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Date:   Thu, 10 Nov 2022 12:58:54 +0800
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Subject: Re: [PATCH bpf-next v2 0/4] Add ftrace direct call for arm64
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To:     Florent Revest <revest@chromium.org>,
        Masami Hiramatsu <mhiramat@kernel.org>
Cc:     Steven Rostedt <rostedt@goodmis.org>,
        Xu Kuohai <xukuohai@huawei.com>,
        Mark Rutland <mark.rutland@arm.com>,
        Catalin Marinas <catalin.marinas@arm.com>,
        Daniel Borkmann <daniel@iogearbox.net>,
        Xu Kuohai <xukuohai@huaweicloud.com>,
        linux-arm-kernel@lists.infradead.org, linux-kernel@vger.kernel.org,
        bpf@vger.kernel.org, Will Deacon <will@kernel.org>,
        Jean-Philippe Brucker <jean-philippe@linaro.org>,
        Ingo Molnar <mingo@redhat.com>,
        Oleg Nesterov <oleg@redhat.com>,
        Alexei Starovoitov <ast@kernel.org>,
        Andrii Nakryiko <andrii@kernel.org>,
        Martin KaFai Lau <martin.lau@linux.dev>,
        Song Liu <song@kernel.org>, Yonghong Song <yhs@fb.com>,
        John Fastabend <john.fastabend@gmail.com>,
        KP Singh <kpsingh@kernel.org>,
        Stanislav Fomichev <sdf@google.com>,
        Hao Luo <haoluo@google.com>, Jiri Olsa <jolsa@kernel.org>,
        Zi Shen Lim <zlim.lnx@gmail.com>,
        Pasha Tatashin <pasha.tatashin@soleen.com>,
        Ard Biesheuvel <ardb@kernel.org>,
        Marc Zyngier <maz@kernel.org>, Guo Ren <guoren@kernel.org>
References: <20220913162732.163631-1-xukuohai@huaweicloud.com>
 <f1e14934-dc54-9bf7-501a-89affdb7371e@iogearbox.net>
 <YzG51Jyd5zhvygtK@arm.com> <YzHk1zRf1Dp8YTEe@FVFF77S0Q05N>
 <970a25e4-9b79-9e0c-b338-ed1a934f2770@huawei.com>
 <YzR5WSLux4mmFIXg@FVFF77S0Q05N>
 <2cb606b4-aa8b-e259-cdfd-1bfc61fd7c44@huawei.com>
 <CABRcYmKPchvtkkgWhOJ6o3pHVqTWeenGawHfZ2ug8Akdh6NfnQ@mail.gmail.com>
 <7f34d333-3b2a-aea5-f411-d53be2c46eee@huawei.com>
 <20221005110707.55bd9354@gandalf.local.home>
 <CABRcYmJGY6fp0CtUBYN8BjEDN=r42BPLSBcrxqu491bTRmfm7g@mail.gmail.com>
 <20221005113019.18aeda76@gandalf.local.home>
 <CABRcYmL0bDkgYP3tSwhZYaGUSbsUR3Gq85QCRiUAdXt65czzmg@mail.gmail.com>
 <20221006122922.53802a5c@gandalf.local.home>
 <CABRcYm+d=xY9nBCJo-6JW_=F41g4X32QM9WOPChaOTfs6d6KCA@mail.gmail.com>
 <20221021203158.4464ac19d8b19b6da6a40852@kernel.org>
 <CABRcYmKzwAFr_0NOxeWhXcCiT5wwi_qkm5Czc0C4CVCAs8stFw@mail.gmail.com>
From:   wuqiang <wuqiang.matt@bytedance.com>
In-Reply-To: <CABRcYmKzwAFr_0NOxeWhXcCiT5wwi_qkm5Czc0C4CVCAs8stFw@mail.gmail.com>
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On 2022/10/22 00:49, Florent Revest wrote:
> On Fri, Oct 21, 2022 at 1:32 PM Masami Hiramatsu <mhiramat@kernel.org> wrote:
>> On Mon, 17 Oct 2022 19:55:06 +0200
>> Florent Revest <revest@chromium.org> wrote:
>>> Mark finished an implementation of his per-callsite-ops and min-args
>>> branches (meaning that we can now skip the expensive ftrace's saving
>>> of all registers and iteration over all ops if only one is attached)
>>> - https://git.kernel.org/pub/scm/linux/kernel/git/mark/linux.git/log/?h=arm64-ftrace-call-ops-20221017
>>>
>>> And Masami wrote similar patches to what I had originally done to
>>> fprobe in my branch:
>>> - https://github.com/mhiramat/linux/commits/kprobes/fprobe-update
>>>
>>> So I could rebase my previous "bpf on fprobe" branch on top of these:
>>> (as before, it's just good enough for benchmarking and to give a
>>> general sense of the idea, not for a thorough code review):
>>> - https://github.com/FlorentRevest/linux/commits/fprobe-min-args-3
>>>
>>> And I could run the benchmarks against my rpi4. I have different
>>> baseline numbers as Xu so I ran everything again and tried to keep the
>>> format the same. "indirect call" refers to my branch I just linked and
>>> "direct call" refers to the series this is a reply to (Xu's work)
>>
>> Thanks for sharing the measurement results. Yes, fprobes/rethook
>> implementation is just porting the kretprobes implementation, thus
>> it may not be so optimized.
>>
>> BTW, I remember Wuqiang's patch for kretprobes.
>>
>> https://lore.kernel.org/all/20210830173324.32507-1-wuqiang.matt@bytedance.com/T/#u
> 
> Oh that's a great idea, thanks for pointing it out Masami!
> 
>> This is for the scalability fixing, but may possible to improve
>> the performance a bit. It is not hard to port to the recent kernel.
>> Can you try it too?
> 
> I rebased it on my branch
> https://github.com/FlorentRevest/linux/commits/fprobe-min-args-3
> 
> And I got measurements again. Unfortunately it looks like this does not help :/
> 
> New benchmark results: https://paste.debian.net/1257856/
> New perf report: https://paste.debian.net/1257859/
> 
> The fprobe based approach is still significantly slower than the
> direct call approach.

