linux-kernel - Re: [PATCH v5 bpf-next 2/3] bpf: introduce helper bpf_get_branch

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Date:   Thu, 2 Sep 2021 18:03:41 -0700
From:   Alexei Starovoitov <alexei.starovoitov@...il.com>
To:     Song Liu <songliubraving@...com>
Cc:     bpf <bpf@...r.kernel.org>, LKML <linux-kernel@...r.kernel.org>,
        Arnaldo Carvalho de Melo <acme@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Ingo Molnar <mingo@...hat.com>, kjain@...ux.ibm.com,
        Kernel Team <kernel-team@...com>
Subject: Re: [PATCH v5 bpf-next 2/3] bpf: introduce helper bpf_get_branch_snapshot

On Thu, Sep 2, 2021 at 9:58 AM Song Liu <songliubraving@...com> wrote:
>
> +BPF_CALL_3(bpf_get_branch_snapshot, void *, buf, u32, size, u64, flags)
> +{
> +#ifndef CONFIG_X86
> +       return -ENOENT;
> +#else
> +       static const u32 br_entry_size = sizeof(struct perf_branch_entry);
> +       u32 entry_cnt = size / br_entry_size;
> +
> +       if (unlikely(flags))
> +               return -EINVAL;
> +
> +       if (!buf || (size % br_entry_size != 0))
> +               return -EINVAL;
> +
> +       entry_cnt = static_call(perf_snapshot_branch_stack)(buf, entry_cnt);

Not taken branches will not be counted even with PERF_SAMPLE_BRANCH_ANY,
right?
Probably the first unlikely(flags) will be a fallthrough in asm.
Maybe worth adding unlikely to 2nd condition as well to make
sure that the compiler will generate default fallthrough code for it ?
So both will not appear in lbr entries?
Or maybe do:
if (unlikely(!buf))
   return -EINVAL;
entry_cnt = static_call
if (size % br_entry_size)
   return -EINVAL;

The lbr trace will be collected anyway.
If there are jmps in lbr due to earlier "if"s we can move them
after static_call. Like if (unlikely(flags) can be done afterwards too.

Bigger bang for the buck would be static-inline-ing migrate_disable, though.