netdev - Re: [PATCH v2 net-next 1/6] bpf: introduce BPF_PROG_TEST

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Date:   Sat, 1 Apr 2017 08:45:01 -0700
From:   Alexei Starovoitov <ast@...com>
To:     Jesper Dangaard Brouer <brouer@...hat.com>
CC:     "David S . Miller" <davem@...emloft.net>,
        Daniel Borkmann <daniel@...earbox.net>,
        Wang Nan <wangnan0@...wei.com>,
        Martin KaFai Lau <kafai@...com>, <netdev@...r.kernel.org>,
        <kernel-team@...com>
Subject: Re: [PATCH v2 net-next 1/6] bpf: introduce BPF_PROG_TEST_RUN command

On 4/1/17 12:14 AM, Jesper Dangaard Brouer wrote:
> On Thu, 30 Mar 2017 21:45:38 -0700
> Alexei Starovoitov <ast@...com> wrote:
>
>> static u32 bpf_test_run(struct bpf_prog *prog, void *ctx, u32 repeat, u32 *time)
>> +{
>> +	u64 time_start, time_spent = 0;
>> +	u32 ret = 0, i;
>> +
>> +	if (!repeat)
>> +		repeat = 1;
>> +	time_start = ktime_get_ns();
>
> I've found that is useful to record the CPU cycles, as it is more
> useful for comparing between CPUs.  The nanosec time measurement varies
> too much between CPUs and GHz.  I do use nanosec measurements myself a
> lot, but that is mostly because it is easier to relate to pps rates.
> For eBPF code execution I think it is more useful to get a cycles cost
> count?

for micro-benchmarking of an instruction or small primitives
like spin_lock and irq_save/restore, yes. Cycles are more interesting
to look at. Here it's the whole program which in case of networking
likely does at least a few map lookups.
Also this duration field is more of sanity test then actual metric.

> I've been using tsc[1] (rdtsc) to get the CPU cycles, I believe
> get_cycles() the more generic call, which have arch specific impl. (but
> can return 0 if no arch support).
>
> The best solution would be to use the perf infrastructure and PMU
> counter to get both PMU cycles and instructions, as that also tell you
> about the pipeline efficiency like instructions per cycles.  I only got
> this partly working in [1][2].

to use get_cycles() or perf_event_create_kernel_counter() the current
simple loop would become kthread pinned to cpu and so on.
imo it's an overkill.
The only reason 'duration' being reported is a sanity test with user
space measurements.
What this command allows to do is:
$ time ./my_bpf_benchmark
The reported time should match the kernel reported 'duration'.
The tiny difference will come from resched. That's sanity part.
Now we can also do
$ perf record ./my_bpf_benchmark
and get all perf goodness for free without adding any kernel code.
I want this test_run command to stay execution only. All pmu and
performance metrics should stay on perf side.
In case of performance optimization of bpf programs we're trying
to improve perf by changing the way program is written, hence
we need perf to point out which line of C code is costly.
Second is improving performance by changing JIT, map implementations
and so on. Here we also want full perf tool power.
Unfortunately there is an issue with perf today, since as soon as
my_bpf_benchmark exits, bpf prog is unloaded and ksym is gone, so
'perf report' cannot associate addresses back to source code.
We discussed a solution with Arnaldo. So that's orthogonal work in
progress which is needed regardless of this test_run command.

User space can also pin itself to cpu instead of asking kernel to
do it and run the same program on multiple cpus in parallel testing
interaction between concurrent map accesses and so on.
So by keeping test_run command as execution only primitive we allow
user space to do all the fancy tricks and measurements.