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

[Jiri Olsa <olsajiri@gmail.com>]

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@kernel.org>

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@gmail.com>
> ---
>  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
>