Re: [RFC PATCH v4 10/29] bpf tools: Collect map definitions from 'maps' section

From: Wangnan (F)
Date: Mon Jun 01 2015 - 01:20:46 EST



On 2015/6/1 10:12, Namhyung Kim wrote:
Hi Alexei and Wang,

On Thu, May 28, 2015 at 08:35:19PM -0700, Alexei Starovoitov wrote:
On Thu, May 28, 2015 at 03:14:44PM +0800, Wangnan (F) wrote:
On 2015/5/28 14:09, Alexei Starovoitov wrote:
On Thu, May 28, 2015 at 11:09:50AM +0800, Wangnan (F) wrote:
For me, enable eBPF program to read PMU counter is the first thing need to
be done.
The other thing is enabling eBPF programs to bring some information to perf
sample.

Here is an example to show my idea.

I have a program which:

int main()
{
while(1) {
read(...);
/* do A */
write(...);
/* do B */
}
}

Then by using following script:

SEC("enter=sys_write $outdata:u64")
int enter_sys_write(...) {
u64 cycles_cnt = bpf_read_pmu(&cycles_pmu);
bpf_store_value(cycles_cnt);
return 1;
}

SEC("enter=sys_read $outdata:u64")
int enter_sys_read(...) {
u64 cycles_cnt = bpf_read_pmu(&cycles_pmu);
bpf_store_value(cycles_cnt);
return 1;
}

by 'perf script', we can check the counter of cycles at each points, then we
are allowed
to compute the number of cycles between any two sampling points. This way we
can compute
how many cycles taken by A and B. If instruction counter is also recorded,
we will know
the IPC of A and B.
Agree. That's useful. That's exactly what I meant by
"compute a number of cache misses between two kprobe events".
The overhead is less when bpf program computes the cycle and instruction
delta, computes IPC and passes only final IPC numbers to the user space.
It can even average IPC over time.
For some very frequent events it can read cycle_cnt on sys_entry_read,
then read it on sys_exit_read, compute delta and average it into the map.
User space can read the map every second or every 10 seconds and print
nice graph.
Looks very interesting and useful indeed!

As far as 'bpf_store_value' goes... I was thinking to expose perf ring_buffer
to bpf programs, so that program can stream any data to perf that receives
it via mmap. Then you don't need this '$outdata' hack.
Then we need to define and pass the format of such data so that perf
tools can read and process the data. IIRC Masami suggested to have an
additional user event type for inserting/injecting non-perf events -
like PERF_RECORD_USER_DEFINED_TYPE? And its contents is something
similar to tracepoint event format file so that we can reuse existing
code to parse the event definition.

Is it possible to expose such format through /sys/kernel/debug/tracing/events/*/*/format
so we can avoid extra work on perf side and make it accessable by both perf and ftrace?

Currently we do this by opening an internal PMU and adding a common
field in trace_define_common_fields(). By reading that PMU in
tracing_generic_entry_update() we are able to collect its value by both perf and ftrace, both
kprobe events and tracepoints (the implementation is ugly. We have to hardwire the
PMU because alerting common field dynamically is hard. If we want to trace multiple PMUs then
recompiling is required). In serval usecase, we found that using ftrace should be better
because the cost of perf is higher.

Although currently BPF programs can only get executed if it traced by perf, I think we can
extend it to ftrace (but not sure how to do it now...).

Currently I'm still working on perf bpf stuffs. I think it has almost done. The next step
should be solving arguments passing problem. After that we should enable eBPF programs to
read hardware PMU. Outputting should be the final step. I'm glad to see many people are
thinking on it. Please keep me in the loop if you have any new idea on this area.

Thank you.

Thanks,
Namhyung


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