On Thu, Sep 2, 2021 at 9:58 AM Song Liu <songliubraving@xxxxxx> 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.
