On 4/9/21 6:02 AM, Pavel Hrdina wrote:
On Thu, Apr 08, 2021 at 10:00:32PM -0400, Eric Farman wrote:
On 4/8/21 8:12 AM, Pavel Hrdina wrote:
On Wed, Apr 07, 2021 at 11:28:48PM -0400, Eric Farman wrote:
On 4/7/21 9:07 AM, Pavel Hrdina wrote:
On Fri, Mar 26, 2021 at 05:25:03PM +0100, Eric Farman wrote:
The introduction of nested cgroups used a little macro
virCgroupGetNested() to retrieve the nested cgroup
pointer, if one exists. But this macro isn't used when
removing cgroups, resulting in some messages:
Mar 25 20:55:17 fedora33 libvirtd[955]: unable to open '/sys/fs/cgroup/machine.slice/machine-qemu\x2d1\x2dguest.scope/': No such file or directory
Mar 25 20:55:17 fedora33 libvirtd[955]: Failed to remove cgroup for guest
That directory exists while the guest is running, as it
was created by systemd/machined, so the code probably meant
to open the libvirt/ subdirectory from that point.
Similarly, there happen to be BPF-related file descriptors
that don't get cleaned up in this process too, because they
are anchored off the nested cgroup location:
[test@fedora33 ~]# ls /proc/$(pgrep libvirtd)/fd/* | wc -l
35
[test@fedora33 ~]# virsh create guest.xml
Domain 'guest' created from guest.xml
[test@fedora33 ~]# ls /proc/$(pgrep libvirtd)/fd/* | wc -l
42
[test@fedora33 ~]# virsh shutdown guest
Domain 'guest' is being shutdown
[test@fedora33 ~]# ls /proc/$(pgrep libvirtd)/fd/* | wc -l
37
[test@fedora33 ~]# virsh create guest.xml
Domain 'guest' created from guest.xml
[test@fedora33 ~]# ls /proc/$(pgrep libvirtd)/fd/* | wc -l
44
[test@fedora33 ~]# virsh shutdown guest
Domain 'guest' is being shutdown
[test@fedora33 ~]# ls /proc/$(pgrep libvirtd)/fd/* | wc -l
39
Let's fix this by using the same macro when removing cgroups,
so that it picks up the right structure and can remove the
associated resources properly.
Fixes: 184245f53b94 ("vircgroup: introduce nested cgroup to properly work with systemd")
Signed-off-by: Eric Farman <farman@xxxxxxxxxxxxx>
---
src/util/vircgroup.c | 5 +++--
1 file changed, 3 insertions(+), 2 deletions(-)
I don't thing this patch is correct. With systemd we would get the same
error without the nested cgroup as well. It's because we terminate the
qemu process which makes systemd remove the VM root cgroup as well.
I don't experience any problems reverting the blamed patch. The qemu cgroup
code doesn't make any distinction about systemd or not; it just calls the
virCgroupRemove() to clean up the resources that were set up here during
init:
qemuInitCgroup()
virCgroupNewMachine()
virCgroupNewMachineSystemd()
virCgroupNewNested()
The group pointer that's stashed in qemu's struct is that of the
machine-qemu...scope group, rather than the nested group, but nothing in the
cleanup path touches group->nested. My initial patch is certainly flawed (as
you explain below), so maybe something like this is better?
@@ -2615,6 +2615,9 @@ virCgroupRemove(virCgroupPtr group)
{
size_t i;
+ if (group->nested)
+ virCgroupRemove(group->nested);
+
for (i = 0; i < VIR_CGROUP_BACKEND_TYPE_LAST; i++) {
if (group->backends[i]) {
int rc = group->backends[i]->remove(group);
Not great, since it cleans up the nested group but then still attempts to
clean up the machine-qemu...scope group that was setup by systemd. This
group wasn't setup by virCgroupV2MakeGroup(), so calling virCgroupV2Remove()
seems wrong too. Not sure how to address that.
I'm not sure how this will help. As I've already pointed out calling
virCgroupRemove(group) results in calling one or both functions:
virCgroupV1Remove()
virCgroupV2Remove()
Where both of these functions will call virCgroupRemoveRecursively()
which will recursively remove all subdirectories including the nested
one.
So the extra
if (group->nested)
virCgroupRemove(group->nested);
will not help with anything.
Looking at the code (I did not test it) it looks like the error message
is produced by following this path:
qemuProcessStop()
qemuRemoveCgroup()
virCgroupTerminateMachine() this will make systemd to remove the cgroup
virCgroupRemove()
virCgroupV2Remove()
virCgroupV2DevicesRemoveProg()
virCgroupV2DevicesDetectProg()
open()
Yes, this is the path where exit out, and thus never get to the
RemoveRecursively routine you mentioned above. But it's not because of the
virCgroupTerminateMachine() call, but rather the progfd and mapfd fields in
the cgroup passed to virCgroupV2DevicesDetectProg().
With the blamed patch, these fields are zero, so we go ahead and try to do
all that other work. Prior to that patch, and with my proposed patch, these
fd's are non-zero, and so it exits immediately back to
virCgroupV2DevicesRemoveProg() which does a VIR_FORCE_CLOSE on them. The
non-zero fd's are related to BPF (see below), and are stashed in that nested
cgroup nowadays.
