Re: [PATCH 32/45] xfs: use the CIL space used counter for emptiness checks

Darrick J. Wong
2021-03-10T23:01:51Z

On Fri, Mar 05, 2021 at 04:11:30PM +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
>
> In the next patches we are going to make the CIL list itself
> per-cpu, and so we cannot use list_empty() to check is the list is
> empty. Replace the list_empty() checks with a flag in the CIL to
> indicate we have committed at least one transaction to the CIL and
> hence the CIL is not empty.
>
> We need this flag to be an atomic so that we can clear it without
> holding any locks in the commit fast path, but we also need to be
> careful to avoid atomic operations in the fast path. Hence we use
> the fact that test_bit() is not an atomic op to first check if the
> flag is set and then run the atomic test_and_clear_bit() operation
> to clear it and steal the initial unit reservation for the CIL
> context checkpoint.
>
> When we are switching to a new context in a push, we place the
> setting of the XLOG_CIL_EMPTY flag under the xc_push_lock. THis
> allows all the other places that need to check whether the CIL is
> empty to use test_bit() and still be serialised correctly with the
> CIL context swaps that set the bit.
>
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---
> fs/xfs/xfs_log_cil.c | 49 +++++++++++++++++++++++--------------------
> fs/xfs/xfs_log_priv.h | 4 ++++
> 2 files changed, 30 insertions(+), 23 deletions(-)
>
> diff --git a/fs/xfs/xfs_log_cil.c b/fs/xfs/xfs_log_cil.c
> index 4047f95a0fc4..e6e36488f0c7 100644
> --- a/fs/xfs/xfs_log_cil.c
> +++ b/fs/xfs/xfs_log_cil.c
> @@ -70,6 +70,7 @@ xlog_cil_ctx_switch(
> struct xfs_cil *cil,
> struct xfs_cil_ctx *ctx)
> {
> + set_bit(XLOG_CIL_EMPTY, &cil->xc_flags);
> ctx->sequence = ++cil->xc_current_sequence;
> ctx->cil = cil;
> cil->xc_ctx = ctx;
> @@ -436,13 +437,12 @@ xlog_cil_insert_items(
> list_splice_init(&tp->t_busy, &ctx->busy_extents);
>
> /*
> - * Now transfer enough transaction reservation to the context ticket
> - * for the checkpoint. The context ticket is special - the unit
> - * reservation has to grow as well as the current reservation as we
> - * steal from tickets so we can correctly determine the space used
> - * during the transaction commit.
> + * We need to take the CIL checkpoint unit reservation on the first
> + * commit into the CIL. Test the XLOG_CIL_EMPTY bit first so we don't
> + * unnecessarily do an atomic op in the fast path here.
> */
> - if (ctx->ticket->t_curr_res == 0) {
> + if (test_bit(XLOG_CIL_EMPTY, &cil->xc_flags) &&
> + test_and_clear_bit(XLOG_CIL_EMPTY, &cil->xc_flags)) {

Hm, it'll be amusing to see where this goes. Usually I tell myself in
siutations like these "I think this is ok, let's see where we are in
another 4-5 patches" but now I'm 7 patches out and my brain is getting
close to ENOSPC so I'll tentatively say:

Reviewed-by: Darrick J. Wong <djwong@kernel.org>

With the caveat that I could have more to say after the fact...

