| Message ID | 20220707235259.1097443-7-david@fromorbit.com (mailing list archive) |
|---|---|
| State | Accepted |
| Headers | show |
| Series | xfs: lockless buffer lookups | expand |
On Fri, Jul 08, 2022 at 09:52:59AM +1000, Dave Chinner wrote: > From: Dave Chinner <dchinner@redhat.com> > > 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. > > Signed-off-by: Dave Chinner <dchinner@redhat.com> > Reviewed-by: Christoph Hellwig <hch@lst.de> Looks good, will test.... Reviewed-by: Darrick J. Wong <djwong@kernel.org> --D > --- > 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 1a6542e01bec..6dac5583977f 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_lookup( > 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 >
diff --git a/fs/xfs/xfs_buf.c b/fs/xfs/xfs_buf.c index 1a6542e01bec..6dac5583977f 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_lookup( 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 */