[PATCH v4 tip/perf/core 4/4] uprobes: add speculative lockless VMA-to-inode-to-uprobe resolution

Andrii Nakryiko
2024-10-28T01:09:19Z

Given filp_cachep is marked SLAB_TYPESAFE_BY_RCU (and FMODE_BACKING
files, a special case, now goes through RCU-delated freeing), we can
safely access vma->vm_file->f_inode field locklessly under just
rcu_read_lock() protection, which enables looking up uprobe from
uprobes_tree completely locklessly and speculatively without the need to
acquire mmap_lock for reads. In most cases, anyway, assuming that there
are no parallel mm and/or VMA modifications. The underlying struct
file's memory won't go away from under us (even if struct file can be
reused in the meantime).

We rely on newly added mmap_lock_speculation_{begin,end}() helpers to
validate that mm_struct stays intact for entire duration of this
speculation. If not, we fall back to mmap_lock-protected lookup.
The speculative logic is written in such a way that it will safely
handle any garbage values that might be read from vma or file structs.

Benchmarking results speak for themselves.

BEFORE (latest tip/perf/core)
=============================
uprobe-nop ( 1 cpus): 3.384 ± 0.004M/s ( 3.384M/s/cpu)
uprobe-nop ( 2 cpus): 5.456 ± 0.005M/s ( 2.728M/s/cpu)
uprobe-nop ( 3 cpus): 7.863 ± 0.015M/s ( 2.621M/s/cpu)
uprobe-nop ( 4 cpus): 9.442 ± 0.008M/s ( 2.360M/s/cpu)
uprobe-nop ( 5 cpus): 11.036 ± 0.013M/s ( 2.207M/s/cpu)
uprobe-nop ( 6 cpus): 10.884 ± 0.019M/s ( 1.814M/s/cpu)
uprobe-nop ( 7 cpus): 7.897 ± 0.145M/s ( 1.128M/s/cpu)
uprobe-nop ( 8 cpus): 10.021 ± 0.128M/s ( 1.253M/s/cpu)
uprobe-nop (10 cpus): 9.932 ± 0.170M/s ( 0.993M/s/cpu)
uprobe-nop (12 cpus): 8.369 ± 0.056M/s ( 0.697M/s/cpu)
uprobe-nop (14 cpus): 8.678 ± 0.017M/s ( 0.620M/s/cpu)
uprobe-nop (16 cpus): 7.392 ± 0.003M/s ( 0.462M/s/cpu)
uprobe-nop (24 cpus): 5.326 ± 0.178M/s ( 0.222M/s/cpu)
uprobe-nop (32 cpus): 5.426 ± 0.059M/s ( 0.170M/s/cpu)
uprobe-nop (40 cpus): 5.262 ± 0.070M/s ( 0.132M/s/cpu)
uprobe-nop (48 cpus): 6.121 ± 0.010M/s ( 0.128M/s/cpu)
uprobe-nop (56 cpus): 6.252 ± 0.035M/s ( 0.112M/s/cpu)
uprobe-nop (64 cpus): 7.644 ± 0.023M/s ( 0.119M/s/cpu)
uprobe-nop (72 cpus): 7.781 ± 0.001M/s ( 0.108M/s/cpu)
uprobe-nop (80 cpus): 8.992 ± 0.048M/s ( 0.112M/s/cpu)

