[External] Re: [PATCH bpf-next v2 1/9] bpf: tracing: add support to record and check the accessed args
Andrii Nakryiko
andrii.nakryiko at gmail.com
Fri Mar 29 16:28:33 PDT 2024
On Thu, Mar 28, 2024 at 8:10 AM Steven Rostedt <rostedt at goodmis.org> wrote:
>
> On Thu, 28 Mar 2024 22:43:46 +0800
> 梦龙董 <dongmenglong.8 at bytedance.com> wrote:
>
> > I have done a simple benchmark on creating 1000
> > trampolines. It is slow, quite slow, which consume up to
> > 60s. We can't do it this way.
> >
> > Now, I have a bad idea. How about we introduce
> > a "dynamic trampoline"? The basic logic of it can be:
> >
> > """
> > save regs
> > bpfs = trampoline_lookup_ip(ip)
> > fentry = bpfs->fentries
> > while fentry:
> > fentry(ctx)
> > fentry = fentry->next
> >
> > call origin
> > save return value
> >
> > fexit = bpfs->fexits
> > while fexit:
> > fexit(ctx)
> > fexit = fexit->next
> >
> > xxxxxx
> > """
> >
> > And we lookup the "bpfs" by the function ip in a hash map
> > in trampoline_lookup_ip. The type of "bpfs" is:
> >
> > struct bpf_array {
> > struct bpf_prog *fentries;
> > struct bpf_prog *fexits;
> > struct bpf_prog *modify_returns;
> > }
> >
> > When we need to attach the bpf progA to function A/B/C,
> > we only need to create the bpf_arrayA, bpf_arrayB, bpf_arrayC
> > and add the progA to them, and insert them to the hash map
> > "direct_call_bpfs", and attach the "dynamic trampoline" to
> > A/B/C. If bpf_arrayA exist, just add progA to the tail of
> > bpf_arrayA->fentries. When we need to attach progB to
> > B/C, just add progB to bpf_arrayB->fentries and
> > bpf_arrayB->fentries.
> >
> > Compared to the trampoline, extra overhead is introduced
> > by the hash lookuping.
> >
> > I have not begun to code yet, and I am not sure the overhead is
> > acceptable. Considering that we also need to do hash lookup
> > by the function in kprobe_multi, maybe the overhead is
> > acceptable?
>
> Sounds like you are just recreating the function management that ftrace
> has. It also can add thousands of trampolines very quickly, because it does
> it in batches. It takes special synchronization steps to attach to fentry.
> ftrace (and I believe multi-kprobes) updates all the attachments for each
> step, so the synchronization needed is only done once.
>
> If you really want to have thousands of functions, why not just register it
> with ftrace itself. It will give you the arguments via the ftrace_regs
> structure. Can't you just register a program as the callback?
>
> It will probably make your accounting much easier, and just let ftrace
> handle the fentry logic. That's what it was made to do.
>
I thought I'll just ask instead of digging through code, sorry for
being lazy :) Is there any way to pass pt_regs/ftrace_regs captured
before function execution to a return probe (fexit/kretprobe)? I.e.,
how hard is it to pass input function arguments to a kretprobe? That's
the biggest advantage of fexit over kretprobe, and if we can make
these original pt_regs/ftrace_regs available to kretprobe, then
multi-kretprobe will effectively be this multi-fexit.
