Message-ID: <CAEf4BzYvpgfFGckcKdzkC_g1J1SFi7xBe=_cjdVy4KEMikvGMw@mail.gmail.com>
Date: Mon, 12 Aug 2024 10:49:56 -0700
From: Andrii Nakryiko <andrii.nakryiko@...il.com>
To: "Liao, Chang" <liaochang1@...wei.com>
Cc: peterz@...radead.org, mingo@...hat.com, acme@...nel.org, 
	namhyung@...nel.org, mark.rutland@....com, alexander.shishkin@...ux.intel.com, 
	jolsa@...nel.org, irogers@...gle.com, adrian.hunter@...el.com, 
	kan.liang@...ux.intel.com, 
	"oleg@...hat.com >> Oleg Nesterov" <oleg@...hat.com>, Andrii Nakryiko <andrii@...nel.org>, 
	Masami Hiramatsu <mhiramat@...nel.org>, Steven Rostedt <rostedt@...dmis.org>, paulmck@...nel.org, 
	linux-kernel@...r.kernel.org, linux-perf-users@...r.kernel.org, 
	bpf@...r.kernel.org, linux-trace-kernel@...r.kernel.org
Subject: Re: [PATCH] uprobes: Optimize the allocation of insn_slot for performance

On Mon, Aug 12, 2024 at 4:11 AM Liao, Chang <liaochang1@...wei.com> wrote:
>
>
>
> 在 2024/8/9 2:26, Andrii Nakryiko 写道:
> > On Thu, Aug 8, 2024 at 1:45 AM Liao, Chang <liaochang1@...wei.com> wrote:
> >>
> >> Hi Andrii and Oleg.
> >>
> >> This patch sent by me two weeks ago also aim to optimize the performance of uprobe
> >> on arm64. I notice recent discussions on the performance and scalability of uprobes
> >> within the mailing list. Considering this interest, I've added you and other relevant
> >> maintainers to the CC list for broader visibility and potential collaboration.
> >>
> >
> > Hi Liao,
> >
> > As you can see there is an active work to improve uprobes, that
> > changes lifetime management of uprobes, removes a bunch of locks taken
> > in the uprobe/uretprobe hot path, etc. It would be nice if you can
> > hold off a bit with your changes until all that lands. And then
> > re-benchmark, as costs might shift.
> >
> > But also see some remarks below.
> >
> >> Thanks.
> >>
> >> 在 2024/7/27 17:44, Liao Chang 写道:
> >>> The profiling result of single-thread model of selftests bench reveals
> >>> performance bottlenecks in find_uprobe() and caches_clean_inval_pou() on
> >>> ARM64. On my local testing machine, 5% of CPU time is consumed by
> >>> find_uprobe() for trig-uprobe-ret, while caches_clean_inval_pou() take
> >>> about 34% of CPU time for trig-uprobe-nop and trig-uprobe-push.
> >>>
> >>> This patch introduce struct uprobe_breakpoint to track previously
> >>> allocated insn_slot for frequently hit uprobe. it effectively reduce the
> >>> need for redundant insn_slot writes and subsequent expensive cache
> >>> flush, especially on architecture like ARM64. This patch has been tested
> >>> on Kunpeng916 (Hi1616), 4 NUMA nodes, 64 cores@ 2.4GHz. The selftest
> >>> bench and Redis GET/SET benchmark result below reveal obivious
> >>> performance gain.
> >>>
> >>> before-opt
> >>> ----------
> >>> trig-uprobe-nop:  0.371 ± 0.001M/s (0.371M/prod)
> >>> trig-uprobe-push: 0.370 ± 0.001M/s (0.370M/prod)
> >>> trig-uprobe-ret:  1.637 ± 0.001M/s (1.647M/prod)
> >
> > I'm surprised that nop and push variants are much slower than ret
> > variant. This is exactly opposite on x86-64. Do you have an
> > explanation why this might be happening? I see you are trying to
> > optimize xol_get_insn_slot(), but that is (at least for x86) a slow
> > variant of uprobe that normally shouldn't be used. Typically uprobe is
> > installed on nop (for USDT) and on function entry (which would be push
> > variant, `push %rbp` instruction).
> >
> > ret variant, for x86-64, causes one extra step to go back to user
> > space to execute original instruction out-of-line, and then trapping
> > back to kernel for running uprobe. Which is what you normally want to
> > avoid.
> >
> > What I'm getting at here. It seems like maybe arm arch is missing fast
> > emulated implementations for nops/push or whatever equivalents for
> > ARM64 that is. Please take a look at that and see why those are slow
> > and whether you can make those into fast uprobe cases?
>
> Hi Andrii,
>
> As you correctly pointed out, the benchmark result on Arm64 is counterintuitive
> compared to X86 behavior. My investigation revealed that the root cause lies in
> the arch_uprobe_analyse_insn(), which excludes the Arm64 equvialents instructions
> of 'nop' and 'push' from the emulatable instruction list. This forces the kernel
> to handle these instructions out-of-line in userspace upon breakpoint exception
> is handled, leading to a significant performance overhead compared to 'ret' variant,
> which is already emulated.
>
> To address this issue, I've developed a patch supports  the emulation of 'nop' and
> 'push' variants. The benchmark results below indicates the performance gain of
> emulation is obivious.
>
> xol (1 cpus)
> ------------
> uprobe-nop:  0.916 ± 0.001M/s (0.916M/prod)
> uprobe-push: 0.908 ± 0.001M/s (0.908M/prod)
> uprobe-ret:  1.855 ± 0.000M/s (1.855M/prod)
> uretprobe-nop:  0.640 ± 0.000M/s (0.640M/prod)
> uretprobe-push: 0.633 ± 0.001M/s (0.633M/prod)
> uretprobe-ret:  0.978 ± 0.003M/s (0.978M/prod)
>
> emulation (1 cpus)
> -------------------
> uprobe-nop:  1.862 ± 0.002M/s  (1.862M/s/cpu)
> uprobe-push: 1.743 ± 0.006M/s  (1.743M/s/cpu)
> uprobe-ret:  1.840 ± 0.001M/s  (1.840M/s/cpu)
> uretprobe-nop:  0.964 ± 0.004M/s  (0.964M/s/cpu)
> uretprobe-push: 0.936 ± 0.004M/s  (0.936M/s/cpu)
> uretprobe-ret:  0.940 ± 0.001M/s  (0.940M/s/cpu)
>
> As you can see, the performance gap between nop/push and ret variants has been significantly
> reduced. Due to the emulation of 'push' instruction need to access userspace memory, it spent
> more cycles than the other.

