On Sat, 9 Jun 2012 01:14:46 +0300 "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx> wrote: > On Fri, Jun 08, 2012 at 03:02:53PM -0700, Andrew Morton wrote: > > On Sat, 9 Jun 2012 00:41:03 +0300 > > "Kirill A. Shutemov" <kirill.shutemov@xxxxxxxxxxxxxxx> wrote: > > > > > There's no reason to call rcu_barrier() on every deactivate_locked_super(). > > > We only need to make sure that all delayed rcu free inodes are flushed > > > before we destroy related cache. > > > > > > Removing rcu_barrier() from deactivate_locked_super() affects some > > > fas paths. E.g. on my machine exit_group() of a last process in IPC > > > namespace takes 0.07538s. rcu_barrier() takes 0.05188s of that time. > > > > What an unpleasant patch. Is final-process-exiting-ipc-namespace a > > sufficiently high-frequency operation to justify the change? This, please. > > I don't really understand what's going on here. Are you saying that > > there is some filesystem against which we run deactivate_locked_super() > > during exit_group(), and that this filesystem doesn't use rcu-freeing > > of inodes? The description needs this level of detail, please. You still haven't explained where this deactivate_locked_super() call is coming from. Oh well. > I think the rcu_barrier() is in wrong place. We need it to safely destroy > inode cache. deactivate_locked_super() is part of umount() path, but all > filesystems I've checked have inode cache for whole filesystem, not > per-mount. Well from a design perspective, putting the rcu_barrier() in the vfs is the *correct* place. Individual filesystems shouldn't be hard-coding knowledge about vfs internal machinery. A neater implementation might be to add a kmem_cache* argument to unregister_filesystem(). If that is non-NULL, unregister_filesystem() does the rcu_barrier() and destroys the cache. That way we get to delete (rather than add) a bunch of code from all filesystems and new and out-of-tree filesystems cannot forget to perform the rcu_barrier(). > > The implementation would be less unpleasant if we could do the > > rcu_barrier() in kmem_cache_destroy(). I can't see a way of doing that > > without adding a dedicated slab flag, which would require editing all > > the filesystems anyway. > > I think rcu_barrier() for all kmem_cache_destroy() would be too expensive. That is not what I proposed. -- To unsubscribe from this list: send the line "unsubscribe linux-ext4" in the body of a message to majordomo@xxxxxxxxxxxxxxx More majordomo info at http://vger.kernel.org/majordomo-info.html
