Re: [RFC][PATCH] CPUSets: Move most calls to rebuild_sched_domains() to the workqueue

["Paul Menage" <menage@google.com>]

On Thu, Jun 26, 2008 at 10:10 PM, Max Krasnyansky <maxk@qualcomm.com> wrote:
>
> Instead of changing cpu_hotplug locking should we maybe try to avoid using
> cgroup_lock in rebuild_sched_domains() ?

Yes, that would be good too.

> There is a comment in cpuset.c that says
>  * If a task is only holding callback_mutex, then it has read-only
>  * access to cpusets.
>
> I'm not sure if it's still valid. rebuild_sched_domains() only needs read only
> access, it does not really modify any cpuset structures.

The comment is still valid, if you interpret it narrowly enough.
Holding callback_mutex gives you read-only access to structures that
are under the control of cpusets. But rebuild_sched_domains() needs to
traverse the hierarchy of cpusets, and that hierarchy is controlled by
cgroups. Currently the only synchronization primitives exposed by
cgroups are:

- cgroup_lock()/cgroup_unlock() to prevent all cgroup modifications
(also used as the main synchronization primitive by some subsystems,
i.e. it's in danger of becoming the cgroups equivalent of the BKL).

- task_lock()/task_unlock() to prevent a specific task from changing cgroups

Possible options for richer locking support include:

- lock/unlock a hierarchy, to prevent creation/deletion of cgroups in
that hierarchy

- lock/unlock a cgroup to prevent deletion of that cgroup

- lock/unlock a cgroup to prevent task movement in/out of that cgroup

For the case of rebuild_sched_domains, we need the first of these
options. This lock would be taken in cgroup.c at the points where it
attached and removed cgroups from a cgroup tree, and could be taken by
something like cpusets that needed to keep the hierarchy stable while
scanning it. I think it would be fine to make it a mutex rather than a
spinlock.

cpu_hotplug.lock has to nest outside this hierarchy lock due to it
being taken at the root of the hotplug/unplug path. So as long as we
can ensure that we can always nest the hierarchy lock inside any
get_online_cpus()/put_online_cpus() pairs, we should be OK.

Paul