Re: [PATCH] uprobes: reduce contention on uprobes_tree access

[Andrii Nakryiko]

On Thu, Mar 21, 2024 at 7:57 AM Jonathan Haslam
<jonathan.haslam@xxxxxxxxx> wrote:
>
> Active uprobes are stored in an RB tree and accesses to this tree are
> dominated by read operations. Currently these accesses are serialized by
> a spinlock but this leads to enormous contention when large numbers of
> threads are executing active probes.
>
> This patch converts the spinlock used to serialize access to the
> uprobes_tree RB tree into a reader-writer spinlock. This lock type
> aligns naturally with the overwhelmingly read-only nature of the tree
> usage here. Although the addition of reader-writer spinlocks are
> discouraged [0], this fix is proposed as an interim solution while an
> RCU based approach is implemented (that work is in a nascent form). This
> fix also has the benefit of being trivial, self contained and therefore
> simple to backport.

Yep, makes sense, I think we'll want to backport this ASAP to some of
the old kernels we have. Thanks!

Acked-by: Andrii Nakryiko <andrii@xxxxxxxxxx>

>
> This change has been tested against production workloads that exhibit
> significant contention on the spinlock and an almost order of magnitude
> reduction for mean uprobe execution time is observed (28 -> 3.5 microsecs).
>
> [0] https://docs.kernel.org/locking/spinlocks.html
>
> Signed-off-by: Jonathan Haslam <jonathan.haslam@xxxxxxxxx>
> ---
>  kernel/events/uprobes.c | 22 +++++++++++-----------
>  1 file changed, 11 insertions(+), 11 deletions(-)
>
> diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
> index 929e98c62965..42bf9b6e8bc0 100644
> --- a/kernel/events/uprobes.c
> +++ b/kernel/events/uprobes.c
> @@ -39,7 +39,7 @@ static struct rb_root uprobes_tree = RB_ROOT;
>   */
>  #define no_uprobe_events()     RB_EMPTY_ROOT(&uprobes_tree)
>
> -static DEFINE_SPINLOCK(uprobes_treelock);      /* serialize rbtree access */
> +static DEFINE_RWLOCK(uprobes_treelock);        /* serialize rbtree access */
>
>  #define UPROBES_HASH_SZ        13
>  /* serialize uprobe->pending_list */
> @@ -669,9 +669,9 @@ static struct uprobe *find_uprobe(struct inode *inode, loff_t offset)
>  {
>         struct uprobe *uprobe;
>
> -       spin_lock(&uprobes_treelock);
> +       read_lock(&uprobes_treelock);
>         uprobe = __find_uprobe(inode, offset);
> -       spin_unlock(&uprobes_treelock);
> +       read_unlock(&uprobes_treelock);
>
>         return uprobe;
>  }
> @@ -701,9 +701,9 @@ static struct uprobe *insert_uprobe(struct uprobe *uprobe)
>  {
>         struct uprobe *u;
>
> -       spin_lock(&uprobes_treelock);
> +       write_lock(&uprobes_treelock);
>         u = __insert_uprobe(uprobe);
> -       spin_unlock(&uprobes_treelock);
> +       write_unlock(&uprobes_treelock);
>
>         return u;
>  }
> @@ -935,9 +935,9 @@ static void delete_uprobe(struct uprobe *uprobe)
>         if (WARN_ON(!uprobe_is_active(uprobe)))
>                 return;
>
> -       spin_lock(&uprobes_treelock);
> +       write_lock(&uprobes_treelock);
>         rb_erase(&uprobe->rb_node, &uprobes_tree);
> -       spin_unlock(&uprobes_treelock);
> +       write_unlock(&uprobes_treelock);
>         RB_CLEAR_NODE(&uprobe->rb_node); /* for uprobe_is_active() */
>         put_uprobe(uprobe);
>  }
> @@ -1298,7 +1298,7 @@ static void build_probe_list(struct inode *inode,
>         min = vaddr_to_offset(vma, start);
>         max = min + (end - start) - 1;
>
> -       spin_lock(&uprobes_treelock);
> +       read_lock(&uprobes_treelock);
>         n = find_node_in_range(inode, min, max);
>         if (n) {
>                 for (t = n; t; t = rb_prev(t)) {
> @@ -1316,7 +1316,7 @@ static void build_probe_list(struct inode *inode,
>                         get_uprobe(u);
>                 }
>         }
> -       spin_unlock(&uprobes_treelock);
> +       read_unlock(&uprobes_treelock);
>  }
>
>  /* @vma contains reference counter, not the probed instruction. */
> @@ -1407,9 +1407,9 @@ vma_has_uprobes(struct vm_area_struct *vma, unsigned long start, unsigned long e
>         min = vaddr_to_offset(vma, start);
>         max = min + (end - start) - 1;
>
> -       spin_lock(&uprobes_treelock);
> +       read_lock(&uprobes_treelock);
>         n = find_node_in_range(inode, min, max);
> -       spin_unlock(&uprobes_treelock);
> +       read_unlock(&uprobes_treelock);
>
>         return !!n;
>  }
> --
> 2.43.0
>