Re: [PATCH 40/45] xfs: convert CIL to unordered per cpu lists

Darrick J. Wong
2021-03-11T01:15:43Z

On Fri, Mar 05, 2021 at 04:11:38PM +1100, Dave Chinner wrote:
> From: Dave Chinner <dchinner@redhat.com>
>
> So that we can remove the cil_lock which is a global serialisation
> point. We've already got ordering sorted, so all we need to do is
> treat the CIL list like the busy extent list and reconstruct it
> before the push starts.
>
> This is what we're trying to avoid:
>
> - 75.35% 1.83% [kernel] [k] xfs_log_commit_cil
> - 46.35% xfs_log_commit_cil
> - 41.54% _raw_spin_lock
> - 67.30% do_raw_spin_lock
> 66.96% __pv_queued_spin_lock_slowpath
>
> Which happens on a 32p system when running a 32-way 'rm -rf'
> workload. After this patch:
>
> - 20.90% 3.23% [kernel] [k] xfs_log_commit_cil
> - 17.67% xfs_log_commit_cil
> - 6.51% xfs_log_ticket_ungrant
> 1.40% xfs_log_space_wake
> 2.32% memcpy_erms
> - 2.18% xfs_buf_item_committing
> - 2.12% xfs_buf_item_release
> - 1.03% xfs_buf_unlock
> 0.96% up
> 0.72% xfs_buf_rele
> 1.33% xfs_inode_item_format
> 1.19% down_read
> 0.91% up_read
> 0.76% xfs_buf_item_format
> - 0.68% kmem_alloc_large
> - 0.67% kmem_alloc
> 0.64% __kmalloc
> 0.50% xfs_buf_item_size
>
> It kinda looks like the workload is running out of log space all
> the time. But all the spinlock contention is gone and the
> transaction commit rate has gone from 800k/s to 1.3M/s so the amount
> of real work being done has gone up a *lot*.
>
> Signed-off-by: Dave Chinner <dchinner@redhat.com>
> ---
> fs/xfs/xfs_log_cil.c | 61 ++++++++++++++++++++-----------------------
> fs/xfs/xfs_log_priv.h | 2 --
> 2 files changed, 29 insertions(+), 34 deletions(-)
>
> diff --git a/fs/xfs/xfs_log_cil.c b/fs/xfs/xfs_log_cil.c
> index 7420389f4cee..3d43a5088154 100644
> --- a/fs/xfs/xfs_log_cil.c
> +++ b/fs/xfs/xfs_log_cil.c
> @@ -448,10 +448,9 @@ xlog_cil_insert_items(
> /*
> * 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. We don't need to
> - * hold the xc_cil_lock here to clear the XLOG_CIL_EMPTY bit as we are
> - * under the xc_ctx_lock here and that needs to be held exclusively to
> - * reset the XLOG_CIL_EMPTY bit.
> + * unnecessarily do an atomic op in the fast path here. We can clear the
> + * XLOG_CIL_EMPTY bit as we are under the xc_ctx_lock here and that
> + * needs to be held exclusively to reset the XLOG_CIL_EMPTY bit.
> */
> if (test_bit(XLOG_CIL_EMPTY, &cil->xc_flags) &&
> test_and_clear_bit(XLOG_CIL_EMPTY, &cil->xc_flags))
> @@ -505,24 +504,6 @@ xlog_cil_insert_items(
> /* attach the transaction to the CIL if it has any busy extents */
> if (!list_empty(&tp->t_busy))
> list_splice_init(&tp->t_busy, &cilpcp->busy_extents);
> - put_cpu_ptr(cilpcp);
> -
> - /*
> - * If we've overrun the reservation, dump the tx details before we move
> - * the log items. Shutdown is imminent...
> - */
> - tp->t_ticket->t_curr_res -= ctx_res + len;
> - if (WARN_ON(tp->t_ticket->t_curr_res < 0)) {
> - xfs_warn(log->l_mp, "Transaction log reservation overrun:");
> - xfs_warn(log->l_mp,
> - " log items: %d bytes (iov hdrs: %d bytes)",
> - len, iovhdr_res);
> - xfs_warn(log->l_mp, " split region headers: %d bytes",
> - split_res);
> - xfs_warn(log->l_mp, " ctx ticket: %d bytes", ctx_res);
> - xlog_print_trans(tp);
> - }
> -
> /*
> * Now update the order of everything modified in the transaction
> * and insert items into the CIL if they aren't already there.
> @@ -530,7 +511,6 @@ xlog_cil_insert_items(
> * the transaction commit.
> */
> order = atomic_inc_return(&ctx->order_id);
> - spin_lock(&cil->xc_cil_lock);
> list_for_each_entry(lip, &tp->t_items, li_trans) {
>
> /* Skip items which aren't dirty in this transaction. */
> @@ -540,10 +520,26 @@ xlog_cil_insert_items(
> lip->li_order_id = order;
> if (!list_empty(&lip->li_cil))
> continue;
> - list_add(&lip->li_cil, &cil->xc_cil);
> + list_add(&lip->li_cil, &cilpcp->log_items);

Ok, so if I understand this correctly -- every time a transaction
commits, it marks every dirty log item with a monotonically increasing
counter. If the log item isn't already on another CPU's CIL list, it
gets added to the current CPU's CIL list...

