[PATCH 44/45] xfs: xlog_sync() manually adjusts grant head space

Dave Chinner
2021-03-05T05:12:58Z

From: Dave Chinner <dchinner@redhat.com>

When xlog_sync() rounds off the tail the iclog that is being
flushed, it manually subtracts that space from the grant heads. This
space is actually reserved by the transaction ticket that covers
the xlog_sync() call from xlog_write(), but we don't plumb the
ticket down far enough for it to account for the space consumed in
the current log ticket.

The grant heads are hot, so we really should be accounting this to
the ticket is we can, rather than adding thousands of extra grant
head updates every CIL commit.

Interestingly, this actually indicates a potential log space overrun
can occur when we force the log. By the time that xfs_log_force()
pushes out an active iclog and consumes the roundoff space, the
reservation for that roundoff space has been returned to the grant
heads and is no longer covered by a reservation. In theory the
roundoff added to log force on an already full log could push the
write head past the tail. In practice, the CIL commit that writes to
the log and needs the iclog pushed will have reserved space for
roundoff, so when it releases the ticket there will still be
physical space for the roundoff to be committed to the log, even
though it is no longer reserved. This roundoff won't be enough space
to allow a transaction to be woken if the log is full, so overruns
should not actually occur in practice.

That said, it indicates that we should not release the CIL context
log ticket until after we've released the commit iclog. It also
means that xlog_sync() still needs the direct grant head
manipulation if we don't provide it with a ticket. Log forces are
rare when we are in fast paths running 1.5 million transactions/s
that make the grant heads hot, so let's optimise the hot case and
pass CIL log tickets down to the xlog_sync() code.

Signed-off-by: Dave Chinner <dchinner@redhat.com>
---
fs/xfs/xfs_log.c | 39 +++++++++++++++++++++++++--------------
fs/xfs/xfs_log_cil.c | 19 ++++++++++++++-----
fs/xfs/xfs_log_priv.h | 3 ++-
3 files changed, 41 insertions(+), 20 deletions(-)

diff --git a/fs/xfs/xfs_log.c b/fs/xfs/xfs_log.c
index fd58c3213ebf..1c7d522b12cd 100644
--- a/fs/xfs/xfs_log.c
+++ b/fs/xfs/xfs_log.c
@@ -55,7 +55,8 @@ xlog_grant_push_ail(
STATIC void
xlog_sync(
struct xlog *log,
- struct xlog_in_core *iclog);
+ struct xlog_in_core *iclog,
+ struct xlog_ticket *ticket);
#if defined(DEBUG)
STATIC void
xlog_verify_dest_ptr(
@@ -535,7 +536,8 @@ __xlog_state_release_iclog(
int
xlog_state_release_iclog(
struct xlog *log,
- struct xlog_in_core *iclog)
+ struct xlog_in_core *iclog,
+ struct xlog_ticket *ticket)
{
lockdep_assert_held(&log->l_icloglock);

@@ -545,7 +547,7 @@ xlog_state_release_iclog(
if (atomic_dec_and_test(&iclog->ic_refcnt) &&
__xlog_state_release_iclog(log, iclog)) {
spin_unlock(&log->l_icloglock);
- xlog_sync(log, iclog);
+ xlog_sync(log, iclog, ticket);
spin_lock(&log->l_icloglock);
}

@@ -898,7 +900,7 @@ xlog_unmount_write(
else
ASSERT(iclog->ic_state == XLOG_STATE_WANT_SYNC ||
iclog->ic_state == XLOG_STATE_IOERROR);
- error = xlog_state_release_iclog(log, iclog);
+ error = xlog_state_release_iclog(log, iclog, tic);
xlog_wait_on_iclog(iclog);

if (tic) {
@@ -1930,7 +1932,8 @@ xlog_calc_iclog_size(
STATIC void
xlog_sync(
struct xlog *log,
- struct xlog_in_core *iclog)
+ struct xlog_in_core *iclog,
+ struct xlog_ticket *ticket)
{
unsigned int count; /* byte count of bwrite */
unsigned int roundoff; /* roundoff to BB or stripe */
@@ -1941,12 +1944,20 @@ xlog_sync(

count = xlog_calc_iclog_size(log, iclog, &roundoff);

- /* move grant heads by roundoff in sync */
- xlog_grant_add_space(log, &log->l_reserve_head.grant, roundoff);
- xlog_grant_add_space(log, &log->l_write_head.grant, roundoff);
+ /*
+ * If we have a ticket, account for the roundoff via the ticket
+ * reservation to avoid touching the hot grant heads needlessly.
+ * Otherwise, we have to move grant heads directly.
+ */
+ if (ticket) {
+ ticket->t_curr_res -= roundoff;
+ } else {
+ xlog_grant_add_space(log, &log->l_reserve_head.grant, roundoff);
+ xlog_grant_add_space(log, &log->l_write_head.grant, roundoff);
+ }

/* put cycle number in every block */
- xlog_pack_data(log, iclog, roundoff);
+ xlog_pack_data(log, iclog, roundoff);

