE could mitigate the cross CPU runqueue lock contention.
Since commit f3dd3f674555 ("sched: Remove the limitation of WF_ON_CPU
on wakelist if wakee cpu is idle"), TTWU_QUEUE offloads the work from
the waker and leverages the idle CPU to queue the wakee. However, a long
idle duration is still observed. The idle task spends quite some time
on sched_ttwu_pending() before it switches out. This long idle
duration would mislead SIS_UTIL, then SIS_UTIL suggests the waker scan
for more CPUs. The time spent searching for an idle CPU would make
wakee waiting for more time, which in turn leads to more idle time.
The NEWLY_IDLE balance fails to pull tasks to the idle CPU, which
might be caused by no runnable wakee being found.

[Proposal]
If a system is busy, and if the workloads are doing frequent context
switches, it might not be a good idea to spread the wakee on different
CPUs. Instead, consider the task running time and enhance wake affine
might be applicable.

This idea has been suggested by Rik at LPC 2019 when discussing
the latency nice. He asked the following question: if P1 is a small-time
slice task on CPU, can we put the waking task P2 on the CPU and wait for
P1 to release the CPU, without wasting time to search for an idle CPU?
At LPC 2021 Vincent Guittot has proposed:
1. If the wakee is a long-running task, should we skip the short idle CPU?
2. If the wakee is a short-running task, can we put it onto a lightly loaded
local CPU?

Current proposal is a variant of 2:
If the target CPU is running a short-time slice task, and the wakee
is also a short-time slice task, the target CPU could be chosen as the
candidate when the system is busy.

The definition of a short-time slice task is: The average running time
of the task during each run is no more than sysctl_sched_min_granularity.
If a task switches in and then voluntarily relinquishes the CPU
quickly, it is regarded as a short-running task. Choosing
sysctl_sched_min_granularity because it is the minimal slice if there
are too many runnable tasks.

Reuse the nr_idle_scan of SIS_UTIL to decide if the system is busy.
If yes, then a compromised "idle" CPU might be acceptable.

The reason is that, if the waker is a short running task, it might
relinquish the CPU soon, the wakee has the chance to be scheduled.
On the other hand, if the wakee is also a short-running task, the
impact it brings to the target CPU is small. If the system is
already busy, maybe we could lower the bar to find an idle CPU.
The effect is, the wake affine is enhanced.

[Benchmark results]
The baseline is 6.0-rc4.

The throughput of will-it-scale.context_switch1 has been increased by
331.13% with this patch applied.

netperf
=======
case load baseline(std%) compare%( std%)
TCP_RR 28 threads 1.00 ( 0.57) +0.29 ( 0.59)
TCP_RR 56 threads 1.00 ( 0.49) +0.43 ( 0.43)
TCP_RR 84 threads 1.00 ( 0.34) +0.24 ( 0.34)
TCP_RR 112 threads 1.00 ( 0.26) +1.57 ( 0.20)
TCP_RR 140 threads 1.00 ( 0.20) +178.05 ( 8.83)
TCP_RR 168 threads 1.00 ( 10.14) +0.87 ( 10.03)
TCP_RR 196 threads 1.00 ( 13.51) +0.90 ( 11.84)
TCP_RR 224 threads 1.00 ( 7.12) +0.66 ( 8.28)
UDP_RR 28 threads 1.00 ( 0.96) -0.10 ( 0.97)
UDP_RR 56 threads 1.00 ( 10.93) +0.24 ( 0.82)
UDP_RR 84 threads 1.00 ( 8.99) +0.40 ( 0.71)
UDP_RR 112 threads 1.00 ( 0.15) +0.72 ( 7.77)
UDP_RR 140 threads 1.00 ( 11.11) +135.81 ( 13.86)
UDP_RR 168 threads 1.00 ( 12.58) +147.63 ( 12.72)
UDP_RR 196 threads 1.00 ( 19.47) -0.34 ( 16.14)
UDP_RR 224 threads 1.00 ( 12.88) -0.35 ( 12.73)

