Re: [PATCH v5 bpf-next 2/3] bpf: introduce helper bpf_get_branch_snapshot

[Peter Zijlstra]

On Thu, Sep 02, 2021 at 09:57:05AM -0700, Song Liu wrote:
> +BPF_CALL_3(bpf_get_branch_snapshot, void *, buf, u32, size, u64, flags)
> +{
> +	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);

That's at least 2, possibly 3 branches just from the sanity checks, plus
at least one from starting the BPF prog and one from calling this
function, gets you at ~5 branch entries gone before you even do the
snapshot thing.

Less if you're in branch-stack mode.

Can't the validator help with getting rid of the some of that?

I suppose you have to have this helper function because the JIT cannot
emit static_call()... although in this case one could cheat and simply
emit a call to static_call_query() and not bother with dynamic updates
(because there aren't any).