OOM and High CPU utilization in update_blocked_averages because of too many cfs_rqs in rq->leaf_cfs_rq_list

[Gavin Guo]

[Impact]
The CPU utilization keeps high and the flamegraph[1] shows that the CPU
is busy updating the load average in the for loop inside
update_blocked_averages() function. Also, the OOM happens because of
the decayed cfs_rqs are not released.

[Fix]
commit a9e7f6544b9cebdae54d29f87a7ba2a83c0471b5
Author: Tejun Heo <email address hidden>
Date: Tue Apr 25 17:43:50 2017 -0700

sched/fair: Fix O(nr_cgroups) in load balance path

Currently, rq->leaf_cfs_rq_list is a traversal ordered list of all
live cfs_rqs which have ever been active on the CPU; unfortunately,
this makes update_blocked_averages() O(# total cgroups) which isn't
scalable at all.

This shows up as a small CPU consumption and scheduling latency
increase in the load balancing path in systems with CPU controller
enabled across most cgroups. In an edge case where temporary cgroups
were leaking, this caused the kernel to consume good several tens of
percents of CPU cycles running update_blocked_averages(), each run
taking multiple millisecs.

This patch fixes the issue by taking empty and fully decayed cfs_rqs
off the rq->leaf_cfs_rq_list.

[Test]
1). Running the script
#/bin/bash

for i in $(seq 1 10); do
        ( for j in $(seq 1 3000); do ssh -S none u@localhost date;done; echo "done $i" ) &
done

2). Observe the cfs_rqs
$ watch -n1 "grep cfs_rq /proc/sched_debug| wc -l"

3). Observe the CPU utilization rate
$ sudo htop

The patched kernel[2] shows that the CPU utilization rate is normal, the
cfs_rqs is decreased periodically, and the memory can be limited.

[Reference]
[1]. http://kernel.ubuntu.com/~gavinguo/168887/2018-01-31_07-38-45.perf.data.svg
[2]. https://launchpad.net/~mimi0213kimo/+archive/ubuntu/cfs-rq-clean

[Ubuntu Kernel Bot (ubuntu-kernel-bot)]

This bug is missing log files that will aid in diagnosing the problem. While running an Ubuntu kernel (not a mainline or third-party kernel) please enter the following command in a terminal window:

apport-collect 1747896

and then change the status of the bug to 'Confirmed'.

If, due to the nature of the issue you have encountered, you are unable to run this command, please add a comment stating that fact and change the bug status to 'Confirmed'.

This change has been made by an automated script, maintained by the Ubuntu Kernel Team.

[Stefan Bader (smb)]

The proposed patch was part of 4.13, so is included in Artful and later already. Thus setting to fix released for the main task.

[Stefan Bader (smb)]

This bug is awaiting verification that the kernel in -proposed solves the problem. Please test the kernel and update this bug with the results. If the problem is solved, change the tag 'verification-needed-xenial' to 'verification-done-xenial'. If the problem still exists, change the tag 'verification-needed-xenial' to 'verification-failed-xenial'.

If verification is not done by 5 working days from today, this fix will be dropped from the source code, and this bug will be closed.

See https://wiki.ubuntu.com/Testing/EnableProposed for documentation how to enable and use -proposed. Thank you!

[Launchpad Janitor (janitor)]

This bug was fixed in the package linux - 4.4.0-119.143

---------------
linux (4.4.0-119.143) xenial; urgency=medium

  * linux: 4.4.0-119.143 -proposed tracker (LP: #1760327)

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    - Revert "Bluetooth: btusb: fix QCA Rome suspend/resume"

linux (4.4.0-118.142) xenial; urgency=medium

  * linux: 4.4.0-118.142 -proposed tracker (LP: #1759607)

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linux (4.4.0-117.141) xenial; urgency=medium

  * linux: 4.4.0-117.141 -proposed tracker (LP: #1755208)

  * Xenial update to 4.4.114 stable release (LP: #1754592)
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    - usbip: prevent vhci_hcd driver from leaking a socket pointer address
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    - ACPICA: Namespace: fix operand cache leak
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      in 64bit kernel
    - netfilter: nf_dup_ipv6: set again FLOWI_FLAG_KNOWN_NH at flowi6_flags
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