[PATCH v3 00/13] uprobes: RCU-protected hot path optimizations

Andrii Nakryiko
2024-08-13T04:29:40Z

( In addition to previously posted first 8 patches, I'm sending 5 more as an
RFC for people to get the general gist of where this work heading and what
uprobe performance is now achievable. I think first 8 patches are ready to be
applied and I'd appreciate early feedback on the remaining 5 ones.

Patches #9 and #10 implement SRCU-based refcounting avoidance for
return_instance and single-stepped uprobes, and lockless SRCU expiration by
timeout. Please look at hprobe_*() helpers for most of the details.

Last three patches are implementing lockless VMA -> uprobe resolution logic by
relying on a) SLAB_TYPESAFE_BY_RCU protection for files_cachep and b) on
Suren's mm_lock_seq changes. Note benchmarking numbers in the last patch.

The remainder of cover letter is written as applied to only first 8 patches. )

This patch set is heavily inspired by Peter Zijlstra's uprobe optimization
patches ([0]) and continue that work, albeit trying to keep complexity to the
minimum, and attepting to reuse existing primitives as much as possible. The
goal here is to optimize obvious uprobe triggering hot path, while keeping the
rest of locking mostly intact.

I've added uprobe_unregister_sync() into the error handling code path inside
uprobe_unregister(). This is due to recent refactorings from Oleg Nesterov
([1]), which necessitates this addition.

Except for refcounting change patch (which I stongly believe is a good
improvement we should do and forget about quasi-refcounting schema of
uprobe->consumers list), the rest of the changes are similar to Peter's
initial changes in [0].

Main differences would be:
- no special RCU protection for mmap and fork handling, we just stick to
refcounts there, as those are infrequent and not performance-sensitive
code, while being complex and thus benefiting from proper locking;
- the above means we don't need to do any custom SRCU additions to handle
forking code path;
- I handled UPROBE_HANDLER_REMOVE problem in handler_chain() differently,
again, leveraging existing locking scheam;
- I kept refcount usage for uretprobe and single-stepping uprobes, I plan to
address that in a separate follow up patches. The plan is to avoid
task_work, but I need to sit down and write and test the code.
- finally, I dutifully was using SRCU throughout all the changes, and only
last patch switches SRCU to RCU Tasks Trace and demonstrates significant
performance and scalability gains from this.

The changes in this patch set were tested using BPF selftests and using
uprobe-stress ([2]) tool. One recent BPF selftest (uprobe_multi/consumers),
only recently added by Jiri Olsa will need a single-line adjustment to the
counting logic, but the patch itself is in bpf-next/master, so we'll have to
address that once linux-trace or tip and bpf-next trees merge. I'll take care
of that when this happens.

Now, for the benchmarking results. I've used the following script (which
utilizes BPF selftests-based bench tool). The CPU used was 80-core Intel Xeon
Gold 6138 CPU @ 2.00GHz running kernel with production-like config. I minimized
background noise by stopping any service I could identify and stop, so results
are pretty stable and variability is pretty small, overall.

Benchmark script:

#!/bin/bash

set -eufo pipefail

for i in uprobe-nop uretprobe-nop; do
for p in 1 2 4 8 16 32 64; do
summary=$(sudo ./bench -w3 -d5 -p$p -a trig-$i | tail -n1)
total=$(echo "$summary" | cut -d'(' -f1 | cut -d' ' -f3-)
percpu=$(echo "$summary" | cut -d'(' -f2 | cut -d')' -f1 | cut -d'/' -f1)
printf "%-15s (%2d cpus): %s (%s/s/cpu)\n" $i $p "$total" "$percpu"
done
echo
done

With all the lock-avoiding changes done in this patch set, we get a pretty
decent improvement in performance and scalability of uprobes with number of
CPUs, even though we are still nowhere near linear scalability. This is due to
the remaining mmap_lock, which is currently taken to resolve interrupt address
to inode+offset and then uprobe instance. And, of course, uretprobes still need
similar RCU to avoid refcount in the hot path, which will be addressed in the
follow up patches. (Again, note, I left the benchmark numbers with lockless
RB-tree patches in.)

