Bug 2155424

Summary:	ListUnitFiles impacts systemd a lot, causing high CPU consumption and delay in operations
Product:	Red Hat Enterprise Linux 9	Reporter:	Renaud Métrich <rmetrich>
Component:	systemd	Assignee:	systemd maint <systemd-maint>
Status:	CLOSED MIGRATED	QA Contact:	Frantisek Sumsal <fsumsal>
Severity:	medium	Docs Contact:
Priority:	medium
Version:	9.0	CC:	dtardon, fkrska, larutiun, msekleta, rgreene, sbalasub, smahanga, systemd-maint, systemd-maint-list, systemd-maint, ykhutale
Target Milestone:	rc	Keywords:	MigratedToJIRA, Performance, Reproducer, Triaged
Target Release:	---
Hardware:	All
OS:	Linux
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Fixed In Version:		Doc Type:	If docs needed, set a value
Doc Text:		Story Points:	---
Clone Of:		Environment:
Last Closed:	2023-09-21 12:20:03 UTC	Type:	Bug
Regression:	---	Mount Type:	---
Documentation:	---	CRM:
Verified Versions:		Category:	---
oVirt Team:	---	RHEL 7.3 requirements from Atomic Host:
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Embargoed:

Description Renaud Métrich 2022-12-21 08:38:26 UTC

Description of problem:

This is a respin of BZ #1828759 ("systemctl list-unit-files" kills the system usability).

With RHEL8.7 / systemd-239-68.el8 (didn't check with older ones yet), we can see that the ListUnitFiles DBus operation is very costly.
When executing systemd under strace, we can observe that on a system having 145 units ("minimal installation" of RHEL8.7), the readlinkat/openat/getdents64 syscalls are called hundreds or even thousands of times on same files (see reproducer below), causing high CPU consumption.


Version-Release number of selected component (if applicable):

systemd-239-68.el8

How reproducible:

Always

Steps to Reproduce:
1. Execute the DBus ListUnitFiles operation while stracing systemd

    # strace -CttTvyy -s 128 -o list-unit-files.strace -p 1 &
    # time dbus-send --system --print-reply --reply-timeout=20000 --type=method_call --dest=org.freedesktop.systemd1 /org/freedesktop/systemd1 org.freedesktop.systemd1.Manager.ListUnitFiles
    # kill %1

2. Check number of syscalls

    # tail -20 list-unit-files.strace
    % time     seconds  usecs/call     calls    errors syscall
    ------ ----------- ----------- --------- --------- ----------------
     30.08    0.277966           2    119048           readlinkat
     21.62    0.199787           2     79554     16869 openat
     21.01    0.194170           2     89605           getdents64
     10.02    0.092602           1     90426           fcntl
    ...

3. Check number of times readlinkat() is executed on a given path

    # grep " readlinkat(" list-unit-files.strace | cut -f2 -d'"' | sort | uniq -c | sort -k1 -rn > list-unit-files.strace.readlinkat
    # head list-unit-files.strace.readlinkat
    820 /etc/systemd/system/syslog.service
    819 /etc/systemd/system/multi-user.target.wants/remote-fs.target
    818 /etc/systemd/system/multi-user.target.wants/crond.service
    817 /etc/systemd/system/multi-user.target.wants/NetworkManager.service
    815 /etc/systemd/system/multi-user.target.wants/dnf-makecache.timer
    814 /etc/systemd/system/multi-user.target.wants/sssd.service
    ...

    # grep "NetworkManager.service" list-unit-files.strace.readlinkat
    817 /etc/systemd/system/multi-user.target.wants/NetworkManager.service
    793 /etc/systemd/system/dbus-org.freedesktop.NetworkManager.service

Actual results:

Slowness, CPU consumption

Expected results:

Files/Symlinks resolved once.

Comment 23 RHEL Program Management 2023-09-21 12:16:22 UTC

Issue migration from Bugzilla to Jira is in process at this time. This will be the last message in Jira copied from the Bugzilla bug.

Comment 24 RHEL Program Management 2023-09-21 12:20:03 UTC

This BZ has been automatically migrated to the issues.redhat.com Red Hat Issue Tracker. All future work related to this report will be managed there.

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