[Devel] Re: [PATCH 5/6] Makes procs file writable to move all threads by tgid at once

[Paul Menage]

On Fri, Jul 24, 2009 at 8:50 AM, Matt Helsley<matthltc at us.ibm.com> wrote:
>
> There is much ado about not taking additional "global locks" in fork()
> paths.
>
> * The fork and exit callbacks cgroup_fork() and cgroup_exit(), don't
> * (usually) take cgroup_mutex.  These are the two most performance
> * critical pieces of code here.
> ...
>
> and as I recall cgroup_fork() doesn't ever take cgroup_mutex because it is
> so performance critical.

cgroup_mutex is a much bigger and heavier mutex than the new rwsem
being introduced in this patch. It's sort of the BKL of cgroups,
although where possible I'm encouraging use of finer-grained
alternatives (such as subsystem-specific locks, the per-hierarchy
lock, etc).

> Assuming the above comments in kernel/cgroup.c
> are correct then this patch adds a performance regression by introducing a
> global mutex in the fork path, doesn't it?

Yes, although to what magnitude isn't clear.

Alternatives that we looked at were:

- add a clone_rwsem to task_struct, and require that a clone operation
that's adding to the same thread group take a read lock on the
leader's clone_rwsem; then the effect would be localised to a single
process; but for a system that has one big multi-threaded server on
it, the effect would still be similar to a global lock

- move the housekeeping done by cgroup_fork() inside the tasklist_lock
critical section in do_fork(); then cgroup_attach_proc() can rely on
the existing global tasklist_lock to provide the necessary
synchronization, rather than introducing a second global lock; the
downside is that it slightly increases the size of the section where
tasklist_lock is held for write.

Paul
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