AFTER
=====
uprobe-nop ( 1 cpus): 3.534 ± 0.033M/s ( 3.534M/s/cpu)
uprobe-nop ( 2 cpus): 6.701 ± 0.007M/s ( 3.351M/s/cpu)
uprobe-nop ( 3 cpus): 10.031 ± 0.007M/s ( 3.344M/s/cpu)
uprobe-nop ( 4 cpus): 13.003 ± 0.012M/s ( 3.251M/s/cpu)
uprobe-nop ( 5 cpus): 16.274 ± 0.006M/s ( 3.255M/s/cpu)
uprobe-nop ( 6 cpus): 19.563 ± 0.024M/s ( 3.261M/s/cpu)
uprobe-nop ( 7 cpus): 22.696 ± 0.054M/s ( 3.242M/s/cpu)
uprobe-nop ( 8 cpus): 24.534 ± 0.010M/s ( 3.067M/s/cpu)
uprobe-nop (10 cpus): 30.475 ± 0.117M/s ( 3.047M/s/cpu)
uprobe-nop (12 cpus): 33.371 ± 0.017M/s ( 2.781M/s/cpu)
uprobe-nop (14 cpus): 38.864 ± 0.004M/s ( 2.776M/s/cpu)
uprobe-nop (16 cpus): 41.476 ± 0.020M/s ( 2.592M/s/cpu)
uprobe-nop (24 cpus): 64.696 ± 0.021M/s ( 2.696M/s/cpu)
uprobe-nop (32 cpus): 85.054 ± 0.027M/s ( 2.658M/s/cpu)
uprobe-nop (40 cpus): 101.979 ± 0.032M/s ( 2.549M/s/cpu)
uprobe-nop (48 cpus): 110.518 ± 0.056M/s ( 2.302M/s/cpu)
uprobe-nop (56 cpus): 117.737 ± 0.020M/s ( 2.102M/s/cpu)
uprobe-nop (64 cpus): 124.613 ± 0.079M/s ( 1.947M/s/cpu)
uprobe-nop (72 cpus): 133.239 ± 0.032M/s ( 1.851M/s/cpu)
uprobe-nop (80 cpus): 142.037 ± 0.138M/s ( 1.775M/s/cpu)

Previously total throughput was maxing out at 11mln/s, and gradually
declining past 8 cores. With this change, it now keeps growing with each
added CPU, reaching 142mln/s at 80 CPUs (this was measured on a 80-core
Intel(R) Xeon(R) Gold 6138 CPU @ 2.00GHz).

Reviewed-by: Oleg Nesterov <oleg@redhat.com>
Suggested-by: Matthew Wilcox <willy@infradead.org>
Suggested-by: Peter Zijlstra <peterz@infradead.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
---
kernel/events/uprobes.c | 45 +++++++++++++++++++++++++++++++++++++++++
1 file changed, 45 insertions(+)

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index 290c445768fa..efcd62f7051d 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -2074,6 +2074,47 @@ static int is_trap_at_addr(struct mm_struct *mm, unsigned long vaddr)
return is_trap_insn(&opcode);
}

+static struct uprobe *find_active_uprobe_speculative(unsigned long bp_vaddr)
+{
+ struct mm_struct *mm = current->mm;
+ struct uprobe *uprobe = NULL;
+ struct vm_area_struct *vma;
+ struct file *vm_file;
+ loff_t offset;
+ unsigned int seq;
+
+ guard(rcu)();
+
+ if (!mmap_lock_speculation_begin(mm, &seq))
+ return NULL;
+
+ vma = vma_lookup(mm, bp_vaddr);
+ if (!vma)
+ return NULL;
+
+ /*
+ * vm_file memory can be reused for another instance of struct file,
+ * but can't be freed from under us, so it's safe to read fields from
+ * it, even if the values are some garbage values; ultimately
+ * find_uprobe_rcu() + mmap_lock_speculation_end() check will ensure
+ * that whatever we speculatively found is correct
+ */
+ vm_file = READ_ONCE(vma->vm_file);
+ if (!vm_file)
+ return NULL;
+
+ offset = (loff_t)(vma->vm_pgoff << PAGE_SHIFT) + (bp_vaddr - vma->vm_start);
+ uprobe = find_uprobe_rcu(vm_file->f_inode, offset);
+ if (!uprobe)
+ return NULL;
+
+ /* now double check that nothing about MM changed */
+ if (!mmap_lock_speculation_end(mm, seq))
+ return NULL;
+
+ return uprobe;
+}
+
/* assumes being inside RCU protected region */
static struct uprobe *find_active_uprobe_rcu(unsigned long bp_vaddr, int *is_swbp)
{
@@ -2081,6 +2122,10 @@ static struct uprobe *find_active_uprobe_rcu(unsigned long bp_vaddr, int *is_swb
struct uprobe *uprobe = NULL;
struct vm_area_struct *vma;

+ uprobe = find_active_uprobe_speculative(bp_vaddr);
+ if (uprobe)
+ return uprobe;
+
mmap_read_lock(mm);
vma = vma_lookup(mm, bp_vaddr);
if (vma) {
--
2.43.5