trace-cmd/SOURCES/Various-fixes-for-trace-cmd-flightrecorder-systemd.patch

From 1132270594314acb2eed250f1690d5b10d0fb47c Mon Sep 17 00:00:00 2001
From: John Kacur <jkacur@redhat.com>
Date: Tue, 30 Jun 2015 14:30:47 +0200
Subject: [PATCH] Various fixes for trace-cmd flightrecorder - systemd files

1) Allow systemctl enable trace-cmd.service to start trace-cmd.service automatically at system boot
2) Don't stop trace-cmd.service at system shutdown
3) Don't stop trace-cmd.service at each kernel module loading
4) Start trace-cmd.service before any normal services
1) Allow systemctl enable trace-cmd.service to start trace-cmd.service automatically at system boot

I added [Install] section in trace-cmd.service. This is necessary to
make trace-cmd.service automatically get started at system
boot. Without the [Install] section, systemctl enable
trace-cmd.service fails as below:

  ~]# systemctl enable trace-cmd.service
  The unit files have no [Install] section. They are not meant to be enabled
  using systemctl.
  Possible reasons for having this kind of units are:
  1) A unit may be statically enabled by being symlinked from another unit's
     .wants/ or .requires/ directory.
  2) A unit's purpose may be to act as a helper for some other unit which has
     a requirement dependency on it.
  3) A unit may be started when needed via activation (socket, path, timer,
     D-Bus, udev, scripted systemctl call, ...).

WantedBy=multi-user.target in trace-cmd.service means that
trace-cmd.service is started if multi-user.target, traditional
runlevel 3, is active.

2) Don't stop trace-cmd.service at system shutdown

In the current configuration, trace-cmd reset is executed when
trace-cmd.service is stopped at system shutdown. Then, trace buffer is
cleared. This could become a problem if system crash happens at system
shutdown. Even if we get a crash dump at the crash during system
shutdown, the crash dump doesn't contain useful trace data.

To fix the issue, it's necessary to make trace-cmd.service not stop at
system shutdown.

To achieve this, I added DefaultDependencies=no into trace-cmd.service.

To understand this, first you need to understand how systemd achieves
system shutdown. Look at the description of shutdown.target in man 7
systemd.special.

     shutdown.target
         A special target unit that terminates the services on
         system shutdown.

         Services that shall be terminated on system shutdown
         shall add Conflicts= dependencies to this unit for their
         service unit, which is implicitly done when
         DefaultDependencies=yes is set (the default).

As this explains, systemd adds Conflicts=shutdown.target dependency to
all the unit files by default. By this, if we shutdown system for
example by systemctl poweroff, then all the currently active services
with the default dependency get stopped when shutdown.target is pulled
in.

Thus, in order not to stop trace-cmd.service at system shutdown, it's
necessary to remove the default Conflicts=shutdown.target
dependency. This is achieved by DefaultDependencies=no.

Also, we need to consider a default dependency around basic.target
that is explained in man 7 systemd.special as follows:

     basic.target
         A special target unit covering basic boot-up.

         systemd automatically adds dependencies of the types
         Requires= and After= for this target unit to all
         services (except for those with DefaultDependencies=no).

         Usually this should pull-in all mount points, swap
         devices, sockets, timers, and path units and other basic
         initialization necessary for general purpose daemons.

As this explains, Requires=basic.target is assigned to each service by
default.

Requires=basic.target means that the service tries to pull in the
basic.target when it's pulled in and gets stopped if the basic.target
gets stopped.

This means that trace-cmd.service gets stopped at system shutdown via
Requires=basic.target when basic.target is stopped.

Thus, not to stop trace-cmd.service at system shutdown, it's necessary
to remove the default Requires=basic.target dependency. This is again
achieved by DefaultDependencies=no.

3) Don't stop trace-cmd.service at each kernel module loading

We have now udev configuration that restarts trace-cmd.service each
time some kernel module is loaded. However, systemctl restart
trace-cmd.service means that systemctl stop trace-cmd.service and then
systemctl start trace-cmd.service. trace data gets lost when the
systemctl stop trace-cmd.service is executed as I explained above.

To fix the issue, we remove trace-cmd reset from ExecReload= in
trace-cmd.service and use systemctl reload trace-cmd.service in the
udev script instead of systemctl try-restart trace-cmd.service.

Note that it's enough to trace-cmd start to enable trace events of
newly loaded kernel modules.

Also note that systemctl is-active trace-cmd.service in the udev
script is needed to avoid executing systemctl reload trace-cmd.service
when trace-cmd.service is inactive. Without systemctl is-active
trace-cmd.service, systemctl reload trace-cmd.service is executed even
when trace-cmd.service is inactive and then logs the following
message:

  Jun 12 20:24:25 localhost systemd: Unit trace-cmd.service cannot be reloaded because it is inactive.

4) Start trace-cmd.service before any normal services

trace-cmd.service needs to start early enough to collect useful kernel
event information. It's too late if any of the normal services has
started.

To achieve this, DefaultDependencies=no is again needed to remove
After=basic.target dependency assigned by default that is explained in
man 7 systemd.special.

Also, we need Before=basic.target to start trace-cmd.service *before*
any normal services, i.e. any services with the default
After=basic.target dependency.

Signed-off-by: John Kacur <jkacur@redhat.com>
---
 98-trace-cmd.rules | 2 +-
 trace-cmd.service  | 7 ++++++-
 2 files changed, 7 insertions(+), 2 deletions(-)

diff --git a/98-trace-cmd.rules b/98-trace-cmd.rules
index 7c073692a0ab..9575bd819a8e 100644
--- a/98-trace-cmd.rules
+++ b/98-trace-cmd.rules
@@ -1 +1 @@
-SUBSYSTEM=="module", ACTION=="add", PROGRAM="/usr/bin/systemctl try-restart trace-cmd.service"
+SUBSYSTEM=="module", ACTION=="add", PROGRAM="/usr/bin/systemctl is-active trace-cmd.service", PROGRAM="/usr/bin/systemctl reload trace-cmd.service"
diff --git a/trace-cmd.service b/trace-cmd.service
index 6439b490f8f4..a5cdd046ca0c 100644
--- a/trace-cmd.service
+++ b/trace-cmd.service
@@ -1,5 +1,7 @@
 [Unit]
 Description=trace-cmd Flightrecorder
+DefaultDependencies=no
+Before=basic.target
 
 [Service]
 Type=oneshot
@@ -7,4 +9,7 @@ RemainAfterExit=yes
 EnvironmentFile=/etc/sysconfig/trace-cmd.conf
 ExecStart=/usr/bin/trace-cmd start $OPTS
 ExecStop=/usr/bin/trace-cmd reset
-ExecReload=/usr/bin/trace-cmd reset; /usr/bin/trace-cmd start $OPTS
+ExecReload=/usr/bin/trace-cmd start $OPTS
+
+[Install]
+WantedBy=multi-user.target
-- 
1.8.3.1