hackbench
=========
case load baseline(std%) compare%( std%)
process-pipe 1 group 1.00 ( 1.02) +0.14 ( 0.62)
process-pipe 2 groups 1.00 ( 0.73) +0.29 ( 0.51)
process-pipe 4 groups 1.00 ( 0.16) +0.24 ( 0.31)
process-pipe 8 groups 1.00 ( 0.06) +11.56 ( 0.11)
process-sockets 1 group 1.00 ( 1.59) +0.06 ( 0.77)
process-sockets 2 groups 1.00 ( 1.13) -1.86 ( 1.31)
process-sockets 4 groups 1.00 ( 0.14) +1.76 ( 0.29)
process-sockets 8 groups 1.00 ( 0.27) +2.73 ( 0.10)
threads-pipe 1 group 1.00 ( 0.43) +0.83 ( 2.20)
threads-pipe 2 groups 1.00 ( 0.52) +1.03 ( 0.55)
threads-pipe 4 groups 1.00 ( 0.44) -0.08 ( 0.31)
threads-pipe 8 groups 1.00 ( 0.04) +11.86 ( 0.05)
threads-sockets 1 groups 1.00 ( 1.89) +3.51 ( 0.57)
threads-sockets 2 groups 1.00 ( 0.04) -1.12 ( 0.69)
threads-sockets 4 groups 1.00 ( 0.14) +1.77 ( 0.18)
threads-sockets 8 groups 1.00 ( 0.03) +2.75 ( 0.03)

tbench
======
case load baseline(std%) compare%( std%)
loopback 28 threads 1.00 ( 0.08) +0.51 ( 0.25)
loopback 56 threads 1.00 ( 0.15) -0.89 ( 0.16)
loopback 84 threads 1.00 ( 0.03) +0.35 ( 0.07)
loopback 112 threads 1.00 ( 0.06) +2.84 ( 0.01)
loopback 140 threads 1.00 ( 0.07) +0.69 ( 0.11)
loopback 168 threads 1.00 ( 0.09) +0.14 ( 0.18)
loopback 196 threads 1.00 ( 0.04) -0.18 ( 0.20)
loopback 224 threads 1.00 ( 0.25) -0.37 ( 0.03)

Other benchmarks are under testing.

Discussed below are the results from running standard benchmarks on
a dual socket Zen3 (2 x 64C/128T) machine configured in different
NPS modes.

NPS Modes are used to logically divide single socket into
multiple NUMA region.
Following is the NUMA configuration for each NPS mode on the system:

NPS1: Each socket is a NUMA node.
Total 2 NUMA nodes in the dual socket machine.

Node 0: 0-63, 128-191
Node 1: 64-127, 192-255

NPS2: Each socket is further logically divided into 2 NUMA regions.
Total 4 NUMA nodes exist over 2 socket.
Node 0: 0-31, 128-159
Node 1: 32-63, 160-191
Node 2: 64-95, 192-223
Node 3: 96-127, 223-255

NPS4: Each socket is logically divided into 4 NUMA regions.
Total 8 NUMA nodes exist over 2 socket.
Node 0: 0-15, 128-143
Node 1: 16-31, 144-159
Node 2: 32-47, 160-175
Node 3: 48-63, 176-191
Node 4: 64-79, 192-207
Node 5: 80-95, 208-223
Node 6: 96-111, 223-231
Node 7: 112-127, 232-255

Benchmark Results:

Kernel versions:
- tip: 5.19.0 tip sched/core
- shortrun: 5.19.0 tip sched/core + this patch

When we started testing, the tip was at:
commit 7e9518baed4c ("sched/fair: Move call to list_last_entry() in detach_tasks")

~~~~~~~~~~~~~
~ hackbench ~
~~~~~~~~~~~~~

NPS1

Test: tip shortrun
1-groups: 4.23 (0.00 pct) 4.24 (-0.23 pct)
2-groups: 4.93 (0.00 pct) 5.68 (-15.21 pct)
4-groups: 5.32 (0.00 pct) 6.21 (-16.72 pct)
8-groups: 5.46 (0.00 pct) 6.49 (-18.86 pct)
16-groups: 7.31 (0.00 pct) 7.78 (-6.42 pct)

NPS2

Test: tip shortrun
1-groups: 4.19 (0.00 pct) 4.19 (0.00 pct)
2-groups: 4.77 (0.00 pct) 5.43 (-13.83 pct)
4-groups: 5.15 (0.00 pct) 6.20 (-20.38 pct)
8-groups: 5.47 (0.00 pct) 6.54 (-19.56 pct)
16-groups: 6.63 (0.00 pct) 7.28 (-9.80 pct)

NPS4

Test: tip shortrun
1-groups: 4.23 (0.00 pct) 4.39 (-3.78 pct)
2-groups: 4.78 (0.00 pct) 5.48 (-14.64 pct)
4-groups: 5.17 (0.00 pct) 6.14 (-18.76 pct)
8-groups: 5.63 (0.00 pct) 6.51 (-15.63 pct)
16-groups: 7.88 (0.00 pct) 7.03 (10.78 pct)

~~~~~~~~~~~~
~ schbench ~
~~~~~~~~~~~~

