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

From: Google
Date: Mon Apr 22 2024 - 11:13:02 EST

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On Mon, 22 Apr 2024 13:39:32 +0200
Jiri Olsa <olsajiri@xxxxxxxxx> wrote:

> 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 Jiri!

This looks good to me too.
Let me pick this for probes/for-next.

Thank you,

>
> 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
> >
>

--
Masami Hiramatsu (Google) <mhiramat@xxxxxxxxxx>

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