(virsh create/shutdown a guest three times)
# readlink /proc/$(pgrep libvirtd)/fd/* | grep bpf
anon_inode:bpf-map
anon_inode:bpf-map
anon_inode:bpf-prog
anon_inode:bpf-map
anon_inode:bpf-prog
anon_inode:bpf-prog
I still fail to see how calling virCgroupRemove(group->nested) in
virCgroupRemove() would help at all. The original issue you mentioned in
the commit message is that we log this error:
unable to open '/sys/fs/cgroup/machine.slice/machine-qemu\x2d1\x2dguest.scope/': No such file or directory
So calling virCgroupRemove(group->nested) would also fail with:
unable to open '/sys/fs/cgroup/machine.slice/machine-qemu\x2d1\x2dguest.scope/libvirt/': No such file or directory
because if the VM root cgroup ('machine-qemu\x2d1\x2dguest.scope')
doesn't exist the nested cgroup ('libvirt') will not exist as well.
While you are correct that the nested cgroup doesn't exist in sysfs, the
pointers being handled by libvirt still do since virCgroupFree() hasn't
yet been called. This message shows up because the cgroup we are
processing (the VM root cgroup) contains zeros for progfd and mapfd, and
virCgroupV2DevicesDetectProg() attempts to open the corresponding path
that systemd already cleaned up.
The nested cgroup ('libvirt') has nonzero file descriptors, so the check
at the top of virCgroupV2DevicesDetectProg() would return before
attempting to open the nonexistent '/sys/fs/cgroup/.../libvirt/' path.
In that case, the message would NOT be generated, and the code will just
go back to virCgroupV2DevicesRemoveProg() to close the file descriptors.
Now to the progfd and mapfd being zero, that depends whether libvirtd
was restarted while the VM was running.
Huh? No, I see the progfd and mapfd fields zero in the VM root cgroup
and stored with the BPF file descriptors in the nested ('libvirt')
cgroup with your patch. Previously, they were in the VM root cgroup itself.
When a VM is started on host
with cgroups v2 libvirt will create BPF program and BPF map to limit
access to system devices and stores both in the mentioned variables.
But if you restart libvirtd it will load the BPF prog ID and BPF map ID
only on demand, for example when destroying the VM.
Now on hosts with systemd the owner of the VM root cgroup is systemd
and because we use call talk to systemd-machined and register the VM
there the VM root cgroup is created for us by systemd-machined and it is
associated with qemu PID. When destroying the VM we will kill the qemu
process which will trigger systemd-machined to automatically cleanup the
cgroup. Once that happens kernel should eventually cleanup both BPF prog
and BPF map that were associated with the nested cgroup because it no
longer exist and there are no other references to the prog or map.
It may take some time before kernel actually removes the prog and map.
This all may be true, but as I said in my commit message libvirt's
leaving open two file descriptors for the BPF prog and map that aren't
being closed when the guest is shut down. I was attempting to trigger a
different bug by doing a virsh create/shutdown in a loop, and eventually
my creates would fail because I'd exhausted the number of open files.
The only thing on systemd based host we can do is to skip the whole BPF
detection code if the directory was already removed which I've already
proposed by handling ENOENT in virCgroupV2DevicesDetectProg().
Yes. That removes the message, but doesn't clean up the file descriptors
being left open.
All of the above applies to systemd based hosts. If we consider system
without systemd then there is an actual bug as you already pointed out
that the BPF prog and BPF map are now associated with the nested cgroup,
to fix that we should change only virCgroupV2Remove:
diff --git a/src/util/vircgroupv2.c b/src/util/vircgroupv2.c
index 248d4047e5..4664492c34 100644
--- a/src/util/vircgroupv2.c
+++ b/src/util/vircgroupv2.c
@@ -523,6 +523,7 @@ static int
virCgroupV2Remove(virCgroupPtr group)
{
g_autofree char *grppath = NULL;
+ virCgroupPtr parent = virCgroupGetNested(group);
int controller;
/* Don't delete the root group, if we accidentally
@@ -534,7 +535,7 @@ virCgroupV2Remove(virCgroupPtr group)
if (virCgroupV2PathOfController(group, controller, "", &grppath) < 0)
return 0;
- if (virCgroupV2DevicesRemoveProg(group) < 0)
+ if (virCgroupV2DevicesRemoveProg(parent) < 0)
return -1;
return virCgroupRemoveRecursively(grppath);
This addresses both symptoms I'm experiencing, but it feels weird to be
the only place outside of vircgroup.c that needs to navigate/understand
the difference between group and group->nested. This is why I'd proposed
that logic in the caller, so it's covered by both v1 and v2, but if it's
only the v2 code that needs this then okay. I don't have a non-systemd
system nearby to try it with.
Eric
Unfortunately we cannot simply ignore ENOENT in the
virCgroupV2DevicesDetectProg() because it is used in different places
where such error should be reported.
What we can do is to introduce new parameter `bool quiet` into
virCgroupV2DevicesDetectProg() that would silence the ENOENT error if
set to true and we could use it from virCgroupV2DevicesRemoveProg()
or something similar.
This smells fishy to me. I tried it, and it does indeed get us to
virCgroupRemoveRecursively() later down that callchain. The downside is it
is of course called with the same cgroup
(grppath="/sys/fs/cgroup/machine.slice/machine-qemu\x2d1\x2dguest.scope/")
and returns without doing anything when virDirOpenQuiet() also returns
ENOENT.
Well that's the only thing we can do if the directories no longer exist.
Pavel
Eric
Pavel
This happens only on cgroup controllers managed by systemd. For example
if you switch to cgroups v1 where each controller is in separate
directory not all controllers supported by libvirt are also supported by
systemd. In this case libvirt creates all the cgroups by itself and is
responsible to cleanup as well. With this patch we would not remove the
VM root cgroups in these controllers. This would affect following
controllers:
cpuset
freezer
net_cls
net_prio
perf_event
You can verify what happens with cgroups v1 by adding
systemd.unified_cgroup_hierarchy=0 to your kernel cmdline.
Neat, thanks for that tip.
Thanks,
Eric
Pavel
