On Fri, Jul 24, 2009 at 09:01:46AM -0700, Paul Menage wrote: > On Fri, Jul 24, 2009 at 8:50 AM, Matt Helsley<matthltc@xxxxxxxxxx> 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. OK. > 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 Except most processes aren't big multi-threaded servers and not everyone runs such processes. They'll experience the overhead of a global lock when they don't have to. Again, it's a question of magnitudes we don't know I think. > > - 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. Well, I imagine holding tasklist_lock is worse than cgroup_mutex in some ways since it's used even more widely. Makes sense not to use it here.. Cheers, -Matt Helsley _______________________________________________ Containers mailing list Containers@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/containers