/* real byte length */
size = iclog->ic_offset;
@@ -2187,7 +2198,7 @@ xlog_write_get_more_iclog_space(
xlog_state_finish_copy(log, iclog, *record_cnt, *data_cnt);
ASSERT(iclog->ic_state == XLOG_STATE_WANT_SYNC ||
iclog->ic_state == XLOG_STATE_IOERROR);
- error = xlog_state_release_iclog(log, iclog);
+ error = xlog_state_release_iclog(log, iclog, ticket);
spin_unlock(&log->l_icloglock);
if (error)
return error;
@@ -2470,7 +2481,7 @@ xlog_write(
ASSERT(optype & XLOG_COMMIT_TRANS);
*commit_iclog = iclog;
} else {
- error = xlog_state_release_iclog(log, iclog);
+ error = xlog_state_release_iclog(log, iclog, ticket);
}
spin_unlock(&log->l_icloglock);

@@ -2929,7 +2940,7 @@ xlog_state_get_iclog_space(
* reference to the iclog.
*/
if (!atomic_add_unless(&iclog->ic_refcnt, -1, 1))
- error = xlog_state_release_iclog(log, iclog);
+ error = xlog_state_release_iclog(log, iclog, ticket);
spin_unlock(&log->l_icloglock);
if (error)
return error;
@@ -3157,7 +3168,7 @@ xfs_log_force(
atomic_inc(&iclog->ic_refcnt);
lsn = be64_to_cpu(iclog->ic_header.h_lsn);
xlog_state_switch_iclogs(log, iclog, 0);
- if (xlog_state_release_iclog(log, iclog))
+ if (xlog_state_release_iclog(log, iclog, NULL))
goto out_error;

if (be64_to_cpu(iclog->ic_header.h_lsn) != lsn)
@@ -3250,7 +3261,7 @@ xlog_force_lsn(
}
atomic_inc(&iclog->ic_refcnt);
xlog_state_switch_iclogs(log, iclog, 0);
- if (xlog_state_release_iclog(log, iclog))
+ if (xlog_state_release_iclog(log, iclog, NULL))
goto out_error;
if (log_flushed)
*log_flushed = 1;
diff --git a/fs/xfs/xfs_log_cil.c b/fs/xfs/xfs_log_cil.c
index d60c72ad391a..aef60f19ab05 100644
--- a/fs/xfs/xfs_log_cil.c
+++ b/fs/xfs/xfs_log_cil.c
@@ -804,6 +804,7 @@ xlog_cil_push_work(
int cpu;
struct xlog_cil_pcp *cilpcp;
LIST_HEAD (log_items);
+ struct xlog_ticket *ticket;

new_ctx = xlog_cil_ctx_alloc();
new_ctx->ticket = xlog_cil_ticket_alloc(log);
@@ -1037,12 +1038,10 @@ xlog_cil_push_work(
if (error)
goto out_abort_free_ticket;

- xfs_log_ticket_ungrant(log, ctx->ticket);
-
spin_lock(&commit_iclog->ic_callback_lock);
if (commit_iclog->ic_state == XLOG_STATE_IOERROR) {
spin_unlock(&commit_iclog->ic_callback_lock);
- goto out_abort;
+ goto out_abort_free_ticket;
}
ASSERT_ALWAYS(commit_iclog->ic_state == XLOG_STATE_ACTIVE ||
commit_iclog->ic_state == XLOG_STATE_WANT_SYNC);
@@ -1073,12 +1072,23 @@ xlog_cil_push_work(
commit_iclog->ic_flags &= ~XLOG_ICL_NEED_FLUSH;
}

+ /*
+ * Pull the ticket off the ctx so we can ungrant it after releasing the
+ * commit_iclog. The ctx may be freed by the time we return from
+ * releasing the commit_iclog (i.e. checkpoint has been completed and
+ * callback run) so we can't reference the ctx after the call to
+ * xlog_state_release_iclog().
+ */
+ ticket = ctx->ticket;
+
/* release the hounds! */
spin_lock(&log->l_icloglock);
if (commit_iclog_sync && commit_iclog->ic_state == XLOG_STATE_ACTIVE)
xlog_state_switch_iclogs(log, commit_iclog, 0);
- xlog_state_release_iclog(log, commit_iclog);
+ xlog_state_release_iclog(log, commit_iclog, ticket);
spin_unlock(&log->l_icloglock);
+
+ xfs_log_ticket_ungrant(log, ticket);
return;

out_skip:
@@ -1089,7 +1099,6 @@ xlog_cil_push_work(

out_abort_free_ticket:
xfs_log_ticket_ungrant(log, ctx->ticket);
-out_abort:
ASSERT(XLOG_FORCED_SHUTDOWN(log));
xlog_cil_committed(ctx);
}
diff --git a/fs/xfs/xfs_log_priv.h b/fs/xfs/xfs_log_priv.h
index 6a4160200417..3d43d3940757 100644
--- a/fs/xfs/xfs_log_priv.h
+++ b/fs/xfs/xfs_log_priv.h
@@ -487,7 +487,8 @@ int xlog_commit_record(struct xlog *log, struct xlog_ticket *ticket,
struct xlog_in_core **iclog, xfs_lsn_t *lsn);
void xlog_state_switch_iclogs(struct xlog *log, struct xlog_in_core *iclog,
int eventual_size);
-int xlog_state_release_iclog(struct xlog *xlog, struct xlog_in_core *iclog);
+int xlog_state_release_iclog(struct xlog *xlog, struct xlog_in_core *iclog,
+ struct xlog_ticket *ticket);

void xfs_log_ticket_ungrant(struct xlog *log, struct xlog_ticket *ticket);
void xfs_log_ticket_regrant(struct xlog *log, struct xlog_ticket *ticket);
--
2.28.0