[tip: sched/core] sched/fair: Disallow sync wakeup from interrupt context

tip-bot2 for Libo Chen
2022-07-21T08:44:46Z

The following commit has been merged into the sched/core branch of tip:

Commit-ID: 14b3f2d9ee8df3b6040f7e21f9fcd1d848938fd9
Gitweb: https://git.kernel.org/tip/14b3f2d9ee8df3b6040f7e21f9fcd1d848938fd9
Author: Libo Chen <libo.chen@oracle.com>
AuthorDate: Mon, 11 Jul 2022 15:47:04 -07:00
Committer: Peter Zijlstra <peterz@infradead.org>
CommitterDate: Thu, 21 Jul 2022 10:39:39 +02:00

sched/fair: Disallow sync wakeup from interrupt context

Barry Song first pointed out that replacing sync wakeup with regular wakeup
seems to reduce overeager wakeup pulling and shows noticeable performance
improvement.[1]

This patch argues that allowing sync for wakeups from interrupt context
is a bug and fixing it can improve performance even when irq/softirq is
evenly spread out.

For wakeups from ISR, the waking CPU is just the CPU of ISR and the so-called
waker can be any random task that happens to be running on that CPU when the
interrupt comes in. This is completely different from other wakups where the
task running on the waking CPU is the actual waker. For example, two tasks
communicate through a pipe or mutiple tasks access the same critical section,
etc. This difference is important because with sync we assume the waker will
get off the runqueue and go to sleep immedately after the wakeup. The
assumption is built into wake_affine() where it discounts the waker's presence
from the runqueue when sync is true. The random waker from interrupts bears no
relation to the wakee and don't usually go to sleep immediately afterwards
unless wakeup granularity is reached. Plus the scheduler no longer enforces the
preepmtion of waker for sync wakeup as it used to before
patch f2e74eeac03ffb7 ("sched: Remove WAKEUP_SYNC feature"). Enforcing sync
wakeup preemption for wakeups from interrupt contexts doesn't seem to be
appropriate too but at least sync wakeup will do what it's supposed to do.

Add a check to make sure that sync can only be set when in_task() is true. If
a wakeup is from interrupt context, sync flag will be 0 because in_task()
returns 0.

Tested in two scenarios: wakeups from 1) task contexts, expected to see no
performance changes; 2) interrupt contexts, expected to see better performance
under low/medium load and no regression under heavy load.

Use hackbench for scenario 1 and pgbench for scenarios 2 both from mmtests on
a 2-socket Xeon E5-2699v3 box with 256G memory in total. Running 5.18 kernel
with SELinux disabled. Scheduler/MM tunables are all default. Irqbalance
daemon is active.

Hackbench (config-scheduler-unbound)
=========
process-pipes:
Baseline Patched
Amean 1 0.4300 ( 0.00%) 0.4420 ( -2.79%)
Amean 4 0.8757 ( 0.00%) 0.8774 ( -0.20%)
Amean 7 1.3712 ( 0.00%) 1.3789 ( -0.56%)
Amean 12 2.3541 ( 0.00%) 2.3714 ( -0.73%)
Amean 21 4.2229 ( 0.00%) 4.2439 ( -0.50%)
Amean 30 5.9113 ( 0.00%) 5.9451 ( -0.57%)
Amean 48 9.3873 ( 0.00%) 9.4898 ( -1.09%)
Amean 79 15.9281 ( 0.00%) 16.1385 ( -1.32%)
Amean 110 22.0961 ( 0.00%) 22.3433 ( -1.12%)
Amean 141 28.2973 ( 0.00%) 28.6209 ( -1.14%)
Amean 172 34.4709 ( 0.00%) 34.9347 ( -1.35%)
Amean 203 40.7621 ( 0.00%) 41.2497 ( -1.20%)
Amean 234 47.0416 ( 0.00%) 47.6470 ( -1.29%)
Amean 265 53.3048 ( 0.00%) 54.1625 ( -1.61%)
Amean 288 58.0595 ( 0.00%) 58.8096 ( -1.29%)

process-sockets:
Baseline Patched
Amean 1 0.6776 ( 0.00%) 0.6611 ( 2.43%)
Amean 4 2.6183 ( 0.00%) 2.5769 ( 1.58%)
Amean 7 4.5662 ( 0.00%) 4.4801 ( 1.89%)
Amean 12 7.7638 ( 0.00%) 7.6201 ( 1.85%)
Amean 21 13.5335 ( 0.00%) 13.2915 ( 1.79%)
Amean 30 19.3369 ( 0.00%) 18.9811 ( 1.84%)
Amean 48 31.0724 ( 0.00%) 30.6015 ( 1.52%)
Amean 79 51.1881 ( 0.00%) 50.4251 ( 1.49%)
Amean 110 71.3399 ( 0.00%) 70.4578 ( 1.24%)
Amean 141 91.4675 ( 0.00%) 90.3769 ( 1.19%)
Amean 172 111.7463 ( 0.00%) 110.3947 ( 1.21%)
Amean 203 131.6927 ( 0.00%) 130.3270 ( 1.04%)
Amean 234 151.7459 ( 0.00%) 150.1320 ( 1.06%)
Amean 265 171.9101 ( 0.00%) 169.9751 ( 1.13%)
Amean 288 186.9231 ( 0.00%) 184.7706 ( 1.15%)