NPS1

#workers: tip shortrun
1: 22.00 (0.00 pct) 36.00 (-63.63 pct)
2: 34.00 (0.00 pct) 38.00 (-11.76 pct)
4: 37.00 (0.00 pct) 36.00 (2.70 pct)
8: 55.00 (0.00 pct) 51.00 (7.27 pct)
16: 69.00 (0.00 pct) 68.00 (1.44 pct)
32: 113.00 (0.00 pct) 116.00 (-2.65 pct)
64: 219.00 (0.00 pct) 232.00 (-5.93 pct)
128: 506.00 (0.00 pct) 1019.00 (-101.38 pct)
256: 45440.00 (0.00 pct) 44864.00 (1.26 pct)
512: 76672.00 (0.00 pct) 73600.00 (4.00 pct)

NPS2

#workers: tip shortrun
1: 31.00 (0.00 pct) 36.00 (-16.12 pct)
2: 36.00 (0.00 pct) 36.00 (0.00 pct)
4: 45.00 (0.00 pct) 39.00 (13.33 pct)
8: 47.00 (0.00 pct) 48.00 (-2.12 pct)
16: 66.00 (0.00 pct) 71.00 (-7.57 pct)
32: 114.00 (0.00 pct) 123.00 (-7.89 pct)
64: 215.00 (0.00 pct) 248.00 (-15.34 pct)
128: 495.00 (0.00 pct) 531.00 (-7.27 pct)
256: 48576.00 (0.00 pct) 47552.00 (2.10 pct)
512: 79232.00 (0.00 pct) 74624.00 (5.81 pct)

NPS4

#workers: tip shortrun
1: 30.00 (0.00 pct) 36.00 (-20.00 pct)
2: 34.00 (0.00 pct) 38.00 (-11.76 pct)
4: 41.00 (0.00 pct) 44.00 (-7.31 pct)
8: 60.00 (0.00 pct) 53.00 (11.66 pct)
16: 68.00 (0.00 pct) 73.00 (-7.35 pct)
32: 116.00 (0.00 pct) 125.00 (-7.75 pct)
64: 224.00 (0.00 pct) 248.00 (-10.71 pct)
128: 495.00 (0.00 pct) 569.00 (-14.94 pct)
256: 45888.00 (0.00 pct) 38720.00 (15.62 pct)
512: 78464.00 (0.00 pct) 73600.00 (6.19 pct)

~~~~~~~~~~
~ tbench ~
~~~~~~~~~~

NPS1

Clients: tip shortrun
1 550.66 (0.00 pct) 546.56 (-0.74 pct)
2 1009.69 (0.00 pct) 1010.01 (0.03 pct)
4 1795.32 (0.00 pct) 1782.71 (-0.70 pct)
8 2971.16 (0.00 pct) 3035.58 (2.16 pct)
16 4627.98 (0.00 pct) 4816.82 (4.08 pct)
32 8065.15 (0.00 pct) 9269.52 (14.93 pct)
64 14994.32 (0.00 pct) 14704.38 (-1.93 pct)
128 5175.73 (0.00 pct) 5174.77 (-0.01 pct)
256 48763.57 (0.00 pct) 49649.67 (1.81 pct)
512 43780.78 (0.00 pct) 44717.04 (2.13 pct)
1024 40341.84 (0.00 pct) 42078.99 (4.30 pct)

NPS2

Clients: tip shortrun
1 551.06 (0.00 pct) 549.17 (-0.34 pct)
2 1000.76 (0.00 pct) 993.75 (-0.70 pct)
4 1737.02 (0.00 pct) 1773.33 (2.09 pct)
8 2992.31 (0.00 pct) 2971.05 (-0.71 pct)
16 4579.29 (0.00 pct) 4470.71 (-2.37 pct)
32 9120.73 (0.00 pct) 8080.89 (-11.40 pct)
64 14918.58 (0.00 pct) 14395.57 (-3.50 pct)
128 20830.61 (0.00 pct) 20579.09 (-1.20 pct)
256 47708.18 (0.00 pct) 47416.37 (-0.61 pct)
512 43721.79 (0.00 pct) 43754.83 (0.07 pct)
1024 40920.49 (0.00 pct) 40701.90 (-0.53 pct)

