From c28cc36469554dc55540f059fbdc7fa22a2c31fc Mon Sep 17 00:00:00 2001 From: Nick Piggin Date: Fri, 7 Jan 2011 17:49:53 +1100 Subject: fs: fs_struct use seqlock Use a seqlock in the fs_struct to enable us to take an atomic copy of the complete cwd and root paths. Use this in the RCU lookup path to avoid a thread-shared spinlock in RCU lookup operations. Multi-threaded apps may now perform path lookups with scalability matching multi-process apps. Operations such as stat(2) become very scalable for multi-threaded workload. Signed-off-by: Nick Piggin --- fs/fs_struct.c | 10 ++++++++++ 1 file changed, 10 insertions(+) (limited to 'fs/fs_struct.c') diff --git a/fs/fs_struct.c b/fs/fs_struct.c index ed45a9c..60b8531 100644 --- a/fs/fs_struct.c +++ b/fs/fs_struct.c @@ -14,9 +14,11 @@ void set_fs_root(struct fs_struct *fs, struct path *path) struct path old_root; spin_lock(&fs->lock); + write_seqcount_begin(&fs->seq); old_root = fs->root; fs->root = *path; path_get(path); + write_seqcount_end(&fs->seq); spin_unlock(&fs->lock); if (old_root.dentry) path_put(&old_root); @@ -31,9 +33,11 @@ void set_fs_pwd(struct fs_struct *fs, struct path *path) struct path old_pwd; spin_lock(&fs->lock); + write_seqcount_begin(&fs->seq); old_pwd = fs->pwd; fs->pwd = *path; path_get(path); + write_seqcount_end(&fs->seq); spin_unlock(&fs->lock); if (old_pwd.dentry) @@ -52,6 +56,7 @@ void chroot_fs_refs(struct path *old_root, struct path *new_root) fs = p->fs; if (fs) { spin_lock(&fs->lock); + write_seqcount_begin(&fs->seq); if (fs->root.dentry == old_root->dentry && fs->root.mnt == old_root->mnt) { path_get(new_root); @@ -64,6 +69,7 @@ void chroot_fs_refs(struct path *old_root, struct path *new_root) fs->pwd = *new_root; count++; } + write_seqcount_end(&fs->seq); spin_unlock(&fs->lock); } task_unlock(p); @@ -88,8 +94,10 @@ void exit_fs(struct task_struct *tsk) int kill; task_lock(tsk); spin_lock(&fs->lock); + write_seqcount_begin(&fs->seq); tsk->fs = NULL; kill = !--fs->users; + write_seqcount_end(&fs->seq); spin_unlock(&fs->lock); task_unlock(tsk); if (kill) @@ -105,6 +113,7 @@ struct fs_struct *copy_fs_struct(struct fs_struct *old) fs->users = 1; fs->in_exec = 0; spin_lock_init(&fs->lock); + seqcount_init(&fs->seq); fs->umask = old->umask; get_fs_root_and_pwd(old, &fs->root, &fs->pwd); } @@ -144,6 +153,7 @@ EXPORT_SYMBOL(current_umask); struct fs_struct init_fs = { .users = 1, .lock = __SPIN_LOCK_UNLOCKED(init_fs.lock), + .seq = SEQCNT_ZERO, .umask = 0022, }; -- cgit v1.1 From b3e19d924b6eaf2ca7d22cba99a517c5171007b6 Mon Sep 17 00:00:00 2001 From: Nick Piggin Date: Fri, 7 Jan 2011 17:50:11 +1100 Subject: fs: scale mntget/mntput The problem that this patch aims to fix is vfsmount refcounting scalability. We need to take a reference on the vfsmount for every successful path lookup, which often go to the same mount point. The fundamental difficulty is that a "simple" reference count can never be made scalable, because any time a reference is dropped, we must check whether that was the last reference. To do that requires communication with all other CPUs that may have taken a reference count. We can make refcounts more scalable in a couple of ways, involving keeping distributed counters, and checking for the global-zero condition less frequently. - check the global sum once every interval (this will delay zero detection for some interval, so it's probably a showstopper for