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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/44744
Reported-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>
This commit is contained in:
Jeff Layton 2020-04-01 18:27:25 -04:00 committed by Ilya Dryomov
parent 1cf03a68e7
commit d67c72e6cc
1 changed files with 6 additions and 2 deletions
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8
fs/ceph/caps.c
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@ -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);