NPS4

Clients: tip shortrun
1 549.22 (0.00 pct) 548.36 (-0.15 pct)
2 1000.08 (0.00 pct) 1037.74 (3.76 pct)
4 1794.78 (0.00 pct) 1802.11 (0.40 pct)
8 3008.50 (0.00 pct) 2989.22 (-0.64 pct)
16 4804.71 (0.00 pct) 4706.51 (-2.04 pct)
32 9156.57 (0.00 pct) 8253.84 (-9.85 pct)
64 14901.45 (0.00 pct) 15049.51 (0.99 pct)
128 20771.20 (0.00 pct) 13229.50 (-36.30 pct)
256 47033.88 (0.00 pct) 46737.17 (-0.63 pct)
512 43429.01 (0.00 pct) 43246.64 (-0.41 pct)
1024 39271.27 (0.00 pct) 42194.75 (7.44 pct)

~~~~~~~~~~
~ stream ~
~~~~~~~~~~

NPS1

10 Runs:

Test: tip shortrun
Copy: 336311.52 (0.00 pct) 330116.75 (-1.84 pct)
Scale: 212955.82 (0.00 pct) 215330.30 (1.11 pct)
Add: 251518.23 (0.00 pct) 250926.53 (-0.23 pct)
Triad: 262077.88 (0.00 pct) 259618.70 (-0.93 pct)

100 Runs:

Test: tip shortrun
Copy: 339533.83 (0.00 pct) 323452.74 (-4.73 pct)
Scale: 194736.72 (0.00 pct) 215789.55 (10.81 pct)
Add: 218294.54 (0.00 pct) 244916.33 (12.19 pct)
Triad: 262371.40 (0.00 pct) 252997.84 (-3.57 pct)

NPS2

10 Runs:

Test: tip shortrun
Copy: 335277.15 (0.00 pct) 305516.57 (-8.87 pct)
Scale: 220990.24 (0.00 pct) 207061.22 (-6.30 pct)
Add: 264156.13 (0.00 pct) 243368.49 (-7.86 pct)
Triad: 268707.53 (0.00 pct) 223486.30 (-16.82 pct)

100 Runs:

Test: tip shortrun
Copy: 334913.73 (0.00 pct) 319677.81 (-4.54 pct)
Scale: 230522.47 (0.00 pct) 222757.62 (-3.36 pct)
Add: 264567.28 (0.00 pct) 254883.62 (-3.66 pct)
Triad: 272974.23 (0.00 pct) 260561.08 (-4.54 pct)

NPS4

10 Runs:

Test: tip shortrun
Copy: 356452.47 (0.00 pct) 255911.77 (-28.20 pct)
Scale: 242986.42 (0.00 pct) 171587.28 (-29.38 pct)
Add: 268512.09 (0.00 pct) 188244.75 (-29.89 pct)
Triad: 281622.43 (0.00 pct) 193271.97 (-31.37 pct)

100 Runs:

Test: tip shortrun
Copy: 367384.81 (0.00 pct) 273101.20 (-25.66 pct)
Scale: 254289.04 (0.00 pct) 189986.88 (-25.28 pct)
Add: 273683.33 (0.00 pct) 206384.96 (-24.58 pct)
Triad: 285696.90 (0.00 pct) 217214.10 (-23.97 pct)

~~~~~~~~~~~~~~~~~~~~~~~~~~
~ Notes and Observations ~
~~~~~~~~~~~~~~~~~~~~~~~~~~

o Schedstat data for Hackbench with 2 groups in NPS1 mode:

---------------------------------------------------------------------------------------------------
cpu: all_cpus (avg) vs cpu: all_cpus (avg)
---------------------------------------------------------------------------------------------------
kernel: : tip shortrun
sched_yield count : 0, 0
Legacy counter can be ignored : 0, 0
schedule called : 53305, 40615 | -23.81|
schedule left the processor idle : 22406, 16919 | -24.49|
try_to_wake_up was called : 30822, 23625 | -23.35|
try_to_wake_up was called to wake up the local cpu : 984, 2583 | 162.50|
total runtime by tasks on this processor (in jiffies) : 596998654, 481267347 | -19.39| *
total waittime by tasks on this processor (in jiffies) : 514142630, 766745576 | 49.13| * Longer wait time

Agree, the wait length is 766745576 / 481267347 = 1.59 after patched, which is
much bigger than 514142630 / 596998654 = 0.86 before the patch.

total timeslices run on this cpu : 30893, 23691 | -23.31| *
---------------------------------------------------------------------------------------------------