> + }
> + put_cpu_ptr(cilpcp);
> +
> + /*
> + * If we've overrun the reservation, dump the tx details before we move
> + * the log items. Shutdown is imminent...
> + */
> + tp->t_ticket->t_curr_res -= ctx_res + len;
> + if (WARN_ON(tp->t_ticket->t_curr_res < 0)) {
> + xfs_warn(log->l_mp, "Transaction log reservation overrun:");
> + xfs_warn(log->l_mp,
> + " log items: %d bytes (iov hdrs: %d bytes)",
> + len, iovhdr_res);
> + xfs_warn(log->l_mp, " split region headers: %d bytes",
> + split_res);
> + xfs_warn(log->l_mp, " ctx ticket: %d bytes", ctx_res);
> + xlog_print_trans(tp);
> }
>
> - spin_unlock(&cil->xc_cil_lock);
>
> if (tp->t_ticket->t_curr_res < 0)
> xfs_force_shutdown(log->l_mp, SHUTDOWN_LOG_IO_ERROR);
> @@ -806,6 +802,7 @@ xlog_cil_push_work(
> bool commit_iclog_sync = false;
> int cpu;
> struct xlog_cil_pcp *cilpcp;
> + LIST_HEAD (log_items);
>
> new_ctx = xlog_cil_ctx_alloc();
> new_ctx->ticket = xlog_cil_ticket_alloc(log);
> @@ -822,6 +819,9 @@ xlog_cil_push_work(
> list_splice_init(&cilpcp->busy_extents,
> &ctx->busy_extents);
> }
> + if (!list_empty(&cilpcp->log_items)) {
> + list_splice_init(&cilpcp->log_items, &log_items);

...and then at CIL push time, we splice each per-CPU list into a big
list, sort the dirty log items by counter number, and process them.

The first thought I had was that it's a darn shame that _insert_items
can't steal a log item from another CPU's CIL list, because you could
then mergesort the per-CPU CIL lists into @log_items. Unfortunately, I
don't think there's a safe way to steal items from a per-CPU list
without involving locks.

The second thought I had was that we have the xfs_pwork mechanism for
launching a bunch of worker threads. A pwork workqueue is (probably)
too costly when the item list is short or there aren't that many CPUs,
but once list_sort starts getting painful, would it be faster to launch
a bunch of threads in push_work to sort each per-CPU list and then merge
sort them into the final list?

FWIW at least mechanically, the last two patches look reasonable to me.

--D

> + }
> }
>
> spin_lock(&cil->xc_push_lock);
> @@ -907,12 +907,12 @@ xlog_cil_push_work(
> * needed on the transaction commit side which is currently locked out
> * by the flush lock.
> */
> - list_sort(NULL, &cil->xc_cil, xlog_cil_order_cmp);
> + list_sort(NULL, &log_items, xlog_cil_order_cmp);
> lv = NULL;
> - while (!list_empty(&cil->xc_cil)) {
> + while (!list_empty(&log_items)) {
> struct xfs_log_item *item;
>
> - item = list_first_entry(&cil->xc_cil,
> + item = list_first_entry(&log_items,
> struct xfs_log_item, li_cil);
> list_del_init(&item->li_cil);
> item->li_order_id = 0;
> @@ -1099,7 +1099,6 @@ 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.
> */
> - ASSERT(!list_empty(&cil->xc_cil));
> ASSERT(!test_bit(XLOG_CIL_EMPTY, &cil->xc_flags));
>
> /*
> @@ -1491,6 +1490,7 @@ xlog_cil_pcp_alloc(
> for_each_possible_cpu(cpu) {
> cilpcp = per_cpu_ptr(pcptr, cpu);
> INIT_LIST_HEAD(&cilpcp->busy_extents);
> + INIT_LIST_HEAD(&cilpcp->log_items);
> }
>
> if (xlog_cil_pcp_hpadd(cil) < 0) {
> @@ -1531,9 +1531,7 @@ xlog_cil_init(
> return -ENOMEM;
> }
>
> - INIT_LIST_HEAD(&cil->xc_cil);
> INIT_LIST_HEAD(&cil->xc_committing);
> - spin_lock_init(&cil->xc_cil_lock);
> spin_lock_init(&cil->xc_push_lock);
> init_waitqueue_head(&cil->xc_push_wait);
> init_rwsem(&cil->xc_ctx_lock);
> @@ -1559,7 +1557,6 @@ xlog_cil_destroy(
> kmem_free(cil->xc_ctx);
> }
>
> - ASSERT(list_empty(&cil->xc_cil));
> ASSERT(test_bit(XLOG_CIL_EMPTY, &cil->xc_flags));
> xlog_cil_pcp_free(cil, cil->xc_pcp);
> kmem_free(cil);
> diff --git a/fs/xfs/xfs_log_priv.h b/fs/xfs/xfs_log_priv.h
> index 92d9e1a03a07..12a1a36eef7e 100644
> --- a/fs/xfs/xfs_log_priv.h
> +++ b/fs/xfs/xfs_log_priv.h
> @@ -262,8 +262,6 @@ struct xfs_cil {
> struct xlog *xc_log;
> unsigned long xc_flags;
> atomic_t xc_iclog_hdrs;
> - struct list_head xc_cil;
> - spinlock_t xc_cil_lock;
>
> struct rw_semaphore xc_ctx_lock ____cacheline_aligned_in_smp;
> struct xfs_cil_ctx *xc_ctx;
> --
> 2.28.0
>