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

On Wed, Jan 13, 2016 at 11:43:50PM +0800, Ming Lei wrote:

On Wed, Jan 13, 2016 at 1:23 PM, Alexei Starovoitov
[off-list ref] wrote:

quoted

On Wed, Jan 13, 2016 at 10:42:49AM +0800, Ming Lei wrote:

quoted

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.

that would be a total overkill.

quoted

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.

no. stale != invalid.
Some analytics/monitor applications are good with ball park numbers
and for them regular hash map with non-atomic increment is good enough,
but others need accurate numbers. Even though they may be seconds stale.