< -------------------------------------- Wakeup info: -------------------------------------- >
kernel: : tip shortrun
Wakeups on same SMT cpus = all_cpus (avg) : 1470, 1301 | -11.50|
Wakeups on same MC cpus = all_cpus (avg) : 22913, 18606 | -18.80|
Wakeups on same DIE cpus = all_cpus (avg) : 3634, 693 | -80.93|
Wakeups on same NUMA cpus = all_cpus (avg) : 1819, 440 | -75.81|
Affine wakeups on same SMT cpus = all_cpus (avg) : 1025, 1421 | 38.63| * More affine wakeups on possibly
Affine wakeups on same MC cpus = all_cpus (avg) : 14455, 17514 | 21.16| * busy runqueue leading to longer
Affine wakeups on same DIE cpus = all_cpus (avg) : 2828, 701 | -75.21| wait time
Affine wakeups on same NUMA cpus = all_cpus (avg) : 1194, 456 | -61.81|
------------------------------------------------------------------------------------------------

Agree, for SMT and MC domain, the wake affine has been enhanced to suggest to pick
a short running CPU rather than an idle one. Then later SIS_UTIL would prefer to
pick this candidate CPU.

We observe a larger wait time which which the patch which points
to the fact that the tasks are piling on the run queue. I believe
Tim's suggestion will help here where we can avoid a pileup as a
result of waker task being a short run task.

Yes, we'll raise the bar to pick a short running CPU.

o Tracepoint data for Stream for 100 runs in NPS4

Following tracepoints were enabled for Stream threads:
- sched_wakeup_new: To observe initial placement
- sched_waking: To check if migration is in wakeup context or lb contxt
- sched_wakeup: To check if migration is in wakeup context or lb contxt
- sched_migrate_task: To observe task movements

--> tip:

run_stream.sh-3724 [057] d..2. 450.593407: sched_wakeup_new: comm=run_stream.sh pid=3733 prio=120 target_cpu=050 *LLC: 6
<idle>-0 [182] d.s4. 450.594375: sched_waking: comm=stream pid=3733 prio=120 target_cpu=050
<idle>-0 [050] dNh2. 450.594381: sched_wakeup: comm=stream pid=3733 prio=120 target_cpu=050
<idle>-0 [182] d.s4. 450.594657: sched_waking: comm=stream pid=3733 prio=120 target_cpu=050
<idle>-0 [050] dNh2. 450.594661: sched_wakeup: comm=stream pid=3733 prio=120 target_cpu=050
stream-3733 [050] d..2. 450.594893: sched_wakeup_new: comm=stream pid=3735 prio=120 target_cpu=057 *LLC: 7
stream-3733 [050] d..2. 450.594955: sched_wakeup_new: comm=stream pid=3736 prio=120 target_cpu=078 *LLC: 9
stream-3733 [050] d..2. 450.594988: sched_wakeup_new: comm=stream pid=3737 prio=120 target_cpu=045 *LLC: 5
stream-3733 [050] d..2. 450.595016: sched_wakeup_new: comm=stream pid=3738 prio=120 target_cpu=008 *LLC: 1
stream-3733 [050] d..2. 450.595029: sched_waking: comm=stream pid=3737 prio=120 target_cpu=045
<idle>-0 [045] dNh2. 450.595037: sched_wakeup: comm=stream pid=3737 prio=120 target_cpu=045
stream-3737 [045] d..2. 450.595072: sched_waking: comm=stream pid=3733 prio=120 target_cpu=050
