On Sep 3, 2021, at 1:47 AM, Peter Zijlstra [off-list ref] wrote:

On Thu, Sep 02, 2021 at 09:57:05AM -0700, Song Liu wrote:

quoted

+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.

Let me try to shuffle the function and get rid of some of these checks. 

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).

We only JIT some key helper functions. I didn't think about that because 
current version is OK for mainstream and future hardware. I guess we 
can try JIT if it turns out some architecture needs more optimization. 

Thanks,
Song