linux-kernel - Re: [PATCH] powerpc/bpf: Only update ldimm64 during extra pass when it is an address

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Message-Id: <1669284441.66eunvaboi.naveen@linux.ibm.com>
Date:   Thu, 24 Nov 2022 15:43:50 +0530
From:   "Naveen N. Rao" <naveen.n.rao@...ux.ibm.com>
To:     Christophe Leroy <christophe.leroy@...roup.eu>,
        Michael Ellerman <mpe@...erman.id.au>,
        Nicholas Piggin <npiggin@...il.com>
Cc:     Andrii Nakryiko <andrii@...nel.org>,
        Alexei Starovoitov <ast@...nel.org>, bpf@...r.kernel.org,
        Daniel Borkmann <daniel@...earbox.net>,
        Hao Luo <haoluo@...gle.com>,
        John Fastabend <john.fastabend@...il.com>,
        Jiri Olsa <jolsa@...nel.org>, KP Singh <kpsingh@...nel.org>,
        linux-kernel@...r.kernel.org, linuxppc-dev@...ts.ozlabs.org,
        Martin KaFai Lau <martin.lau@...ux.dev>,
        Stanislav Fomichev <sdf@...gle.com>,
        Song Liu <song@...nel.org>, Yonghong Song <yhs@...com>
Subject: Re: [PATCH] powerpc/bpf: Only update ldimm64 during extra pass when
 it is an address

Christophe Leroy wrote:
> ldimm64 is not only used for loading function addresses, and

That's probably true today, but I worry that that can change upstream 
and we may not notice at all.

> the NOPs added for padding are impacting performance, so avoid
> them when not necessary.
> 
> On QEMU mac99, with the patch:
> 
> test_bpf: #829 ALU64_MOV_K: all immediate value magnitudes jited:1 167436810 PASS
> test_bpf: #831 ALU64_OR_K: all immediate value magnitudes jited:1 170702940 PASS
> 
> Without the patch:
> 
> test_bpf: #829 ALU64_MOV_K: all immediate value magnitudes jited:1 173012360 PASS
> test_bpf: #831 ALU64_OR_K: all immediate value magnitudes jited:1 176424090 PASS
> 
> That's a 3.5% performance improvement.

A better approach would be to do a full JIT during the extra pass. 
That's what most other architectures do today. And, as long as we can 
ensure that the JIT'ed program size can never increase during the 
extra pass, we should be ok to do a single extra pass.


- Naveen