netdev - Re: [PATCH v4 bpf-next 6/7] bpf: introduce bpf_prog

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Date:   Thu, 20 Jan 2022 04:48:15 +0000
From:   Song Liu <songliubraving@...com>
To:     Alexei Starovoitov <alexei.starovoitov@...il.com>
CC:     Song Liu <song@...nel.org>,
        "bpf@...r.kernel.org" <bpf@...r.kernel.org>,
        "netdev@...r.kernel.org" <netdev@...r.kernel.org>,
        "linux-kernel@...r.kernel.org" <linux-kernel@...r.kernel.org>,
        "ast@...nel.org" <ast@...nel.org>,
        "daniel@...earbox.net" <daniel@...earbox.net>,
        "andrii@...nel.org" <andrii@...nel.org>,
        Kernel Team <Kernel-team@...com>,
        "peterz@...radead.org" <peterz@...radead.org>,
        "x86@...nel.org" <x86@...nel.org>
Subject: Re: [PATCH v4 bpf-next 6/7] bpf: introduce bpf_prog_pack allocator



> On Jan 19, 2022, at 8:14 PM, Alexei Starovoitov <alexei.starovoitov@...il.com> wrote:
> 
> On Wed, Jan 19, 2022 at 03:06:19PM -0800, Song Liu wrote:
>> 
>> +/*
>> + * BPF program pack allocator.
>> + *
>> + * Most BPF programs are pretty small. Allocating a hole page for each
>> + * program is sometime a waste. Many small bpf program also adds pressure
>> + * to instruction TLB. To solve this issue, we introduce a BPF program pack
>> + * allocator. The prog_pack allocator uses HPAGE_PMD_SIZE page (2MB on x86)
>> + * to host BPF programs.
>> + */
>> +#define BPF_PROG_PACK_SIZE	HPAGE_PMD_SIZE
>> +#define BPF_PROG_MAX_PACK_PROG_SIZE	HPAGE_PMD_SIZE
> 
> We have a synthetic test with 1M bpf instructions. How did it JIT?
> Are you saying we were lucky that every BPF insn was JITed to <2 bytes x86?
> Did I misread the 2MB limit?

The logic is, if the program is bigger than 2MB, we fall back to use 
module_alloc(). This limitation simplifies the bpf_prog_pack allocator. 

Thanks,
Song