** Changed in: qemu
Importance: Undecided => Wishlist
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https://bugs.launchpad.net/bugs/1754656
Title:
Please solve graceful (ACPI) poweroff issue, using signals, most
importantly SIGTERM
Status in QEMU:
New
Bug description:
Version:
QEMU emulator version 2.11.1
Introduction:
This is call for action to get attention of somebody in QEMU
project/organization, who is capable of actually doing something about
this pressing issue. This might be TLDR for some, but that's only
because of the complexity of the issue. Please read this with open
mind.
Problem:
As QEMU users, we need (it is a requirement) to have some mechanism in
place, to somehow convert simple POSIX signal sent form host, into
graceful ACPI shutdown of the guest. This signal, due various
historical reasons and daemon design, must be SIGTERM foremost.
Status quo:
After wading through mailing lists and bug tracker I concluded that
this is "political" problem and I am in search of somebody, a
"politician" within QEMU project, who will help us reach conclusion to
this problem.
First I will present analysis of the situation, and then propose some
suggestions for solutions.
Even then, any of these proposals might be, potentially, seen as
problematic in eyes of QEMU maintainers, developers, dictators, long
term users or their dogs.
That's why we need somebody willing, "higher up the chain" or
whatever, to orchestrate discussion so that we can actually reach
consensus in the matter, solution that is acceptable to **everyone**.
Analysis:
Each QEMU emulated virtual machine (vm), running in the host system,
is represented by single qemu-* process (followed by several threads).
Thus for all intents and purposes, any such instantiated vm, must be
seen as it's own, separate, daemon.
I repeat running qemu-* vm **is a daemon**.
QEMU provides incredible IO redirection capabilities, so we don't need
to get into issues of logging, console and monitor redirections and
such, as this is already a (perfectly) solved problem.
What we cannot currently do, at least easily, reliably and simply, is
to shutdown guest gracefully from "outside".
That is not a problem for those of us, who use some kind of higher
level orchestrator (I think one of them is virsh, but this is not
important in this matter) that takes care of this by communicating
with QEMU directly (I guess this is done by sending commands to
internal monitor by pipe (or socket) held open by mentioned
orchestrator).
However it is a problem for those of us, who run qemu-* processes bare
or supervised.
Let's say I, as administrator, want to implement vm instance as supervised
service.
I can use any supervisor for that, systemd even. Let us not get into into
supervisor wars.
At basic level almost all supervisors are similar. Supervisor usually
is yet another process, that "leads" the qemu-* one.
In case of systemd it is PID1, but in case of other supervision
schemes, like daemontools, runit, s6 or nosh, it is separate
'*supervise' process instead.
When such supervisor is tearing down the service,
"leading"/supervising, parent will send SIGTERM to it's child qemu-*
process.
This behaviour is almost universal among all supervisors. This due the
fact, that it is customary for daemons to cease all operations and
exit cleanly when receiving SGITERM signal. If daemon child fails to
exit within configurable timeframe, supervisor deals with it by the
means of SIGKILL.
As such, one would expect, similarly, for qemu-* process to send ACPI
shutdown event to guest internally (roughly equivalent to
'system_powerdown' monitor command) on SIGTERM reception.
But this is not what happens!
Instead, qemu-* just flushes pending IO and kills the guest instantly.
Then, on next vm "boot", guest detects this as power failure event,
and performs fsck checks and other things, it is supposed to do in
case of power failure. We are not mentioning possible data loss that
might have happened due to this behavior, either.
Some supervisors (like systemd for example) might provide feature to
change "termiante operations" signal to something else like SIGTERM,
but that is not universal supervisor feature by any means. Default
action for any proper daemon is to cleanly terminate on SIGTERM.
That is why we need ability to somehow instruct QEMU to **always**
perform graceful ACPI shutdown on SIGTERM.
Potential reply to this bug saying that one should send
'system_powerdown' over monitor connection won't fly!
As it is not always possible (nor required) to hook into supervisor's
signal processing (main reason being intentional supervisor simplicity
in search of extreme reliability, and de facto standardized behavior
of daemons to exit cleanly on SIGTERM).
More over, in situations like machine reboot, most supervisors won't
play around with signal remapping, they will simply send out SIGTERM
to all supervised processes. We want our qemu-* instances to come up
undamaged from such action (on next host reboot) and not have them
stuck in fscks (or worse - ending up damaged) .
If this can be extended further, inside QEMU, with internal signal to
action remapping, the better, but supporting graceful shutdown on
SIGTERM is hard requirement.
