Re: [PATCH RFC v4 net-next 25/26] samples: bpf: counting eBPF example in C

[Alexei Starovoitov]

On Thu, Aug 14, 2014 at 3:13 PM, Brendan Gregg
<brendan.d.gregg@xxxxxxxxx> wrote:
> On Wed, Aug 13, 2014 at 12:57 AM, Alexei Starovoitov <ast@xxxxxxxxxxxx> wrote:
>> this example has two probes in C that use two different maps.
>>
>> 1st probe is the similar to dropmon.c. It attaches to kfree_skb tracepoint and
>> count number of packet drops at different locations
>>
>> 2nd probe attaches to kprobe/sys_write and computes a histogram of different
>> write sizes
>>
>> Usage:
>> $ sudo ex2
>>
>> Should see:
>> writing bpf-5 -> /sys/kernel/debug/tracing/events/skb/kfree_skb/filter
>> writing bpf-8 -> /sys/kernel/debug/tracing/events/kprobes/sys_write/filter
>> location 0xffffffff816efc67 count 1
>>
>> location 0xffffffff815d8030 count 1
>> location 0xffffffff816efc67 count 3
>>
>> location 0xffffffff815d8030 count 4
>> location 0xffffffff816efc67 count 9
>>
>>            syscall write() stats
>>      byte_size       : count     distribution
>>        1 -> 1        : 3141     |****                                  |
>>        2 -> 3        : 2        |                                      |
>>        4 -> 7        : 14       |                                      |
>>        8 -> 15       : 3268     |*****                                 |
>>       16 -> 31       : 732      |                                      |
>>       32 -> 63       : 20042    |************************************* |
>>       64 -> 127      : 12154    |**********************                |
>>      128 -> 255      : 2215     |***                                   |
>>      256 -> 511      : 9        |                                      |
>>      512 -> 1023     : 0        |                                      |
>>     1024 -> 2047     : 1        |                                      |
>
> This is pretty awesome.
>
> Given that this is tracing two tracepoints at once, I'd like to see a
> similar example where time is stored on the first tracepoint,
> retrieved on the second for a delta calculation, then presented with a
> similar histogram as seen above.

Very good point. The time related helpers are missing. In V5 I'm
thinking to add something like bpf_ktime_get_ns().
To associate begin and end events I think bpf_gettid() would be
needed, but that doesn't feel generic enough for helper function,
so I'm leaning toward 'bpf_get_current()' helper that will return
'current' task pointer. eBPF program can use this pointer for
correlation of events or can go exploring task fields with
bpf_fetch_() helpers...

Thank you very much for trying things out and for your feedback!
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