On Mon, Jun 27, 2022 at 04:08:41PM +1000, Dave Chinner wrote:
> From: Dave Chinner <dchinner@xxxxxxxxxx>
>
> Now that we have a standalone fast path for buffer lookup, we can
> easily convert it to use rcu lookups. When we continually hammer the
> buffer cache with trylock lookups, we end up with a huge amount of
> lock contention on the per-ag buffer hash locks:
>
> - 92.71% 0.05% [kernel] [k] xfs_inodegc_worker
> - 92.67% xfs_inodegc_worker
> - 92.13% xfs_inode_unlink
> - 91.52% xfs_inactive_ifree
> - 85.63% xfs_read_agi
> - 85.61% xfs_trans_read_buf_map
> - 85.59% xfs_buf_read_map
> - xfs_buf_get_map
> - 85.55% xfs_buf_find
> - 72.87% _raw_spin_lock
> - do_raw_spin_lock
> 71.86% __pv_queued_spin_lock_slowpath
> - 8.74% xfs_buf_rele
> - 7.88% _raw_spin_lock
> - 7.88% do_raw_spin_lock
> 7.63% __pv_queued_spin_lock_slowpath
> - 1.70% xfs_buf_trylock
> - 1.68% down_trylock
> - 1.41% _raw_spin_lock_irqsave
> - 1.39% do_raw_spin_lock
> __pv_queued_spin_lock_slowpath
> - 0.76% _raw_spin_unlock
> 0.75% do_raw_spin_unlock
>
> This is basically hammering the pag->pag_buf_lock from lots of CPUs
> doing trylocks at the same time. Most of the buffer trylock
> operations ultimately fail after we've done the lookup, so we're
> really hammering the buf hash lock whilst making no progress.
>
> We can also see significant spinlock traffic on the same lock just
> under normal operation when lots of tasks are accessing metadata
> from the same AG, so let's avoid all this by converting the lookup
> fast path to leverages the rhashtable's ability to do rcu protected
> lookups.
>
> We avoid races with the buffer release path by using
> atomic_inc_not_zero() on the buffer hold count. Any buffer that is
> in the LRU will have a non-zero count, thereby allowing the lockless
> fast path to be taken in most cache hit situations. If the buffer
> hold count is zero, then it is likely going through the release path
> so in that case we fall back to the existing lookup miss slow path.
>
> The slow path will then do an atomic lookup and insert under the
> buffer hash lock and hence serialise correctly against buffer
> release freeing the buffer.
>
> The use of rcu protected lookups means that buffer handles now need
> to be freed by RCU callbacks (same as inodes). We still free the
> buffer pages before the RCU callback - we won't be trying to access
> them at all on a buffer that has zero references - but we need the
> buffer handle itself to be present for the entire rcu protected read
> side to detect a zero hold count correctly.
Hmm, so what still uses pag_buf_lock? Are we still using it to
serialize xfs_buf_rele calls?
--D
> Signed-off-by: Dave Chinner <dchinner@xxxxxxxxxx>
> ---
> fs/xfs/xfs_buf.c | 22 +++++++++++++++-------
> fs/xfs/xfs_buf.h | 1 +
> 2 files changed, 16 insertions(+), 7 deletions(-)
>
> diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c
> index 3461ef3ebc1c..14a2d2d6a4e0 100644
> --- a/fs/xfs/xfs_buf.c
> +++ b/fs/xfs/xfs_buf.c
> @@ -294,6 +294,16 @@ xfs_buf_free_pages(
> bp->b_flags &= ~_XBF_PAGES;
> }
>
> +static void
> +xfs_buf_free_callback(
> + struct callback_head *cb)
> +{
> + struct xfs_buf *bp = container_of(cb, struct xfs_buf, b_rcu);
> +
> + xfs_buf_free_maps(bp);
> + kmem_cache_free(xfs_buf_cache, bp);
> +}
> +
> static void
> xfs_buf_free(
> struct xfs_buf *bp)
> @@ -307,8 +317,7 @@ xfs_buf_free(
> else if (bp->b_flags & _XBF_KMEM)
> kmem_free(bp->b_addr);
>
> - xfs_buf_free_maps(bp);
> - kmem_cache_free(xfs_buf_cache, bp);
> + call_rcu(&bp->b_rcu, xfs_buf_free_callback);
> }
>
> static int
> @@ -567,14 +576,13 @@ xfs_buf_find_fast(
> struct xfs_buf *bp;
> int error;
>
> - spin_lock(&pag->pag_buf_lock);
> + rcu_read_lock();
> bp = rhashtable_lookup(&pag->pag_buf_hash, map, xfs_buf_hash_params);
> - if (!bp) {
> - spin_unlock(&pag->pag_buf_lock);
> + if (!bp || !atomic_inc_not_zero(&bp->b_hold)) {
> + rcu_read_unlock();
> return -ENOENT;
> }
> - atomic_inc(&bp->b_hold);
> - spin_unlock(&pag->pag_buf_lock);
> + rcu_read_unlock();
>
> error = xfs_buf_find_lock(bp, flags);
> if (error) {
> diff --git a/fs/xfs/xfs_buf.h b/fs/xfs/xfs_buf.h
> index 58e9034d51bd..02b3c1635ec3 100644
> --- a/fs/xfs/xfs_buf.h
> +++ b/fs/xfs/xfs_buf.h
> @@ -196,6 +196,7 @@ struct xfs_buf {
> int b_last_error;
>
> const struct xfs_buf_ops *b_ops;
> + struct rcu_head b_rcu;
> };
>
> /* Finding and Reading Buffers */
> --
> 2.36.1
>
