On Thu, Apr 02, 2020 at 07:29:11AM -0400, Jeff Layton wrote:
> 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.
>
> Reported-by: Jan Fajerski <jfajerski@xxxxxxxx>
> Signed-off-by: Jeff Layton <jlayton@xxxxxxxxxx>
> ---
> fs/ceph/caps.c | 8 ++++++--
> 1 file changed, 6 insertions(+), 2 deletions(-)
>
> diff --git a/fs/ceph/caps.c b/fs/ceph/caps.c
> index 95c9b25e45a6..3630f05993b3 100644
> --- a/fs/ceph/caps.c
> +++ b/fs/ceph/caps.c
> @@ -1987,6 +1987,7 @@ 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);
> @@ -2118,6 +2119,9 @@ 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;
> @@ -2128,8 +2132,8 @@ 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);
> --
> 2.25.1
>
FWIW, it looks good to me. I've also tested it and I confirm it improves
(a lot!) the testcase reported by Jan. Maybe it's worth adding the URL to
the tracker.
Cheers,
--
Luis
