Messages in this thread | | | Date | Fri, 22 Jun 2018 12:01:17 +0100 | From | Al Viro <> | Subject | Re: [lkp-robot] [fs] 3deb642f0d: will-it-scale.per_process_ops -8.8% regression |
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On Fri, Jun 22, 2018 at 12:00:14PM +0200, Christoph Hellwig wrote: > And a version with select() also covered:
For fuck sake, if you want vfs_poll() inlined, *make* *it* *inlined*. Is there any reason for not doing that other than EXPORT_SYMBOL_GPL fetish? Because if there isn't, I would like to draw your attention to the fact that _this_ pwecious inchewlekshul pwopewty can be trivially open-coded by out-of-tree shite even if it happens to be non-GPL one.
> mask = vfs_poll(f.file, wait); > + if (f.file->f_op->poll) {
... not to mention that here you forgot to remove the call itself while expanding it.
Said that, you are not attacking the worst part of it - it's a static branch, not the considerably more costly indirect ones. Remember when I asked you about the price of those? Method calls are costly.
Another problem with with ->get_poll_head() calling conventions is that originally you wanted to return ERR_PTR(-mask) as a way to report not needing to call ->poll_mask(); that got shot down since quite a few of those don't fit into 12 bits that ERR_PTR() gives us.
IIRC, the real reason for non-constant ->get_poll_head() was the sockets, with
static struct wait_queue_head *sock_get_poll_head(struct file *file, __poll_t events) { struct socket *sock = file->private_data;
if (!sock->ops->poll_mask) return NULL; sock_poll_busy_loop(sock, events); return sk_sleep(sock->sk); }
The first part isn't a problem (it is constant). The second is static inline void sock_poll_busy_loop(struct socket *sock, __poll_t events) { if (sk_can_busy_loop(sock->sk) && events && (events & POLL_BUSY_LOOP)) { /* once, only if requested by syscall */ sk_busy_loop(sock->sk, 1); } }
and the third -
static inline wait_queue_head_t *sk_sleep(struct sock *sk) { BUILD_BUG_ON(offsetof(struct socket_wq, wait) != 0); return &rcu_dereference_raw(sk->sk_wq)->wait; }
Now, ->sk_wq is modified only in sock_init_data() and sock_graft(); the latter, IIRC, is ->accept() helper. Do we ever call either of those on a sock of already opened file? IOW, is there any real reason for socket ->get_poll_head() not to be constant, other than wanting to keep POLL_BUSY_LOOP handling out of ->poll_mask()? I agree that POLL_BUSY_LOOP is ugly as hell, but you *still* have sock_poll_mask() not free from it...
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