Re: [PATCH v2] uprobes: reduce contention on uprobes_tree access

From: Jiri Olsa
Date: Mon Apr 22 2024 - 07:40:13 EST

On Mon, Apr 22, 2024 at 03:23:05AM -0700, Jonathan Haslam 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.
>
> We have used a uprobe benchmark from the BPF selftests [1] to estimate
> the improvements. Each block of results below show 1 line per execution
> of the benchmark ("the "Summary" line) and each line is a run with one
> more thread added - a thread is a "producer". The lines are edited to
> remove extraneous output.
>
> The tests were executed with this driver script:
>
> for num_threads in {1..20}
> do
> sudo ./bench -a -p $num_threads trig-uprobe-nop | grep Summary
> done
>
> SPINLOCK (BEFORE)
> ==================
> Summary: hits 1.396 ± 0.007M/s ( 1.396M/prod)
> Summary: hits 1.656 ± 0.016M/s ( 0.828M/prod)
> Summary: hits 2.246 ± 0.008M/s ( 0.749M/prod)
> Summary: hits 2.114 ± 0.010M/s ( 0.529M/prod)
> Summary: hits 2.013 ± 0.009M/s ( 0.403M/prod)
> Summary: hits 1.753 ± 0.008M/s ( 0.292M/prod)
> Summary: hits 1.847 ± 0.001M/s ( 0.264M/prod)
> Summary: hits 1.889 ± 0.001M/s ( 0.236M/prod)
> Summary: hits 1.833 ± 0.006M/s ( 0.204M/prod)
> Summary: hits 1.900 ± 0.003M/s ( 0.190M/prod)
> Summary: hits 1.918 ± 0.006M/s ( 0.174M/prod)
> Summary: hits 1.925 ± 0.002M/s ( 0.160M/prod)
> Summary: hits 1.837 ± 0.001M/s ( 0.141M/prod)
> Summary: hits 1.898 ± 0.001M/s ( 0.136M/prod)
> Summary: hits 1.799 ± 0.016M/s ( 0.120M/prod)
> Summary: hits 1.850 ± 0.005M/s ( 0.109M/prod)
> Summary: hits 1.816 ± 0.002M/s ( 0.101M/prod)
> Summary: hits 1.787 ± 0.001M/s ( 0.094M/prod)
> Summary: hits 1.764 ± 0.002M/s ( 0.088M/prod)
>
> RW SPINLOCK (AFTER)
> ===================
> Summary: hits 1.444 ± 0.020M/s ( 1.444M/prod)
> Summary: hits 2.279 ± 0.011M/s ( 1.139M/prod)
> Summary: hits 3.422 ± 0.014M/s ( 1.141M/prod)
> Summary: hits 3.565 ± 0.017M/s ( 0.891M/prod)
> Summary: hits 2.671 ± 0.013M/s ( 0.534M/prod)
> Summary: hits 2.409 ± 0.005M/s ( 0.401M/prod)
> Summary: hits 2.485 ± 0.008M/s ( 0.355M/prod)
> Summary: hits 2.496 ± 0.003M/s ( 0.312M/prod)
> Summary: hits 2.585 ± 0.002M/s ( 0.287M/prod)
> Summary: hits 2.908 ± 0.011M/s ( 0.291M/prod)
> Summary: hits 2.346 ± 0.016M/s ( 0.213M/prod)
> Summary: hits 2.804 ± 0.004M/s ( 0.234M/prod)
> Summary: hits 2.556 ± 0.001M/s ( 0.197M/prod)
> Summary: hits 2.754 ± 0.004M/s ( 0.197M/prod)
> Summary: hits 2.482 ± 0.002M/s ( 0.165M/prod)
> Summary: hits 2.412 ± 0.005M/s ( 0.151M/prod)
> Summary: hits 2.710 ± 0.003M/s ( 0.159M/prod)
> Summary: hits 2.826 ± 0.005M/s ( 0.157M/prod)
> Summary: hits 2.718 ± 0.001M/s ( 0.143M/prod)
> Summary: hits 2.844 ± 0.006M/s ( 0.142M/prod)

nice, I'm assuming Masami will take this one.. in any case:

Acked-by: Jiri Olsa <jolsa@xxxxxxxxxx>

thanks,
jirka

>
> The numbers in parenthesis give averaged throughput per thread which is
> of greatest interest here as a measure of scalability. Improvements are
> in the order of 22 - 68% with this particular benchmark (mean = 43%).
>
> V2:
> - Updated commit message to include benchmark results.
>
> [0] https://docs.kernel.org/locking/spinlocks.html
> [1] https://github.com/torvalds/linux/blob/master/tools/testing/selftests/bpf/benchs/bench_trigger.c
>
> 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 e4834d23e1d1..8ae0eefc3a34 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
>