<idle>-0 [050] dNh2. 450.595078: sched_wakeup: comm=stream pid=3733 prio=120 target_cpu=050
stream-3738 [008] d..2. 450.595102: sched_waking: comm=stream pid=3733 prio=120 target_cpu=050
<idle>-0 [050] dNh2. 450.595111: sched_wakeup: comm=stream pid=3733 prio=120 target_cpu=050
stream-3733 [050] d..2. 450.595151: sched_wakeup_new: comm=stream pid=3739 prio=120 target_cpu=097 *LLC: 12
stream-3733 [050] d..2. 450.595181: sched_wakeup_new: comm=stream pid=3740 prio=120 target_cpu=194 *LLC: 8
stream-3733 [050] d..2. 450.595221: sched_wakeup_new: comm=stream pid=3741 prio=120 target_cpu=080 *LLC: 10
stream-3733 [050] d..2. 450.595249: sched_wakeup_new: comm=stream pid=3742 prio=120 target_cpu=144 *LLC: 2
stream-3733 [050] d..2. 450.595285: sched_wakeup_new: comm=stream pid=3743 prio=120 target_cpu=239 *LLC: 13
stream-3733 [050] d..2. 450.595320: sched_wakeup_new: comm=stream pid=3744 prio=120 target_cpu=130 *LLC: 0
stream-3733 [050] d..2. 450.595364: sched_wakeup_new: comm=stream pid=3745 prio=120 target_cpu=113 *LLC: 14
stream-3744 [130] d..2. 450.595407: sched_waking: comm=stream pid=3733 prio=120 target_cpu=050
<idle>-0 [050] dNh2. 450.595416: sched_wakeup: comm=stream pid=3733 prio=120 target_cpu=050
stream-3733 [050] d..2. 450.595423: sched_waking: comm=stream pid=3745 prio=120 target_cpu=113
<idle>-0 [113] dNh2. 450.595433: sched_wakeup: comm=stream pid=3745 prio=120 target_cpu=113
stream-3733 [050] d..2. 450.595452: sched_wakeup_new: comm=stream pid=3746 prio=120 target_cpu=160 *LLC: 4
stream-3733 [050] d..2. 450.595486: sched_wakeup_new: comm=stream pid=3747 prio=120 target_cpu=255 *LLC: 15
stream-3733 [050] d..2. 450.595513: sched_wakeup_new: comm=stream pid=3748 prio=120 target_cpu=159 *LLC: 3
stream-3746 [160] d..2. 450.595533: sched_waking: comm=stream pid=3733 prio=120 target_cpu=050
<idle>-0 [050] dNh2. 450.595542: sched_wakeup: comm=stream pid=3733 prio=120 target_cpu=050
stream-3747 [255] d..2. 450.595562: sched_waking: comm=stream pid=3733 prio=120 target_cpu=050
<idle>-0 [050] dNh2. 450.595573: sched_wakeup: comm=stream pid=3733 prio=120 target_cpu=050
stream-3733 [050] d..2. 450.595614: sched_wakeup_new: comm=stream pid=3749 prio=120 target_cpu=222 *LLC: 11
stream-3740 [194] d..2. 451.140510: sched_waking: comm=stream pid=3747 prio=120 target_cpu=255
<idle>-0 [255] dNh2. 451.140523: sched_wakeup: comm=stream pid=3747 prio=120 target_cpu=255
stream-3733 [050] d..2. 451.617257: sched_waking: comm=stream pid=3740 prio=120 target_cpu=194
stream-3733 [050] d..2. 451.617267: sched_waking: comm=stream pid=3746 prio=120 target_cpu=160
stream-3733 [050] d..2. 451.617269: sched_waking: comm=stream pid=3739 prio=120 target_cpu=097
stream-3733 [050] d..2. 451.617272: sched_waking: comm=stream pid=3742 prio=120 target_cpu=144
stream-3733 [050] d..2. 451.617275: sched_waking: comm=stream pid=3749 prio=120 target_cpu=222
... (No migrations observed)

