[PATCH v7 2/2] sched/fair: Introduce SIS_CURRENT to wake up short task on current CPU
From: Chen Yu
Date: Fri Apr 21 2023 - 04:12:34 EST
[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 is Intel Sapphire Rapids, which has 2 x 56C/112T and
224 CPUs in total. will-it-scale launches 1, 8, 16 ... 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. It is found that, once the number of
instances is higher than 56, the throughput drops accordingly:
^
throughput|
| X
| X X X
| X X X
| X X
| X X
| X
| X
| X
| X
|
+-----------------.------------------->
56
number of instances
[Symptom analysis]
The performance downgrading was caused by a high system idle
percentage(around 20% ~ 30%). The CPUs waste a lot of time in
idle and do nothing. As a comparison, if set CPU affinity to
these workloads and stops them from migrating among CPUs,
the idle percentage drops to nearly 0%, and the throughput
increases a lot. This indicates room for optimization.
The cause is the race condition between select_task_rq() and
the task enqueue.
Suppose there are nr_cpus pairs of ping-pong scheduling
tasks. For example, p0' and p0 are ping-pong scheduling,
so do p1' <=> p1, and p2'<=> p2. None of these tasks are
bound to any CPUs. The problem can be summarized as:
more than 1 wakers are stacked on 1 CPU, which slows down
waking up their wakees:
CPU0 CPU1 CPU2
p0' p1' => idle p2'
try_to_wake_up(p0) try_to_wake_up(p2);
CPU1 = select_task_rq(p0); CPU1 = select_task_rq(p2);
ttwu_queue(p0, CPU1); ttwu_queue(p2, CPU1);
__ttwu_queue_wakelist(p0, CPU1);
WRITE_ONCE(CPU1->ttwu_pending, 1);
__smp_call_single_queue(CPU1, p0); => ttwu_list->p0
quiting cpuidle_idle_call()
__ttwu_queue_wakelist(p2, CPU1);
WRITE_ONCE(CPU1->ttwu_pending, 1);
ttwu_list->p2->p0 <= __smp_call_single_queue(CPU1, p2);
p0' => idle
sched_ttwu_pending()
enqueue_task(p2 and p0)
idle => p2
...
p2 time slice expires
...
!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
<=== !!! p2 delays the wake up of p0' !!!
!!! causes long idle on CPU0 !!!
p2 => p0 !!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!!
p0 wakes up p0'
idle => p0'
Since there are many waker/wakee pairs in the system, the chain reaction
causes many CPUs to be victims. These idle CPUs wait for their waker to
be scheduled.
Tiancheng has mentioned the above issue here[1].
[Proposal]
The root cause is that there is no strict synchronization of
select_task_rq() and the set of ttwu_pending flag among several CPUs.
And this might be by design because the scheduler prefers parallel
wakeup.
Avoid this problem indirectly.
Wake up the short task on current CPU, if the following conditions are met:
1. waker A's rq->nr_running <= 1
2. waker A is a short duration task (waker will fall asleep soon)
3. wakee B is a short duration task (impact of B is minor to A)
4. A->wakee_flips is 0 and A->last_wakee = B
5. B->wakee_flips is 0 and B->last_wakee = A
The reason is that, if the waker is a short-duration task, it might
relinquish the CPU soon, and the wakee has the chance to be scheduled.
On the other hand, if the wakee is a short duration task, putting it on
non-idle CPU would bring minimal impact to the running task. Besides,
if two tasks wake up each other alternately, it suggests that
they usually share resource and should be put together.
This wake up strategy is regarded as a dynamic WF_CURRENT_CPU[2] proposed
by Andrei Vagin, except that this change treats the current CPU as the
last resort when previous CPU is not idle, and avoid tasks stacking
on current CPU as much as possible.
[Benchmark results]
The baseline is v6.3-rc3 tip:sched/core, on top of
Commit 9b8e17813aec ("sched/core: Make sched_dynamic_mutex static").
The test platform Intel Sapphire Rapids has 2 x 56C/112T and 224 CPUs in total.
C-states deeper than C1E are disabled. Turbo is disabled. CPU frequency governor
is performance.
