Messages in this thread | | | Date | Fri, 24 Jul 2009 09:01:46 -0700 | Subject | Re: [PATCH 5/6] Makes procs file writable to move all threads by tgid at once | From | Paul Menage <> |
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On Fri, Jul 24, 2009 at 8:50 AM, Matt Helsley<matthltc@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.
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