Re: [RFC PATCH v4 3/3] bpf: Introduce function for outputing data to perf event

From: He Kuang
Date: Mon Jul 13 2015 - 00:37:19 EST


hi, Alexei

On 2015/7/11 6:10, Alexei Starovoitov wrote:
On 7/10/15 3:03 AM, He Kuang wrote:
There're scenarios that we need an eBPF program to record not only
kprobe point args, but also the PMU counters, time latencies or the
number of cache misses between two probe points and other information
when the probe point is entered.

This patch adds a new trace event to establish infrastruction for bpf to
output data to perf. Userspace perf tools can detect and use this event
as using the existing tracepoint events.

New bpf trace event entry in debugfs:

/sys/kernel/debug/tracing/events/bpf/bpf_output_data

Userspace perf tools detect the new tracepoint event as:

bpf:bpf_output_data [Tracepoint event]

Nice! This approach looks cleanest so far.

+TRACE_EVENT(bpf_output_data,
+
+ TP_PROTO(u64 *src, int len),
+
+ TP_ARGS(src, len),
+
+ TP_STRUCT__entry(
+ __dynamic_array(u64, buf, len)
+ ),
+
+ TP_fast_assign(
+ memcpy(__get_dynamic_array(buf), src, len * sizeof(u64));

may be make it 'u8' array? The extra multiply and...

OK

So the output of three u64 integers (e.g. 0x2060572485, 0x20667b0ff2,
0x623eb6d) will be this:

dd 994 [000] 139.158180: bpf:bpf_output_data: 85 24 57 60 20 00 00 00
f2 0f 7b 66 20 00 00 00 6d eb 23 06 00 00 00 00

And users are not restricted to u64 type elements. I'll change that.


+static u64 bpf_output_trace_data(u64 r1, u64 r2, u64 r3, u64 r4, u64 r5)
+{
+ void *src = (void *) (long) r1;
+ int size = (int) r2;
+
+ trace_bpf_output_data(src, size / sizeof(u64));

.. and this silent round down could be confusing to use.
With array of u8, the program can push any structured data into it
and let user space interpret it.



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