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SubjectRe: [PATCH v3 0/2] kstats: kernel metric collector
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Luigi Rizzo <lrizzo@google.com> writes:

> On Wed, Feb 26, 2020 at 3:11 PM Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>>
>> Luigi Rizzo <lrizzo@google.com> writes:
>>
>> > - the runtime cost and complexity of hooking bpf code is still a bit
>> > unclear to me. kretprobe or tracepoints are expensive, I suppose that
>> > some lean hook replace register_kretprobe() may exist and the
>> > difference from inline annotations would be marginal (we'd still need
>> > to put in the hooks around the code we want to time, though, so it
>> > wouldn't be a pure bpf solution). Any pointers to this are welcome;
>> > Alexei mentioned fentry/fexit and bpf trampolines, but I haven't found
>> > an example that lets me do something equivalent to kretprobe (take a
>> > timestamp before and one after a function without explicit
>> > instrumentation)
>>
>> As Alexei said, with fentry/fexit the overhead should be on par with
>> your example. This functionality is pretty new, though, so I can
>> understand why it's not obvious how to do things with it yet :)
>>
>> I think the best place to look is currently in selftests/bpf in the
>> kernel sources. Grep for 'fexit' and 'fentry' in the progs/ subdir.
>> test_overhead.c and kfree_skb.c seem to have some examples you may be
>> able to work from.
>
> Thank you for the precise reference, Toke.
> I tweaked test_overhead.c to measure (using kstats) the cost of the various
> hooks and I can confirm that fentry and fexit are pretty fast. The
> following table
> shows the p90 runtime of __set_task_comm() at low (100/s) and high (1M/s) rates:
>
> 90 percentile of __set_task_comm() runtime
> (accuracy: 30ns)
> call rate base kprobe kretprobe tracepoint fentry fexit
> 100/sec 270 870 1220 500 400 450
> >1M/s 60 120 210 90
> 70 80
>
> For high rate operation, the overhead of fentry and fexit is quite good,
> even better than tracepoints, and well below the clock's accuracy
> (more detailed measurements indicate ~5ns for fentry, ~10ns for fexit).
> At very low call rates there is an extra 150-200ns
> but that is expected due to the out of line code.

Great, thank you for the performance numbers! This is indeed quite good :)

-Toke

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