vfsmounts). - keep a local count and only taking the global sum when local reaches 0 (this is difficult for vfsmounts, because we can't hold preempt off for the life of a reference, so a counter would need to be per-thread or tied strongly to a particular CPU which requires more locking). - keep a local difference of increments and decrements, which allows us to sum the total difference and hence find the refcount when summing all CPUs. Then, keep a single integer "long" refcount for slow and long lasting references, and only take the global sum of local counters when the long refcount is 0. This last scheme is what I implemented here. Attached mounts and process root and working directory references are "long" references, and everything else is a short reference. This allows scalable vfsmount references during path walking over mounted subtrees and unattached (lazy umounted) mounts with processes still running in them. This results in one fewer atomic op in the fastpath: mntget is now just a per-CPU inc, rather than an atomic inc; and mntput just requires a spinlock and non-atomic decrement in the common case. However code is otherwise bigger and heavier, so single threaded performance is basically a wash. Signed-off-by: Nick Piggin --- fs/fs_struct.c | 26 ++++++++++++++++---------- 1 file changed, 16 insertions(+), 10 deletions(-) (limited to 'fs/fs_struct.c') diff --git a/fs/fs_struct.c b/fs/fs_struct.c index 60b8531..68ca487 100644 --- a/fs/fs_struct.c +++ b/fs/fs_struct.c @@ -17,11 +17,11 @@ void set_fs_root(struct fs_struct *fs, struct path *path) write_seqcount_begin(&fs->seq); old_root = fs->root; fs->root = *path; - path_get(path); + path_get_long(path); write_seqcount_end(&fs->seq); spin_unlock(&fs->lock); if (old_root.dentry) - path_put(&old_root); + path_put_long(&old_root); } /* @@ -36,12 +36,12 @@ void set_fs_pwd(struct fs_struct *fs, struct path *path) write_seqcount_begin(&fs->seq); old_pwd = fs->pwd; fs->pwd = *path; - path_get(path); + path_get_long(path); write_seqcount_end(&fs->seq); spin_unlock(&fs->lock); if (old_pwd.dentry) - path_put(&old_pwd); + path_put_long(&old_pwd); } void chroot_fs_refs(struct path *old_root, struct path *new_root) @@ -59,13 +59,13 @@ void chroot_fs_refs(struct path *old_root, struct path *new_root) write_seqcount_begin(&fs->seq); if (fs->root.dentry == old_root->dentry && fs->root.mnt == old_root->mnt) { - path_get(new_root); + path_get_long(new_root); fs->root = *new_root; count++; } if (fs->pwd.dentry == old_root->dentry && fs->pwd.mnt == old_root->mnt) { - path_get(new_root); + path_get_long(new_root); fs->pwd = *new_root; count++; } @@ -76,13 +76,13 @@ void chroot_fs_refs(struct path *old_root, struct path *new_root) } while_each_thread(g, p); read_unlock(&tasklist_lock); while (count--) - path_put(old_root); + path_put_long(old_root); } void free_fs_struct(struct fs_struct *fs) { - path_put(&fs->root); - path_put(&fs->pwd); + path_put_long(&fs->root); + path_put_long(&fs->pwd); kmem_cache_free(fs_cachep, fs); } @@ -115,7 +115,13 @@ struct fs_struct *copy_fs_struct(struct fs_struct *old) spin_lock_init(&fs->lock); seqcount_init(&fs->seq); fs->umask = old->umask; - get_fs_root_and_pwd(old, &fs->root, &fs->pwd); + + spin_lock(&old->lock); + fs->root = old->root; + path_get_long(&fs->root); + fs->pwd = old->pwd; + path_get_long(&fs->pwd); + spin_unlock(&old->lock); } return fs; } -- cgit v1.1