thread-pipes:
Baseline Patched
Amean 1 0.4523 ( 0.00%) 0.4535 ( -0.28%)
Amean 4 0.9041 ( 0.00%) 0.9085 ( -0.48%)
Amean 7 1.4111 ( 0.00%) 1.4146 ( -0.25%)
Amean 12 2.3532 ( 0.00%) 2.3688 ( -0.66%)
Amean 21 4.1550 ( 0.00%) 4.1701 ( -0.36%)
Amean 30 6.1043 ( 0.00%) 6.2391 ( -2.21%)
Amean 48 10.2077 ( 0.00%) 10.3511 ( -1.40%)
Amean 79 16.7922 ( 0.00%) 17.0427 ( -1.49%)
Amean 110 23.3350 ( 0.00%) 23.6522 ( -1.36%)
Amean 141 29.6458 ( 0.00%) 30.2617 ( -2.08%)
Amean 172 35.8649 ( 0.00%) 36.4225 ( -1.55%)
Amean 203 42.4477 ( 0.00%) 42.8332 ( -0.91%)
Amean 234 48.7117 ( 0.00%) 49.4042 ( -1.42%)
Amean 265 54.9171 ( 0.00%) 55.6551 ( -1.34%)
Amean 288 59.5282 ( 0.00%) 60.2903 ( -1.28%)

thread-sockets:
Baseline Patched
Amean 1 0.6917 ( 0.00%) 0.6892 ( 0.37%)
Amean 4 2.6651 ( 0.00%) 2.6017 ( 2.38%)
Amean 7 4.6734 ( 0.00%) 4.5637 ( 2.35%)
Amean 12 8.0156 ( 0.00%) 7.8079 ( 2.59%)
Amean 21 14.0451 ( 0.00%) 13.6679 ( 2.69%)
Amean 30 20.0963 ( 0.00%) 19.5657 ( 2.64%)
Amean 48 32.4115 ( 0.00%) 31.6001 ( 2.50%)
Amean 79 53.1104 ( 0.00%) 51.8395 ( 2.39%)
Amean 110 74.0929 ( 0.00%) 72.4391 ( 2.23%)
Amean 141 95.1506 ( 0.00%) 93.0992 ( 2.16%)
Amean 172 116.1969 ( 0.00%) 113.8307 ( 2.04%)
Amean 203 137.4413 ( 0.00%) 134.5247 ( 2.12%)
Amean 234 158.5395 ( 0.00%) 155.2793 ( 2.06%)
Amean 265 179.7729 ( 0.00%) 176.1099 ( 2.04%)
Amean 288 195.5573 ( 0.00%) 191.3977 ( 2.13%)

Pgbench (config-db-pgbench-timed-ro-small)
=======
Baseline Patched
Hmean 1 68.54 ( 0.00%) 69.72 ( 1.71%)
Hmean 6 27725.78 ( 0.00%) 34119.11 ( 23.06%)
Hmean 12 55724.26 ( 0.00%) 63158.22 ( 13.34%)
Hmean 22 72806.26 ( 0.00%) 73389.98 ( 0.80%)
Hmean 30 79000.45 ( 0.00%) 75197.02 ( -4.81%)
Hmean 48 78180.16 ( 0.00%) 75074.09 ( -3.97%)
Hmean 80 75001.93 ( 0.00%) 70590.72 ( -5.88%)
Hmean 110 74812.25 ( 0.00%) 74128.57 ( -0.91%)
Hmean 142 74261.05 ( 0.00%) 72910.48 ( -1.82%)
Hmean 144 75375.35 ( 0.00%) 71295.72 ( -5.41%)

For hackbench, +-3% fluctuation is normal on this two-socket box, it's safe to
conclude that there are no performance differences for wakeups from task context.
For pgbench, after many runs, 10~30% gains are very consistent at lower
client counts (< #cores per socket). For higher client counts, both kernels are
very close, +-5% swings are quite common. Also NET_TX|RX softirq load
does spread out across both NUMA nodes in this test so there is no need to do
any explicit RPS/RFS.

[1]: https://lkml.kernel.org/r/20211105105136.12137-1-21cnbao@gmail.com

Signed-off-by: Libo Chen <libo.chen@oracle.com>
Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20220711224704.1672831-1-libo.chen@oracle.com
---
kernel/sched/fair.c | 5 ++++-
1 file changed, 4 insertions(+), 1 deletion(-)

diff --git a/kernel/sched/fair.c b/kernel/sched/fair.c
index 914096c..2fc4725 100644
--- a/kernel/sched/fair.c
+++ b/kernel/sched/fair.c
@@ -7013,7 +7013,10 @@ unlock:
static int
select_task_rq_fair(struct task_struct *p, int prev_cpu, int wake_flags)
{
- int sync = (wake_flags & WF_SYNC) && !(current->flags & PF_EXITING);
+ /*
+ * Only consider WF_SYNC from task context; since only a task can schedule out.
+ */
+ int sync = in_task() && (wake_flags & WF_SYNC) && !(current->flags & PF_EXITING);
struct sched_domain *tmp, *sd = NULL;
int cpu = smp_processor_id();
int new_cpu = prev_cpu;