Proposed solutions:
0. modify QEMU so that it emits ACPI shutdown event equivalent to
'system_powerdown' monitor command by default
- this seems to be a "no go", with backwards compat. and "current users
expectations"
cited as the reasons
- I won't go into a fact that QEMU changed option handling without BOLD
notice few times
1. add single switch '-graceful-shutdown-on-sgiterm'
- this was rejected when person tried to submit patch implementing
something similar
to what I am requesting, only bound to SIGHUP
- that person (implementing graceful poweroff on SIGHUP) was wrong, almost
no
supervision scheme in existence sends out SIGHUP on service termination
request,
although all supervisors are able to send out SIGHUP when instructed
- in daemons SIGHUP is usually reserved for "daemon reload" which can be
interpreted
like "reboot" in QEMU context
- if we see qemu-* proces for what it is, a daemon, it must react properly
to SIGTERM foremost
2. add ability to map internal monitor action commands to few signals like
SIGTERM, SIGHUP, SIGINTR, SIGUSR1, SIGUSR2, SIGALARM etc
- this seems like best solution to me, that allows us to satisfy both
backwards compat. and "current users" requirement, yet allows us
to use qemu-* with proper supervision, and it even adds something extra
(I know some of these signals are used internally by QEMU)
- QEMU already has options parsing infrastructure in place to handle this
nicely, something like:
: -signal SIGTERM,monitorcommand=system_powerdown -signal
SIGHUP,monitorcommand=system_reset
would be great in this case
3. add ability to map signals to executable scripts
- with this scheme QEMU would spawn child on signal reception, and this
script would then be used to perform the action
- this solution is most complex, most convoluted and most "flexible"
- for example with definition like this:
: -signal SIGTERM,script=signals/sigterm
qemu would perform this sequence of tasks:
- on SIGTERM qemu-* would spawn child script ./signals/sigterm
- this script would then pull out monitor fd descriptor from some
kind of fd holder
- would write 'system_powerdown' command into monitor fd and
terminate
- qemu-* would then read the command from monitor
- qemu-* would then interpret read-in command and gracefully terminate
- option parsing infrastructure is in place and QEMU is able to spawn and
reap it's own children
which is proven by network up/down scripts
Of these, it seems that 0. and 1. are simplest to implement, yet
"politically" unimplementable.
More over QEMU people seem to be hard set on SIGTERM meaning "killing
unresponsive guest".
2. seems like most reasonable proposal that has potential to make
everyone happy. It is also most reliable because internal QEMU command
dispatch would have least chances to fail.
3. is most flexible and can also be combined with 2. Reliability wise,
there is slight chance signal handling script will fail to execute,
leaving qemu-* at the mercy of supervisor (timeouted SIGKILL).
Both 2 and 3 should probably provide configurable timeout after which
QEMU would perform default action (eg. as it does now).
Conclusion:
I hope QEMU project members understand severity of the issue and are
open to listed solutions. It might be that proposed solutions don't
match QEMU project "spirit" perfectly. If so, I urge people capable of
resolving this, to propose their versions.
The fact is, that with proliferation of systemd, popularity of
alternative supervisors is on the rise as well, but even under
systemd, unintuitive handling of SIGTERM by bare QEMU processes is a
problem.
Further Reading:
https://patchwork.kernel.org/patch/9626293/
- Daniel P. Berrange says:
"Because QEMU already designate that as doing an immediate stop - ie it'll
allow QEMU block layer to flush pending I/O, but it will not wait for the
guest to shutdown. If we change that behaviour we'll break anyone who
is already relying on SIGHUP - qemu might never exit at all if the guest
ignores the ACPI request"
- this behaviour is incorrect if we perceive qemu-* process as daemon,
proper,
yet it is, supposedly, entrenched in QEMU userbase
- signals remapping capability would allow us to keep the "old" behavior for
entrenched users
while it would allow administrators and orchestrator writers to select
signal disposition
they actually need
https://bugs.launchpad.net/qemu/+bug/1217339
and
https://lists.nongnu.org/archive/html/qemu-devel/2017-03/msg03039.html
- on my QEMU version of 2.11.1 SIGTERM just kills the guest without proper
shutdown
- although thread says exit is graceful
- dicussion is problematic in several ways:
- SIGTERM is not intended to "terminate unresponsive guest" eg "terminate
daemon uncleanly"
in any sane daemon in existence
- it means "terminate gracefully"
- if "terminate unresponsive guest" was true meaning of SIGTERM,
databases like
mariadb or postgers would kill themselves on SIGTERM, leaving data in
inconsistent state, which they, of course, do not!
- some kind of "signal tapping" similar to "port tapping" is suggested
- this is non-obvious and error prone and nonstandard (no normal
supervisor
will play such signal tapping games)
- signal remapping makes more sense in this regard
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