Re: [PATCH v2 net-next 3/4] bpf: bpf_htab: Add syscall to iterate percpu value of a key

[Ming Lei]

On Wed, Jan 13, 2016 at 1:23 PM, Alexei Starovoitov
<alexei.starovoitov@xxxxxxxxx> wrote:
> On Wed, Jan 13, 2016 at 10:42:49AM +0800, Ming Lei wrote:
>>
>> So I don't think it is good to retrieve value from one CPU via one
>> single system call, and accumulate them finally in userspace.
>>
>> One approach I thought of is to define the function(or sort of)
>>
>>  handle_cpu_value(u32 cpu, void *val_cpu, void *val_total)
>>
>> in bpf kernel code for collecting value from each cpu and
>> accumulating them into 'val_total', and most of situations, the
>> function can be implemented without loop most of situations.
>> kernel can call this function directly, and the total value can be
>> return to userspace by one single syscall.
>>
>> Alexei and anyone, could you comment on this draft idea for
>> perpcu map?
>
> I'm not sure how you expect user space to specify such callback.
> Kernel cannot execute user code.

I mean the above callback function can be built into bpf code and then
run from kernel after loading like in packet filter case by tcpdump, maybe
one new prog type is needed. It is doable in theroy. I need to investigate
a bit to understand how it can be called from kernel, and it might be OK
to call it via kprobe, but not elegent just for accumulating value from each
CPU.

> Also syscall/malloc/etc is a noise comparing to ipi and it
> will still be there, so
> for(all cpus) { syscall+ipi;} will have the same speed.

In the syscall path, lots of slow things, and finally the accumulated
value is often stale and may not reprensent accurate number at any
time, and can be thought as invalid.

> I think in this use case the overhead of ipi is justified,
> since user space needs to read accurate numbers otherwise
> the whole per-cpu is not very useful. One can just use
> normal hash map and do normal increment. All cpus will race
> and the counter may contain complete garbage, but in some
> cases such rough counters are actually good enough.
> Here per-cpu hash gives fast performance and _accurate_
> numbers to userspace.

IMO the 'accurate' value on each CPU at different times
doesn't matter, and from user view, they do care the
accurate/final number accumulated from all CPUs at the
specified time.

> Having said that if you see a way to avoid ipi and still
> get correct numbers to user space, it would be great.

As I mentioned in another thread, the current non-percpu
map has the same problem too between lookup element syscall
and updating element in eBPF prog.

-- 
Ming Lei