Overall there is universal improvement for netperf/tbench/will-it-scale,
under different load. And there is no significant impact on hackbench/schbench.
will-it-scale
=============
case load baseline compare%
context_switch1 224 groups 1.00 +1467.97%
netperf
=======
case load baseline(std%) compare%( std%)
TCP_RR 56-threads 1.00 ( 2.20) +13.15 ( 4.36)
TCP_RR 112-threads 1.00 ( 2.08) +87.39 ( 4.35)
TCP_RR 168-threads 1.00 ( 17.82) +419.70 ( 7.34)
TCP_RR 224-threads 1.00 ( 15.60) +779.49 ( 3.24)
TCP_RR 280-threads 1.00 ( 73.01) +192.72 ( 9.17)
TCP_RR 336-threads 1.00 ( 18.42) +0.21 ( 19.25)
TCP_RR 392-threads 1.00 ( 41.34) +26.30 ( 26.97)
TCP_RR 448-threads 1.00 ( 27.61) +0.02 ( 28.65)
UDP_RR 56-threads 1.00 ( 4.82) -0.60 ( 11.28)
UDP_RR 112-threads 1.00 ( 29.67) -15.97 ( 44.92)
UDP_RR 168-threads 1.00 ( 21.62) +113.99 ( 45.49)
UDP_RR 224-threads 1.00 ( 18.54) +184.40 ( 44.54)
UDP_RR 280-threads 1.00 ( 24.41) +199.72 ( 42.38)
UDP_RR 336-threads 1.00 ( 41.28) +0.40 ( 38.10)
UDP_RR 392-threads 1.00 ( 32.40) +1.73 ( 39.68)
UDP_RR 448-threads 1.00 ( 36.24) +3.69 ( 48.73)
tbench
======
case load baseline(std%) compare%( std%)
loopback 56-threads 1.00 ( 1.13) +11.54 ( 1.29)
loopback 112-threads 1.00 ( 0.18) +2.82 ( 0.27)
loopback 168-threads 1.00 ( 0.02) +154.13 ( 0.20)
loopback 224-threads 1.00 ( 58.38) +82.56 ( 0.49)
loopback 280-threads 1.00 ( 0.06) -0.07 ( 0.28)
loopback 336-threads 1.00 ( 0.37) -1.24 ( 0.33)
loopback 392-threads 1.00 ( 0.11) +0.17 ( 0.09)
loopback 448-threads 1.00 ( 0.29) +0.32 ( 0.14)
hackbench
=========
case load baseline(std%) compare%( std%)
process-pipe 1-groups 1.00 ( 0.95) +0.02 ( 0.60)
process-pipe 2-groups 1.00 ( 8.51) -0.43 ( 3.21)
process-pipe 4-groups 1.00 ( 7.39) -9.09 ( 2.50)
process-sockets 1-groups 1.00 ( 1.28) +1.03 ( 0.95)
process-sockets 2-groups 1.00 ( 0.85) +6.27 ( 10.32)
process-sockets 4-groups 1.00 ( 1.28) -0.51 ( 0.66)
threads-pipe 1-groups 1.00 ( 1.69) +0.66 ( 0.40)
threads-pipe 2-groups 1.00 ( 8.25) -7.78 ( 3.31)
threads-pipe 4-groups 1.00 ( 1.52) +1.50 ( 4.98)
threads-sockets 1-groups 1.00 ( 1.50) +0.96 ( 2.02)
threads-sockets 2-groups 1.00 ( 1.87) -1.15 ( 1.91)
threads-sockets 4-groups 1.00 ( 0.77) -0.73 ( 2.15)
schbench
========
case load baseline(std%) compare%( std%)
normal 1-mthreads 1.00 ( 1.25) +0.00 ( 1.25)
normal 2-mthreads 1.00 ( 0.00) +0.88 ( 3.31)
normal 4-mthreads 1.00 ( 1.29) +0.91 ( 1.30)
normal 8-mthreads 1.00 ( 2.21) +0.00 ( 2.21)
Redis
=====
Launch 224 instances of redis-server on machine A, launch 224 instances
of redis-benchmark on machine B, measure the SET/GET latency on B.
It was tested on a 1G NIC card. The 99th latency before vs after SIS_CURRENT
did not change much.
baseline sis_current
SET 115 ms 116 ms
GET 225 ms 228 ms
Thanks Prateek for testing on a dual socket Zen3 system (2 x 64C/128T).
tbench and netperf show good improvements at 128 clients. SpecJBB shows
some improvement in max-jOPS:
tip SIS_CURRENT
SPECjbb2015 max-jOPS 100.00% 102.78%
SPECjbb2015 Critical-jOPS 100.00% 100.00%
Others are perf neutral.
[Limitations/Miscellaneous]
[a]
Peter has suggested[3] comparing task duration with the cost of searching
for an idle CPU. If the latter is higher, then give up the scan, to
achieve better task affine. However, this method does not fit in the case
encountered in this patch. Because there are plenty of (fast)idle CPUs in
the system, it will not take too long to find an idle CPU. The bottleneck is
caused by the race condition mentioned above.
[b]
The short task threshold is sysctl_sched_min_granularity / 8.
According to get_update_sysctl_factor(), the sysctl_sched_min_granularity
could be 0.75 msec * 4 for SCHED_TUNABLESCALING_LOG,
or 0.75 msec * ncpus for SCHED_TUNABLESCALING_LINEAR.