In most cases, each LLCs is running only 1 stream thread leading to optimal performance.

--> with patch:

run_stream.sh-4383 [070] d..2. 1237.764236: sched_wakeup_new: comm=run_stream.sh pid=4392 prio=120 target_cpu=206 *LLC: 9
stream-4392 [206] d..2. 1237.765121: sched_wakeup_new: comm=stream pid=4394 prio=120 target_cpu=070 *LLC: 8
stream-4392 [206] d..2. 1237.765171: sched_wakeup_new: comm=stream pid=4395 prio=120 target_cpu=169 *LLC: 5
stream-4392 [206] d..2. 1237.765204: sched_wakeup_new: comm=stream pid=4396 prio=120 target_cpu=111 *LLC: 13
stream-4392 [206] d..2. 1237.765243: sched_wakeup_new: comm=stream pid=4397 prio=120 target_cpu=130 *LLC: 0
stream-4392 [206] d..2. 1237.765249: sched_waking: comm=stream pid=4396 prio=120 target_cpu=111
<idle>-0 [111] dNh2. 1237.765260: sched_wakeup: comm=stream pid=4396 prio=120 target_cpu=111
stream-4392 [206] d..2. 1237.765281: sched_wakeup_new: comm=stream pid=4398 prio=120 target_cpu=182 *LLC: 6
stream-4392 [206] d..2. 1237.765318: sched_wakeup_new: comm=stream pid=4399 prio=120 target_cpu=060 *LLC: 7
stream-4392 [206] d..2. 1237.765368: sched_wakeup_new: comm=stream pid=4400 prio=120 target_cpu=124 *LLC: 15
stream-4392 [206] d..2. 1237.765408: sched_wakeup_new: comm=stream pid=4401 prio=120 target_cpu=031 *LLC: 3
stream-4392 [206] d..2. 1237.765439: sched_wakeup_new: comm=stream pid=4402 prio=120 target_cpu=095 *LLC: 11
stream-4392 [206] d..2. 1237.765475: sched_wakeup_new: comm=stream pid=4403 prio=120 target_cpu=015 *LLC: 1
stream-4401 [031] d..2. 1237.765497: sched_waking: comm=stream pid=4392 prio=120 target_cpu=206
stream-4401 [031] d..2. 1237.765506: sched_migrate_task: comm=stream pid=4392 prio=120 orig_cpu=206 dest_cpu=152 *LLC: 9 -> 3
<idle>-0 [152] dNh2. 1237.765540: sched_wakeup: comm=stream pid=4392 prio=120 target_cpu=152
stream-4403 [015] d..2. 1237.765562: sched_waking: comm=stream pid=4392 prio=120 target_cpu=152
stream-4403 [015] d..2. 1237.765570: sched_migrate_task: comm=stream pid=4392 prio=120 orig_cpu=152 dest_cpu=136 *LLC: 3 -> 1
<idle>-0 [136] dNh2. 1237.765602: sched_wakeup: comm=stream pid=4392 prio=120 target_cpu=136
stream-4392 [136] d..2. 1237.765799: sched_wakeup_new: comm=stream pid=4404 prio=120 target_cpu=097 *LLC: 12
stream-4392 [136] d..2. 1237.765893: sched_wakeup_new: comm=stream pid=4405 prio=120 target_cpu=084 *LLC: 10
stream-4392 [136] d..2. 1237.765957: sched_wakeup_new: comm=stream pid=4406 prio=120 target_cpu=119 *LLC: 14
stream-4392 [136] d..2. 1237.766018: sched_wakeup_new: comm=stream pid=4407 prio=120 target_cpu=038 *LLC: 4
stream-4406 [119] d..2. 1237.766044: sched_waking: comm=stream pid=4392 prio=120 target_cpu=136
stream-4406 [119] d..2. 1237.766050: sched_migrate_task: comm=stream pid=4392 prio=120 orig_cpu=136 dest_cpu=240 *LLC: 1 -> 14
<idle>-0 [240] dNh2. 1237.766154: sched_wakeup: comm=stream pid=4392 prio=120 target_cpu=240
stream-4392 [240] d..2. 1237.766361: sched_wakeup_new: comm=stream pid=4408 prio=120 target_cpu=023 *LLC: 2
stream-4399 [060] d..2. 1238.300605: sched_waking: comm=stream pid=4406 prio=120 target_cpu=119 *LLC: 14 <--- Two stream threads are
stream-4399 [060] d..2. 1238.300611: sched_waking: comm=stream pid=4392 prio=120 target_cpu=240 *LLC: 14 <--- on the same LLC leading to
<idle>-0 [119] dNh2. 1238.300620: sched_wakeup: comm=stream pid=4406 prio=120 target_cpu=119 *LLC: 14 cache contention, degrading
<idle>-0 [240] dNh2. 1238.300621: sched_wakeup: comm=stream pid=4392 prio=120 target_cpu=240 *LLC: 14 the Stream throughput.
... (No more migrations observed)

After all the wakeups and migrations, LLC 14 contains two stream threads (pid: 4392 and 4406)
All the migrations happen between the events sched_waking and sched_wakeup showing the migrations
happens during a wakeup and not as a resutl of load balancing.

This patch is more about enhancing the wake affine, rather than improving
the SIS efficiency, so Mel's SIS statistic patch was not deployed for now.

[Limitations]
When the number of CPUs suggested by SIS_UTIL is lower than 60% of the LLC
CPUs, the LLC domain is regarded as relatively busy. However, the 60% is
somewhat hacky, because it indicates that the util_avg% is around 50%,
a half busy LLC. I don't have other lightweight/accurate method in mind to
check if the LLC domain is busy or not.

[Misc]
At LPC we received useful suggestions. The first one is that we should look at
the time from the task is woken up, to the time the task goes back to sleep.
I assume this is aligned with what is proposed here - we consider the average
running time, rather than the total running time. The second one is that we
should consider the long-running task. And this is under investigation.

Besides, Prateek has mentioned that the SIS_UTIL is unable to deal with
burst workload. Because there is a delay to reflect the instantaneous
utilization and SIS_UTIL expects the workload to be stable. If the system
is idle most of the time, but suddenly the workloads burst, the SIS_UTIL
overscans. The current patch might mitigate this symptom somehow, as burst
workload is usually regarded as a short-running task.

