linux-kernel - Re: [PATCH bpf-next v2 02/18] x86,bpf: add bpf_global

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Message-ID: <4737114.cEBGB3zze1@7940hx>
Date: Wed, 16 Jul 2025 21:05:25 +0800
From: Menglong Dong <menglong.dong@...ux.dev>
To: Alexei Starovoitov <alexei.starovoitov@...il.com>
Cc: Menglong Dong <menglong8.dong@...il.com>,
 Steven Rostedt <rostedt@...dmis.org>, Jiri Olsa <jolsa@...nel.org>,
 bpf <bpf@...r.kernel.org>, "H. Peter Anvin" <hpa@...or.com>,
 Martin KaFai Lau <martin.lau@...ux.dev>,
 Eduard Zingerman <eddyz87@...il.com>, Song Liu <song@...nel.org>,
 Yonghong Song <yonghong.song@...ux.dev>,
 John Fastabend <john.fastabend@...il.com>, KP Singh <kpsingh@...nel.org>,
 Stanislav Fomichev <sdf@...ichev.me>, Hao Luo <haoluo@...gle.com>,
 LKML <linux-kernel@...r.kernel.org>,
 Network Development <netdev@...r.kernel.org>
Subject:
 Re: [PATCH bpf-next v2 02/18] x86,bpf: add bpf_global_caller for global
 trampoline

On Wednesday, July 16, 2025 12:35 AM Alexei Starovoitov <alexei.starovoitov@...il.com> write:
> On Tue, Jul 15, 2025 at 1:37 AM Menglong Dong <menglong.dong@...ux.dev> wrote:
> >
> >
> > On 7/15/25 10:25, Alexei Starovoitov wrote:
[......]
> >
> > According to my benchmark, it has ~5% overhead to save/restore
> > *5* variants when compared with *0* variant. The save/restore of regs
> > is fast, but it still need 12 insn, which can produce ~6% overhead.
> 
> I think it's an ok trade off, because with one global trampoline
> we do not need to call rhashtable lookup before entering bpf prog.
> bpf prog will do it on demand if/when it needs to access arguments.
> This will compensate for a bit of lost performance due to extra save/restore.

I don't understand here :/

The rhashtable lookup is done at the beginning of the global trampoline,
which is called before we enter bpf prog. The bpf progs is stored in the
kfunc_md, and we need get them from the hash table.

If this is the only change, it is still OK. But according to my previous, the
rhashtable can cause ~7% addition overhead. So if we change both
them, the performance of tracing-multi is a little far from tracing, which
means ~25% performance gap for the functions that have no arguments.
About the rhashtable part, I'll do more research on it and feedback late.

> 
> PS
> pls don't add your chinatelecom.cn email in cc.
> gmail just cannot deliver there and it's annoying to keep deleting
> it manually in every reply.

Sorry about that. I filtered out such message in my gmail, and
didn't notice it. I'll remove it from the CC in the feature :)

Thanks!
Menglong Dong

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