Great, it's an obvious improvement. Are you going to send patches
upstream? Please cc bpf@...r.kernel.org as well.


I'm also thinking we should update uprobe/uretprobe benchmarks to be
less x86-specific. Right now "-nop" is the happy fastest case, "-push"
is still happy, slightly slower case (due to the need to emulate stack
operation) and "-ret" is meant to be the slow single-step case. We
should adjust the naming and make sure that on ARM64 we hit similar
code paths. Given you seem to know arm64 pretty well, can you please
take a look at updating bench tool for ARM64 (we can also rename
benchmarks to something a bit more generic, rather than using
instruction names)?

>
> >
> >>> trig-uretprobe-nop:  0.331 ± 0.004M/s (0.331M/prod)
> >>> trig-uretprobe-push: 0.333 ± 0.000M/s (0.333M/prod)
> >>> trig-uretprobe-ret:  0.854 ± 0.002M/s (0.854M/prod)
> >>> Redis SET (RPS) uprobe: 42728.52
> >>> Redis GET (RPS) uprobe: 43640.18
> >>> Redis SET (RPS) uretprobe: 40624.54
> >>> Redis GET (RPS) uretprobe: 41180.56
> >>>
> >>> after-opt
> >>> ---------
> >>> trig-uprobe-nop:  0.916 ± 0.001M/s (0.916M/prod)
> >>> trig-uprobe-push: 0.908 ± 0.001M/s (0.908M/prod)
> >>> trig-uprobe-ret:  1.855 ± 0.000M/s (1.855M/prod)
> >>> trig-uretprobe-nop:  0.640 ± 0.000M/s (0.640M/prod)
> >>> trig-uretprobe-push: 0.633 ± 0.001M/s (0.633M/prod)
> >>> trig-uretprobe-ret:  0.978 ± 0.003M/s (0.978M/prod)
> >>> Redis SET (RPS) uprobe: 43939.69
> >>> Redis GET (RPS) uprobe: 45200.80
> >>> Redis SET (RPS) uretprobe: 41658.58
> >>> Redis GET (RPS) uretprobe: 42805.80
> >>>
> >>> While some uprobes might still need to share the same insn_slot, this
> >>> patch compare the instructions in the resued insn_slot with the
> >>> instructions execute out-of-line firstly to decides allocate a new one
> >>> or not.
> >>>
> >>> Additionally, this patch use a rbtree associated with each thread that
> >>> hit uprobes to manage these allocated uprobe_breakpoint data. Due to the
> >>> rbtree of uprobe_breakpoints has smaller node, better locality and less
> >>> contention, it result in faster lookup times compared to find_uprobe().
> >>>
> >>> The other part of this patch are some necessary memory management for
> >>> uprobe_breakpoint data. A uprobe_breakpoint is allocated for each newly
> >>> hit uprobe that doesn't already have a corresponding node in rbtree. All
> >>> uprobe_breakpoints will be freed when thread exit.
> >>>
> >>> Signed-off-by: Liao Chang <liaochang1@...wei.com>
> >>> ---
> >>>  include/linux/uprobes.h |   3 +
> >>>  kernel/events/uprobes.c | 246 +++++++++++++++++++++++++++++++++-------
> >>>  2 files changed, 211 insertions(+), 38 deletions(-)
> >>>
> >
> > [...]
>
> --
> BR
> Liao, Chang

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