Suggested-by: Tim Chen <tim.c.chen@xxxxxxxxx>
Signed-off-by: Chen Yu <yu.c.chen@xxxxxxxxx>
---
kernel/sched/fair.c | 31 ++++++++++++++++++++++++++++++-
1 file changed, 30 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 914096c5b1ae..7519ab5b911c 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6020,6 +6020,19 @@ static int wake_wide(struct task_struct *p)
return 1;
}
+/*
+ * If a task switches in and then voluntarily relinquishes the
+ * CPU quickly, it is regarded as a short running task.
+ * sysctl_sched_min_granularity is chosen as the threshold,
+ * as this value is the minimal slice if there are too many
+ * runnable tasks, see __sched_period().
+ */
+static int is_short_task(struct task_struct *p)
+{
+ return (p->se.sum_exec_runtime <=
+ (p->nvcsw * sysctl_sched_min_granularity));
+}
+
/*
* The purpose of wake_affine() is to quickly determine on which CPU we can run
* soonest. For the purpose of speed we only consider the waking and previous
@@ -6050,7 +6063,8 @@ wake_affine_idle(int this_cpu, int prev_cpu, int sync)
if (available_idle_cpu(this_cpu) && cpus_share_cache(this_cpu, prev_cpu))
return available_idle_cpu(prev_cpu) ? prev_cpu : this_cpu;
- if (sync && cpu_rq(this_cpu)->nr_running == 1)
+ if ((sync && cpu_rq(this_cpu)->nr_running == 1) ||
+ is_short_task(cpu_curr(this_cpu)))

Seems it a bit breaks idle (or will be idle) purpose of wake_affine_idle() here. Maybe we can do it something like this?

if ((sync || is_short_task(cpu_curr(this_cpu))) && cpu_rq(this_cpu)->nr_running == 1)

Thanks,
Honglei

This change seems to optimize for affine wakeup which benefits
tasks with producer-consumer pattern but is not ideal for Stream.
Currently the logic ends will do an affine wakeup even if sync
flag is not set:

stream-4135 [029] d..2. 353.580953: sched_waking: comm=stream pid=4129 prio=120 target_cpu=082
stream-4135 [029] d..2. 353.580957: select_task_rq_fair: wake_affine_idle: Select this_cpu: sync(0) rq->nr_running(1) is_short_task(1)
stream-4135 [029] d..2. 353.580960: sched_migrate_task: comm=stream pid=4129 prio=120 orig_cpu=82 dest_cpu=30
<idle>-0 [030] dNh2. 353.580993: sched_wakeup: comm=stream pid=4129 prio=120 target_cpu=030

I believe a consideration should be made for the sync flag when
going for an affine wakeup. Also the check for short running could
be at the end after checking if prev_cpu is an available_idle_cpu.

We can move the short running check after the prev_cpu check. If we
add the sync flag check would it shrink the coverage of this change?
Since I found that there is limited scenario would enable the sync
flag and we want to make the short running check a generic optimization.
But yes, we can test with/without sync flag constrain to see which one
gives better data.

return this_cpu;
if (available_idle_cpu(prev_cpu))
@@ -6434,6 +6448,21 @@ static int select_idle_cpu(struct task_struct *p, struct sched_domain *sd, bool
/* overloaded LLC is unlikely to have idle cpu/core */
if (nr == 1)
return -1;
+
+ /*
+ * If nr is smaller than 60% of llc_weight, it
+ * indicates that the util_avg% is higher than 50%.
+ * This is calculated by SIS_UTIL in
+ * update_idle_cpu_scan(). The 50% util_avg indicates
+ * a half-busy LLC domain. System busier than this
+ * level could lower its bar to choose a compromised
+ * "idle" CPU. If the waker on target CPU is a short
+ * task and the wakee is also a short task, pick
+ * target directly.
+ */
+ if (!has_idle_core && (5 * nr < 3 * sd->span_weight) &&
+ is_short_task(p) && is_short_task(cpu_curr(target)))
+ return target;

Pileup seen in hackbench could also be a result of an early
bailout here for smaller LLCs but I don't have any data to
substantiate that claim currently.

}
}

Please let me know if you need any more data from the test
system for any of the benchmarks covered or if you would like
me to run any other benchmark on the test system.

Thank you for your testing, I'll enable SNC to divide the LLC domain
into smaller ones, and to see if the issue could be reproduced
on my platform too, then I'll update my finding on this.

thanks,
Chenyu

--
Thanks and Regards,
Prateek

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