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? > 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. 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. > 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. > (kmem_cache_destroy() already has an rcu_barrier(). Can we do away > with the private rcu games in the vfs and switch to > SLAB_DESTROY_BY_RCU?) -- Kirill A. Shutemov
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