ceph: request expedited service on session's last cap flush

When flushing a lot of caps to the MDS's at once (e.g. for syncfs),
we can end up waiting a substantial amount of time for MDS replies, due
to the fact that it may delay some of them so that it can batch them up
together in a single journal transaction. This can lead to stalls when
calling sync or syncfs.

What we'd really like to do is request expedited service on the _last_
cap we're flushing back to the server. If the CHECK_CAPS_FLUSH flag is
set on the request and the current inode was the last one on the
session->s_cap_dirty list, then mark the request with
CEPH_CLIENT_CAPS_SYNC.

Note that this heuristic is not perfect. New inodes can race onto the
list after we've started flushing, but it does seem to fix some common
use cases.

URL: https://tracker.ceph.com/issues/44744Reported-by: Jan Fajerski <jfajerski@suse.com>
Signed-off-by: Jeff Layton <jlayton@kernel.org>
Reviewed-by: "Yan, Zheng" <zyan@redhat.com>
Signed-off-by: Ilya Dryomov <idryomov@gmail.com>

fs/ceph/caps.c

@@ -1997,6 +1997,7 @@ void ceph_check_caps(struct ceph_inode_info *ci, int flags,
 	}
 
 	for (p = rb_first(&ci->i_caps); p; p = rb_next(p)) {
+		int mflags = 0;
 		struct cap_msg_args arg;
 
 		cap = rb_entry(p, struct ceph_cap, ci_node);
@@ -2128,6 +2129,9 @@ void ceph_check_caps(struct ceph_inode_info *ci, int flags,
 			flushing = ci->i_dirty_caps;
 			flush_tid = __mark_caps_flushing(inode, session, false,
 							 &oldest_flush_tid);
+			if (flags & CHECK_CAPS_FLUSH &&
+			    list_empty(&session->s_cap_dirty))
+				mflags |= CEPH_CLIENT_CAPS_SYNC;
 		} else {
 			flushing = 0;
 			flush_tid = 0;
@@ -2138,8 +2142,8 @@ void ceph_check_caps(struct ceph_inode_info *ci, int flags,
 
 		mds = cap->mds;  /* remember mds, so we don't repeat */
 
-		__prep_cap(&arg, cap, CEPH_CAP_OP_UPDATE, 0, cap_used, want,
-			   retain, flushing, flush_tid, oldest_flush_tid);
+		__prep_cap(&arg, cap, CEPH_CAP_OP_UPDATE, mflags, cap_used,
+			   want, retain, flushing, flush_tid, oldest_flush_tid);
 		spin_unlock(&ci->i_ceph_lock);
 
 		__send_cap(mdsc, &arg, ci);