BASELINE (on top of Oleg's clean up patches)
============================================
uprobe-nop ( 1 cpus): 3.032 ± 0.023M/s ( 3.032M/s/cpu)
uprobe-nop ( 2 cpus): 3.452 ± 0.005M/s ( 1.726M/s/cpu)
uprobe-nop ( 4 cpus): 3.663 ± 0.005M/s ( 0.916M/s/cpu)
uprobe-nop ( 8 cpus): 3.718 ± 0.038M/s ( 0.465M/s/cpu)
uprobe-nop (16 cpus): 3.344 ± 0.008M/s ( 0.209M/s/cpu)
uprobe-nop (32 cpus): 2.288 ± 0.021M/s ( 0.071M/s/cpu)
uprobe-nop (64 cpus): 3.205 ± 0.004M/s ( 0.050M/s/cpu)

uretprobe-nop ( 1 cpus): 1.979 ± 0.005M/s ( 1.979M/s/cpu)
uretprobe-nop ( 2 cpus): 2.361 ± 0.005M/s ( 1.180M/s/cpu)
uretprobe-nop ( 4 cpus): 2.309 ± 0.002M/s ( 0.577M/s/cpu)
uretprobe-nop ( 8 cpus): 2.253 ± 0.001M/s ( 0.282M/s/cpu)
uretprobe-nop (16 cpus): 2.007 ± 0.000M/s ( 0.125M/s/cpu)
uretprobe-nop (32 cpus): 1.624 ± 0.003M/s ( 0.051M/s/cpu)
uretprobe-nop (64 cpus): 2.149 ± 0.001M/s ( 0.034M/s/cpu)

Up to second-to-last patch (i.e., SRCU-based optimizations)
===========================================================
uprobe-nop ( 1 cpus): 3.276 ± 0.005M/s ( 3.276M/s/cpu)
uprobe-nop ( 2 cpus): 4.125 ± 0.002M/s ( 2.063M/s/cpu)
uprobe-nop ( 4 cpus): 7.713 ± 0.002M/s ( 1.928M/s/cpu)
uprobe-nop ( 8 cpus): 8.097 ± 0.006M/s ( 1.012M/s/cpu)
uprobe-nop (16 cpus): 6.501 ± 0.056M/s ( 0.406M/s/cpu)
uprobe-nop (32 cpus): 4.398 ± 0.084M/s ( 0.137M/s/cpu)
uprobe-nop (64 cpus): 6.452 ± 0.000M/s ( 0.101M/s/cpu)

uretprobe-nop ( 1 cpus): 2.055 ± 0.001M/s ( 2.055M/s/cpu)
uretprobe-nop ( 2 cpus): 2.677 ± 0.000M/s ( 1.339M/s/cpu)
uretprobe-nop ( 4 cpus): 4.561 ± 0.003M/s ( 1.140M/s/cpu)
uretprobe-nop ( 8 cpus): 5.291 ± 0.002M/s ( 0.661M/s/cpu)
uretprobe-nop (16 cpus): 5.065 ± 0.019M/s ( 0.317M/s/cpu)
uretprobe-nop (32 cpus): 3.622 ± 0.003M/s ( 0.113M/s/cpu)
uretprobe-nop (64 cpus): 3.723 ± 0.002M/s ( 0.058M/s/cpu)

