Re: [lkp-robot] [fs] 3deb642f0d: will-it-scale.per_process_ops -8.8% regression

Christoph Hellwig
2018-06-23T07:05:41Z

On Fri, Jun 22, 2018 at 09:02:55PM +0100, Al Viro wrote:
> > While at the same time corect poll code already checks net_busy_loop_on
> > to set POLL_BUSY_LOOP. So except for sockets where people set the
> > timeout to 0 the code already does the right thing as-is. IMHO not
> > really worth wasting a FMODE_* flag for it, but if you insist I'll add
> > it.
>
> It's not just that - there's also an issue of extra indirect call on the
> fast path for sockets. You get this method of yours + ->poll_mask(),
> which hits another indirect to per-family ->poll_mask(). It might be
> better to have these combined, sparing us an extra indirect call.
>
> Just give it the same calling conventions as ->poll_mask() have...

The problem is that for the busy poll we want the actual busy poll +
__pollwait + ->poll_mask. Which is going to make that new poll_busy_loop
with a return value look exactly like ->poll.

So for now I'm tempted to just do this:

---
From 4abf23f6565ff2a74f1859758f9c894abe476a00 Mon Sep 17 00:00:00 2001
From: Christoph Hellwig <hch@lst.de>
Date: Sat, 23 Jun 2018 09:02:59 +0200
Subject: FOLD: remove ->poll_busy_loop again

Busy looping always comes in from poll(2) or select(2). So instead of
adding a separate method we can just do it at the beginning of ->poll
for now.

Signed-off-by: Christoph Hellwig <hch@lst.de>
---
fs/select.c | 8 --------
include/linux/fs.h | 1 -
net/socket.c | 20 ++++++--------------
3 files changed, 6 insertions(+), 23 deletions(-)

diff --git a/fs/select.c b/fs/select.c
index 25327efca2f9..c68f7cdc777a 100644
--- a/fs/select.c
+++ b/fs/select.c
@@ -38,14 +38,6 @@ __poll_t vfs_poll(struct file *file, struct poll_table_struct *pt)
{
unsigned int events = poll_requested_events(pt);

- /*
- * XXX: might be worth adding a f_mode flag to see if busy looping is
- * supported. Although callers probably only keep setting it when
- * supported, that's why POLL_BUSY_LOOP is reported in the output.
- */
- if ((events & POLL_BUSY_LOOP) && file->f_op->poll_busy_loop)
- file->f_op->poll_busy_loop(file, events);
-
if (file->f_op->poll) {
return file->f_op->poll(file, pt);
} else if (file->f_poll_head) {
diff --git a/include/linux/fs.h b/include/linux/fs.h
index 82133bd1a047..bfaebdc03878 100644
--- a/include/linux/fs.h
+++ b/include/linux/fs.h
@@ -1722,7 +1722,6 @@ struct file_operations {
int (*iterate_shared) (struct file *, struct dir_context *);
__poll_t (*poll) (struct file *, struct poll_table_struct *);
__poll_t (*poll_mask) (struct file *, __poll_t);
- void (*poll_busy_loop)(struct file *file, __poll_t events);
long (*unlocked_ioctl) (struct file *, unsigned int, unsigned long);
long (*compat_ioctl) (struct file *, unsigned int, unsigned long);
int (*mmap) (struct file *, struct vm_area_struct *);
diff --git a/net/socket.c b/net/socket.c
index b52e5b900e09..0aaa49190b30 100644
--- a/net/socket.c
+++ b/net/socket.c
@@ -131,19 +131,6 @@ static ssize_t sock_splice_read(struct file *file, loff_t *ppos,
struct pipe_inode_info *pipe, size_t len,
unsigned int flags);

-#ifdef CONFIG_NET_RX_BUSY_POLL
-static void sock_poll_busy_loop(struct file *file, __poll_t events)
-{
- struct socket *sock = file->private_data;
-
- /* once, only if requested by syscall */
- if (sk_can_busy_loop(sock->sk))
- sk_busy_loop(sock->sk, 1);
-}
-#else
-#define sock_poll_busy_loop NULL
-#endif
-
/*
* Socket files have a set of 'special' operations as well as the generic file ones. These don't appear
* in the operation structures but are done directly via the socketcall() multiplexor.
@@ -155,7 +142,6 @@ static const struct file_operations socket_file_ops = {
.read_iter = sock_read_iter,
.write_iter = sock_write_iter,
.poll_mask = sock_poll_mask,
- .poll_busy_loop = sock_poll_busy_loop,
.poll = sock_poll,
.unlocked_ioctl = sock_ioctl,
#ifdef CONFIG_COMPAT
@@ -1163,6 +1149,12 @@ static __poll_t sock_poll(struct file *file, poll_table *wait)
struct socket *sock = file->private_data;
__poll_t events = poll_requested_events(wait), mask = 0;

+ /*
+ * Poll once, if requested by syscall.
+ */
+ if ((events & POLL_BUSY_LOOP) && sk_can_busy_loop(sock->sk))
+ sk_busy_loop(sock->sk, 1);
+
if (sock->ops->poll) {
mask = sock->ops->poll(file, sock, wait);
} else if (sock->ops->poll_mask) {
--
2.17.1