It can avoid unnecessary task migrataion, see this scenario:
- container init task is on cpu 1, and we assigned it to cpu 1,
but parent cgroup's cpuset.cpus=2
- we created the cgroup dir and inherited cpuset.cpus from parent as 2
- write container init task's pid to cgroup.procs
- [it's possibile the container init task migrated to cpu 2 here]
- set cpuset.cpus as assigned to cpu 1
- [the container init task has to be migrated back to cpu 1]
So we should set cpuset.cpus and cpuset.mems before writing pids
to cgroup.procs to aviod such problem.
Signed-off-by: Qiang Huang <h.huangqiang@huawei.com>
TL;DR: check for IsExist(err) after a failed MkdirAll() is both
redundant and wrong -- so two reasons to remove it.
Quoting MkdirAll documentation:
> MkdirAll creates a directory named path, along with any necessary
> parents, and returns nil, or else returns an error. If path
> is already a directory, MkdirAll does nothing and returns nil.
This means two things:
1. If a directory to be created already exists, no error is
returned.
2. If the error returned is IsExist (EEXIST), it means there exists
a non-directory with the same name as MkdirAll need to use for
directory. Example: we want to MkdirAll("a/b"), but file "a"
(or "a/b") already exists, so MkdirAll fails.
The above is a theory, based on quoted documentation and my UNIX
knowledge.
3. In practice, though, current MkdirAll implementation [1] returns
ENOTDIR in most of cases described in #2, with the exception when
there is a race between MkdirAll and someone else creating the
last component of MkdirAll argument as a file. In this very case
MkdirAll() will indeed return EEXIST.
Because of #1, IsExist check after MkdirAll is not needed.
Because of #2 and #3, ignoring IsExist error is just plain wrong,
as directory we require is not created. It's cleaner to report
the error now.
Note this error is all over the tree, I guess due to copy-paste,
or trying to follow the same usage pattern as for Mkdir(),
or some not quite correct examples on the Internet.
[1] https://github.com/golang/go/blob/f9ed2f75/src/os/path.go
Signed-off-by: Kir Kolyshkin <kir@openvz.org>
Michael Crosby
Qiang Huang
Kir Kolyshkin