RCU Tasks Trace
===============
uprobe-nop ( 1 cpus): 3.396 ± 0.002M/s ( 3.396M/s/cpu)
uprobe-nop ( 2 cpus): 4.271 ± 0.006M/s ( 2.135M/s/cpu)
uprobe-nop ( 4 cpus): 8.499 ± 0.015M/s ( 2.125M/s/cpu)
uprobe-nop ( 8 cpus): 10.355 ± 0.028M/s ( 1.294M/s/cpu)
uprobe-nop (16 cpus): 7.615 ± 0.099M/s ( 0.476M/s/cpu)
uprobe-nop (32 cpus): 4.430 ± 0.007M/s ( 0.138M/s/cpu)
uprobe-nop (64 cpus): 6.887 ± 0.020M/s ( 0.108M/s/cpu)

uretprobe-nop ( 1 cpus): 2.174 ± 0.001M/s ( 2.174M/s/cpu)
uretprobe-nop ( 2 cpus): 2.853 ± 0.001M/s ( 1.426M/s/cpu)
uretprobe-nop ( 4 cpus): 4.913 ± 0.002M/s ( 1.228M/s/cpu)
uretprobe-nop ( 8 cpus): 5.883 ± 0.002M/s ( 0.735M/s/cpu)
uretprobe-nop (16 cpus): 5.147 ± 0.001M/s ( 0.322M/s/cpu)
uretprobe-nop (32 cpus): 3.738 ± 0.008M/s ( 0.117M/s/cpu)
uretprobe-nop (64 cpus): 4.397 ± 0.002M/s ( 0.069M/s/cpu)

For baseline vs SRCU, peak througput increased from 3.7 M/s (million uprobe
triggerings per second) up to about 8 M/s. For uretprobes it's a bit more
modest with bump from 2.4 M/s to 5 M/s.

For SRCU vs RCU Tasks Trace, peak throughput for uprobes increases further from
8 M/s to 10.3 M/s (+28%!), and for uretprobes from 5.3 M/s to 5.8 M/s (+11%),
as we have more work to do on uretprobes side.

Even single-thread (no contention) performance is slightly better: 3.276 M/s to
3.396 M/s (+3.5%) for uprobes, and 2.055 M/s to 2.174 M/s (+5.8%)
for uretprobes.

[0] https://lore.kernel.org/linux-trace-kernel/20240711110235.098009979@infradead.org/
[1] https://lore.kernel.org/linux-trace-kernel/20240729134444.GA12293@redhat.com/
[2] https://github.com/libbpf/libbpf-bootstrap/tree/uprobe-stress

v2->v3:
- undid rcu and rb_node fields colocation which were causing crashes (Oleg);
- ensure synchronize_srcu() on registration failure in patch #4 (Oleg);
v1->v2:
- added back missed kfree() in patch #1 (Oleg);
- forgot the rest, but there were a few small things here and there.

Andrii Nakryiko (10):
uprobes: revamp uprobe refcounting and lifetime management
uprobes: protected uprobe lifetime with SRCU
uprobes: get rid of enum uprobe_filter_ctx in uprobe filter callbacks
uprobes: travers uprobe's consumer list locklessly under SRCU
protection
uprobes: perform lockless SRCU-protected uprobes_tree lookup
uprobes: switch to RCU Tasks Trace flavor for better performance
uprobes: SRCU-protect uretprobe lifetime (with timeout)
uprobes: implement SRCU-protected lifetime for single-stepped uprobe
mm: add SLAB_TYPESAFE_BY_RCU to files_cache
uprobes: add speculative lockless VMA to inode resolution

Peter Zijlstra (2):
perf/uprobe: split uprobe_unregister()
rbtree: provide rb_find_rcu() / rb_find_add_rcu()

Suren Baghdasaryan (1):
mm: introduce mmap_lock_speculation_{start|end}

include/linux/mm_types.h | 3 +
include/linux/mmap_lock.h | 53 +-
include/linux/rbtree.h | 67 ++
include/linux/uprobes.h | 73 +-
kernel/events/uprobes.c | 733 +++++++++++++-----
kernel/fork.c | 7 +-
kernel/trace/bpf_trace.c | 8 +-
kernel/trace/trace_uprobe.c | 15 +-
.../selftests/bpf/bpf_testmod/bpf_testmod.c | 3 +-
9 files changed, 722 insertions(+), 240 deletions(-)

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2.43.5