> -- Steve
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In-Reply-To: <20240328111330.194dcbe5 at gandalf.local.home>
From: Andrii Nakryiko <andrii.nakryiko at gmail.com>
Date: Fri, 29 Mar 2024 16:28:33 -0700
Message-ID: <CAEf4BzYgzOti+Hfdn3SUCjuofGedXRSGApVDD+K2TdG6oNE-pw at mail.gmail.com>
Subject: Re: [External] Re: [PATCH bpf-next v2 1/9] bpf: tracing: add support
to record and check the accessed args
To: Steven Rostedt <rostedt at goodmis.org>
Cc: =?UTF-8?B?5qKm6b6Z6JGj?= <dongmenglong.8 at bytedance.com>,
Alexei Starovoitov <alexei.starovoitov at gmail.com>, Jiri Olsa <jolsa at kernel.org>,
Andrii Nakryiko <andrii at kernel.org>, Alexei Starovoitov <ast at kernel.org>,
Daniel Borkmann <daniel at iogearbox.net>, Martin KaFai Lau <martin.lau at linux.dev>, Eddy Z <eddyz87 at gmail.com>,
Song Liu <song at kernel.org>, Yonghong Song <yonghong.song at linux.dev>,
John Fastabend <john.fastabend at gmail.com>, KP Singh <kpsingh at kernel.org>,
Stanislav Fomichev <sdf at google.com>, Hao Luo <haoluo at google.com>,
Alexander Gordeev <agordeev at linux.ibm.com>, Christian Borntraeger <borntraeger at linux.ibm.com>,
Sven Schnelle <svens at linux.ibm.com>, "David S. Miller" <davem at davemloft.net>,
David Ahern <dsahern at kernel.org>, Dave Hansen <dave.hansen at linux.intel.com>,
X86 ML <x86 at kernel.org>, Mathieu Desnoyers <mathieu.desnoyers at efficios.com>,
Quentin Monnet <quentin at isovalent.com>, bpf <bpf at vger.kernel.org>,
linux-arm-kernel <linux-arm-kernel at lists.infradead.org>, LKML <linux-kernel at vger.kernel.org>,
linux-riscv <linux-riscv at lists.infradead.org>, linux-s390 <linux-s390 at vger.kernel.org>,
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On Thu, Mar 28, 2024 at 8:10 AM Steven Rostedt <rostedt at goodmis.org> wrote:
>
> On Thu, 28 Mar 2024 22:43:46 +0800
> 梦龙董 <dongmenglong.8 at bytedance.com> wrote:
>
> > I have done a simple benchmark on creating 1000
> > trampolines. It is slow, quite slow, which consume up to
> > 60s. We can't do it this way.
> >
> > Now, I have a bad idea. How about we introduce
> > a "dynamic trampoline"? The basic logic of it can be:
> >
> > """
> > save regs
> > bpfs = trampoline_lookup_ip(ip)
> > fentry = bpfs->fentries
> > while fentry:
> > fentry(ctx)
> > fentry = fentry->next
> >
> > call origin
> > save return value
> >
> > fexit = bpfs->fexits
> > while fexit:
> > fexit(ctx)
> > fexit = fexit->next
> >
> > xxxxxx
> > """
> >
> > And we lookup the "bpfs" by the function ip in a hash map
> > in trampoline_lookup_ip. The type of "bpfs" is:
> >
> > struct bpf_array {
> > struct bpf_prog *fentries;
> > struct bpf_prog *fexits;
> > struct bpf_prog *modify_returns;
> > }
> >
> > When we need to attach the bpf progA to function A/B/C,
> > we only need to create the bpf_arrayA, bpf_arrayB, bpf_arrayC
> > and add the progA to them, and insert them to the hash map
> > "direct_call_bpfs", and attach the "dynamic trampoline" to
> > A/B/C. If bpf_arrayA exist, just add progA to the tail of
> > bpf_arrayA->fentries. When we need to attach progB to
> > B/C, just add progB to bpf_arrayB->fentries and
> > bpf_arrayB->fentries.
> >
> > Compared to the trampoline, extra overhead is introduced
> > by the hash lookuping.
> >
> > I have not begun to code yet, and I am not sure the overhead is
> > acceptable. Considering that we also need to do hash lookup
> > by the function in kprobe_multi, maybe the overhead is
> > acceptable?
>
> Sounds like you are just recreating the function management that ftrace
> has. It also can add thousands of trampolines very quickly, because it does
> it in batches. It takes special synchronization steps to attach to fentry.
> ftrace (and I believe multi-kprobes) updates all the attachments for each
> step, so the synchronization needed is only done once.
>
> If you really want to have thousands of functions, why not just register it
> with ftrace itself. It will give you the arguments via the ftrace_regs
> structure. Can't you just register a program as the callback?
>
> It will probably make your accounting much easier, and just let ftrace
> handle the fentry logic. That's what it was made to do.
>
I thought I'll just ask instead of digging through code, sorry for
being lazy :) Is there any way to pass pt_regs/ftrace_regs captured
before function execution to a return probe (fexit/kretprobe)? I.e.,
how hard is it to pass input function arguments to a kretprobe? That's
the biggest advantage of fexit over kretprobe, and if we can make
these original pt_regs/ftrace_regs available to kretprobe, then
multi-kretprobe will effectively be this multi-fexit.
> -- Steve
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