Re: [PATCH 43/45] xfs: avoid cil push lock if possible

Darrick J. Wong
2021-03-11T01:48:07Z

On Fri, Mar 05, 2021 at 04:11:41PM +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
>
> Because now it hurts when the CIL fills up.
>
> - 37.20% __xfs_trans_commit
> - 35.84% xfs_log_commit_cil
> - 19.34% _raw_spin_lock
> - do_raw_spin_lock
> 19.01% __pv_queued_spin_lock_slowpath
> - 4.20% xfs_log_ticket_ungrant
> 0.90% xfs_log_space_wake
>
>
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---
> fs/xfs/xfs_log_cil.c | 14 +++++++++++---
> 1 file changed, 11 insertions(+), 3 deletions(-)
>
> diff --git a/fs/xfs/xfs_log_cil.c b/fs/xfs/xfs_log_cil.c
> index 6dcc23829bef..d60c72ad391a 100644
> --- a/fs/xfs/xfs_log_cil.c
> +++ b/fs/xfs/xfs_log_cil.c
> @@ -1115,10 +1115,18 @@ xlog_cil_push_background(
> ASSERT(!test_bit(XLOG_CIL_EMPTY, &cil->xc_flags));
>
> /*
> - * Don't do a background push if we haven't used up all the
> - * space available yet.
> + * We are done if:
> + * - we haven't used up all the space available yet; or
> + * - we've already queued up a push; and
> + * - we're not over the hard limit; and
> + * - nothing has been over the hard limit.

Er... do these last three bullet points correspond to the last three
lines of the if test? I'm not sure how !waitqueue_active() determines
that nothing has been over the hard limit? Or for that matter how
comparing push_seq against current_seq tells us if we've queued a
push?

--D

> + *
> + * If so, we don't need to take the push lock as there's nothing to do.
> */
> - if (space_used < XLOG_CIL_SPACE_LIMIT(log)) {
> + if (space_used < XLOG_CIL_SPACE_LIMIT(log) ||
> + (cil->xc_push_seq == cil->xc_current_sequence &&
> + space_used < XLOG_CIL_BLOCKING_SPACE_LIMIT(log) &&
> + !waitqueue_active(&cil->xc_push_wait))) {
> up_read(&cil->xc_ctx_lock);
> return;
> }
> --
> 2.28.0
>