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

[Song Liu]

> On Sep 3, 2021, at 1:47 AM, Peter Zijlstra <peterz@xxxxxxxxxxxxx> wrote:
> 
> 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.

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