[PATCH 2/2] uprobes: add speculative lockless VMA-to-inode-to-uprobe resolution

[Andrii Nakryiko <andrii@kernel.org>]

Given filp_cachep is already marked SLAB_TYPESAFE_BY_RCU, 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, under the assumption 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_{start,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).

Suggested-by: Matthew Wilcox <willy@infradead.org>
Signed-off-by: Andrii Nakryiko <andrii@kernel.org>
---
 kernel/events/uprobes.c | 51 +++++++++++++++++++++++++++++++++++++++++
 1 file changed, 51 insertions(+)

diff --git a/kernel/events/uprobes.c b/kernel/events/uprobes.c
index a2e6a57f79f2..b7e0baa83de1 100644
--- a/kernel/events/uprobes.c
+++ b/kernel/events/uprobes.c
@@ -2081,6 +2081,53 @@ 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)
+{
+	const vm_flags_t flags = VM_HUGETLB | VM_MAYEXEC | VM_MAYSHARE;
+	struct mm_struct *mm = current->mm;
+	struct uprobe *uprobe;
+	struct vm_area_struct *vma;
+	struct file *vm_file;
+	struct inode *vm_inode;
+	unsigned long vm_pgoff, vm_start;
+	int seq;
+	loff_t offset;
+
+	if (!mmap_lock_speculation_start(mm, &seq))
+		return NULL;
+
+	rcu_read_lock();
+
+	vma = vma_lookup(mm, bp_vaddr);
+	if (!vma)
+		goto bail;
+
+	vm_file = data_race(vma->vm_file);
+	if (!vm_file || (vma->vm_flags & flags) != VM_MAYEXEC)
+		goto bail;
+
+	vm_inode = data_race(vm_file->f_inode);
+	vm_pgoff = data_race(vma->vm_pgoff);
+	vm_start = data_race(vma->vm_start);
+
+	offset = (loff_t)(vm_pgoff << PAGE_SHIFT) + (bp_vaddr - vm_start);
+	uprobe = find_uprobe_rcu(vm_inode, offset);
+	if (!uprobe)
+		goto bail;
+
+	/* now double check that nothing about MM changed */
+	if (!mmap_lock_speculation_end(mm, seq))
+		goto bail;
+
+	rcu_read_unlock();
+
+	/* happy case, we speculated successfully */
+	return uprobe;
+bail:
+	rcu_read_unlock();
+	return NULL;
+}
+
 /* assumes being inside RCU protected region */
 static struct uprobe *find_active_uprobe_rcu(unsigned long bp_vaddr, int *is_swbp)
 {
@@ -2088,6 +2135,10 @@ 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