Choosing 8 as the divisor is a trade-off. Thanks Honglei for pointing
this out.
[c]
Inspired by Abel's Redis test, the short task definination not only considers
the duration, but also checks the wakee_flips/last_wakee of both waker and
wakee. A waker can only wake up the wakee on current CPU if the wakee_flips is 0
and last_wakee is the wakee. That is to say, task A only wakes up task B, and
task B only wakes up A, A and B can be put together on one CPU.
[d]
Inspired by Andrei's WF_CURRENT_CPU proposal, makes SIS_CURRENT a dynamic version
for WF_CURRENT_CPU: short tasks having close wakeup relationship with each other,
should be put on 1 CPU to benefit cache sharing.
[1] https://lore.kernel.org/lkml/9ed75cad-3718-356f-21ca-1b8ec601f335@xxxxxxxxxxxxxxxxx/
[2] https://lore.kernel.org/lkml/20230308073201.3102738-3-avagin@xxxxxxxxxx/
[3] https://lore.kernel.org/lkml/Y2O8a%2FOhk1i1l8ao@xxxxxxxxxxxxxxxxxxxxxxxxxxxxxxx/
Suggested-by: Tim Chen <tim.c.chen@xxxxxxxxx>
Suggested-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
Tested-by: kernel test robot <yujie.liu@xxxxxxxxx>
Tested-by: K Prateek Nayak <kprateek.nayak@xxxxxxx>
Signed-off-by: Chen Yu <yu.c.chen@xxxxxxxxx>
---
kernel/sched/fair.c | 46 +++++++++++++++++++++++++++++++++++++++++
kernel/sched/features.h | 1 +
2 files changed, 47 insertions(+)
diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 4af5799b90fc..46c1321c0407 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -6501,6 +6501,46 @@ static int wake_wide(struct task_struct *p)
return 1;
}
+/*
+ * Wake up the task on current CPU, if the following conditions are met:
+ *
+ * 1. waker A is the only running task on this_cpu
+ * 3. A is a short duration task (waker will fall asleep soon)
+ * 4. wakee B is a short duration task (impact of B on A is minor)
+ * 5. A and B wake up each other alternately
+ */
+static bool
+wake_on_current(int this_cpu, struct task_struct *p)
+{
+ if (!sched_feat(SIS_CURRENT))
+ return false;
+
+ if (cpu_rq(this_cpu)->nr_running > 1)
+ return false;
+
+ /*
+ * If a task switches in and then voluntarily relinquishes the
+ * CPU quickly, it is regarded as a short duration task. In that
+ * way, the short waker is likely to relinquish the CPU soon, which
+ * provides room for the wakee. Meanwhile, a short wakee would bring
+ * minor impact to the target rq. Put the short waker and wakee together
+ * bring benefit to cache-share task pairs and avoid migration overhead.
+ */
+ if (!current->se.dur_avg || ((current->se.dur_avg * 8) >= sysctl_sched_min_granularity))
+ return false;
+
+ if (!p->se.dur_avg || ((p->se.dur_avg * 8) >= sysctl_sched_min_granularity))
+ return false;
+
+ if (current->wakee_flips || p->wakee_flips)
+ return false;
+
+ if (current->last_wakee != p || p->last_wakee != current)
+ return false;
+
+ return true;
+}
+
/*
* 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
@@ -6594,6 +6634,9 @@ static int wake_affine(struct sched_domain *sd, struct task_struct *p,
if (sched_feat(WA_WEIGHT) && target == nr_cpumask_bits)
target = wake_affine_weight(sd, p, this_cpu, prev_cpu, sync);
+ if (target == nr_cpumask_bits && wake_on_current(this_cpu, p))
+ target = this_cpu;
+
schedstat_inc(p->stats.nr_wakeups_affine_attempts);
if (target != this_cpu)
return prev_cpu;
@@ -7115,6 +7158,9 @@ static int select_idle_sibling(struct task_struct *p, int prev, int target)
}
}
+ if (smp_processor_id() == target && wake_on_current(target, p))
+ return target;
+
i = select_idle_cpu(p, sd, has_idle_core, target);
if ((unsigned)i < nr_cpumask_bits)
return i;
diff --git a/kernel/sched/features.h b/kernel/sched/features.h
index ee7f23c76bd3..a3e05827f7e8 100644
--- a/kernel/sched/features.h
+++ b/kernel/sched/features.h
@@ -62,6 +62,7 @@ SCHED_FEAT(TTWU_QUEUE, true)
*/
SCHED_FEAT(SIS_PROP, false)
SCHED_FEAT(SIS_UTIL, true)
+SCHED_FEAT(SIS_CURRENT, true)
/*
* Issue a WARN when we do multiple update_rq_clock() calls
--
2.25.1