--D

> ctx_res = ctx->ticket->t_unit_res;
> ctx->ticket->t_curr_res = ctx_res;
> tp->t_ticket->t_curr_res -= ctx_res;
> @@ -771,7 +771,7 @@ xlog_cil_push_work(
> * move on to a new sequence number and so we have to be able to push
> * this sequence again later.
> */
> - if (list_empty(&cil->xc_cil)) {
> + if (test_bit(XLOG_CIL_EMPTY, &cil->xc_flags)) {
> cil->xc_push_seq = 0;
> spin_unlock(&cil->xc_push_lock);
> goto out_skip;
> @@ -1019,9 +1019,10 @@ xlog_cil_push_background(
>
> /*
> * The cil won't be empty because we are called while holding the
> - * context lock so whatever we added to the CIL will still be there
> + * context lock so whatever we added to the CIL will still be there.
> */
> ASSERT(!list_empty(&cil->xc_cil));
> + ASSERT(!test_bit(XLOG_CIL_EMPTY, &cil->xc_flags));
>
> /*
> * Don't do a background push if we haven't used up all the
> @@ -1108,7 +1109,8 @@ xlog_cil_push_now(
> * there's no work we need to do.
> */
> spin_lock(&cil->xc_push_lock);
> - if (list_empty(&cil->xc_cil) || push_seq <= cil->xc_push_seq) {
> + if (test_bit(XLOG_CIL_EMPTY, &cil->xc_flags) ||
> + push_seq <= cil->xc_push_seq) {
> spin_unlock(&cil->xc_push_lock);
> return;
> }
> @@ -1128,7 +1130,7 @@ xlog_cil_empty(
> bool empty = false;
>
> spin_lock(&cil->xc_push_lock);
> - if (list_empty(&cil->xc_cil))
> + if (test_bit(XLOG_CIL_EMPTY, &cil->xc_flags))
> empty = true;
> spin_unlock(&cil->xc_push_lock);
> return empty;
> @@ -1289,7 +1291,7 @@ xlog_cil_force_seq(
> * we would have found the context on the committing list.
> */
> if (sequence == cil->xc_current_sequence &&
> - !list_empty(&cil->xc_cil)) {
> + !test_bit(XLOG_CIL_EMPTY, &cil->xc_flags)) {
> spin_unlock(&cil->xc_push_lock);
> goto restart;
> }
> @@ -1320,21 +1322,19 @@ xlog_cil_force_seq(
> */
> bool
> xfs_log_item_in_current_chkpt(
> - struct xfs_log_item *lip)
> + struct xfs_log_item *lip)
> {
> - struct xfs_cil_ctx *ctx;
> + struct xfs_cil *cil = lip->li_mountp->m_log->l_cilp;
>
> - if (list_empty(&lip->li_cil))
> + if (test_bit(XLOG_CIL_EMPTY, &cil->xc_flags))
> return false;
>
> - ctx = lip->li_mountp->m_log->l_cilp->xc_ctx;
> -
> /*
> * li_seq is written on the first commit of a log item to record the
> * first checkpoint it is written to. Hence if it is different to the
> * current sequence, we're in a new checkpoint.
> */
> - if (XFS_LSN_CMP(lip->li_seq, ctx->sequence) != 0)
> + if (XFS_LSN_CMP(lip->li_seq, cil->xc_ctx->sequence) != 0)
> return false;
> return true;
> }
> @@ -1373,13 +1373,16 @@ void
> xlog_cil_destroy(
> struct xlog *log)
> {
> - if (log->l_cilp->xc_ctx) {
> - if (log->l_cilp->xc_ctx->ticket)
> - xfs_log_ticket_put(log->l_cilp->xc_ctx->ticket);
> - kmem_free(log->l_cilp->xc_ctx);
> + struct xfs_cil *cil = log->l_cilp;
> +
> + if (cil->xc_ctx) {
> + if (cil->xc_ctx->ticket)
> + xfs_log_ticket_put(cil->xc_ctx->ticket);
> + kmem_free(cil->xc_ctx);
> }
>
> - ASSERT(list_empty(&log->l_cilp->xc_cil));
> - kmem_free(log->l_cilp);
> + ASSERT(list_empty(&cil->xc_cil));
> + ASSERT(test_bit(XLOG_CIL_EMPTY, &cil->xc_flags));
> + kmem_free(cil);
> }
>
> diff --git a/fs/xfs/xfs_log_priv.h b/fs/xfs/xfs_log_priv.h
> index 003c11653955..b0dc3bc9de59 100644
> --- a/fs/xfs/xfs_log_priv.h
> +++ b/fs/xfs/xfs_log_priv.h
> @@ -248,6 +248,7 @@ struct xfs_cil_ctx {
> */
> struct xfs_cil {
> struct xlog *xc_log;
> + unsigned long xc_flags;
> struct list_head xc_cil;
> spinlock_t xc_cil_lock;
>
> @@ -263,6 +264,9 @@ struct xfs_cil {
> wait_queue_head_t xc_push_wait; /* background push throttle */
> } ____cacheline_aligned_in_smp;
>
> +/* xc_flags bit values */
> +#define XLOG_CIL_EMPTY 1
> +
> /*
> * The amount of log space we allow the CIL to aggregate is difficult to size.
> * Whatever we choose, we have to make sure we can get a reservation for the
> --
> 2.28.0
>