FYI, a new version was released, basing on ring-array, which brings a 6.96%
increase in throughput of 1-thread case for ARM64.

https://lore.kernel.org/all/20221108071443.258794-1-wuqiang.matt@bytedance.com/

Could you share more details of the test ? I'll give it a try.

>> Anyway, eventually, I would like to remove the current kretprobe
>> based implementation and unify fexit hook with function-graph
>> tracer. It should make more better perfromance on it.
> 
> That makes sense. :) How do you imagine the unified solution ?
> Would both the fgraph and fprobe APIs keep existing but under the hood
> one would be implemented on the other ? (or would one be gone ?) Would
> we replace the rethook freelist with the function graph's per-task
> shadow stacks ? (or the other way around ?))

How about a private pool designate for local cpu ? If the fprobed routine
sticks to the same CPU when returning, the object allocation and reclaim
can go a quick path, that should bring same performance as shadow stack.
Otherwise the return of an object will go a slow path (slow as current
freelist or objpool).

>>> Note that I can't really make sense of the perf report with indirect
>>> calls. it always reports it spent 12% of the time in
>>> rethook_trampoline_handler but I verified with both a WARN in that
>>> function and a breakpoint with a debugger, this function does *not*
>>> get called when running this "bench trig-fentry" benchmark. Also it
>>> wouldn't make sense for fprobe_handler to call it so I'm quite
>>> confused why perf would report this call and such a long time spent
>>> there. Anyone know what I could be missing here ?
> 
> I made slight progress on this. If I put the vmlinux file in the cwd
> where I run perf report, the reports no longer contain references to
> rethook_trampoline_handler. Instead, they have a few
> 0xffff800008xxxxxx addresses under fprobe_handler. (like in the
> pastebin I just linked)
> 
> It's still pretty weird because that range is the vmalloc area on
> arm64 and I don't understand why anything under fprobe_handler would
> execute there. However, I'm also definitely sure that these 12% are
> actually spent getting buffers from the rethook memory pool because if
> I replace rethook_try_get and rethook_recycle calls with the usage of
> a dummy static bss buffer (for the sake of benchmarking the
> "theoretical best case scenario") these weird perf report traces are
> gone and the 12% are saved. https://paste.debian.net/1257862/
> 
> This is why I would be interested in seeing rethook's memory pool
> reimplemented on top of something like
> https://lwn.net/Articles/788923/ If we get closer to the performance
> of the the theoretical best case scenario where getting a blob of
> memory is ~free (and I think it could be the case with a per task
> shadow stack like fgraph's), then a bpf on fprobe implementation would
> start to approach the performances of a direct called trampoline on
> arm64: https://paste.debian.net/1257863/