[RFC PATCH] sched/fair: Choose the CPU where short task is running during wake up
From: Chen Yu
Date: Thu Sep 15 2022 - 12:54:51 EST
[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.
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)))
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;
}
}
--
2.25.1