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. 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 _______________________________________________ Containers mailing list Containers@xxxxxxxxxxxxxxxxxxxxxxxxxx https://lists.linux-foundation.org/mailman/listinfo/containers
