netdev - Re: [PATCH net-next 1/4] bpf: add helper bpf_perf_read_counter

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Date:   Fri, 1 Sep 2017 22:50:40 +0200
From:   Peter Zijlstra <peterz@...radead.org>
To:     Alexei Starovoitov <ast@...com>
Cc:     Yonghong Song <yhs@...com>, rostedt@...dmis.org,
        daniel@...earbox.net, netdev@...r.kernel.org, kernel-team@...com
Subject: Re: [PATCH net-next 1/4] bpf: add helper bpf_perf_read_counter_time
 for perf event array map

On Fri, Sep 01, 2017 at 01:29:17PM -0700, Alexei Starovoitov wrote:

> >+BPF_CALL_4(bpf_perf_read_counter_time, struct bpf_map *, map, u64, flags,
> >+	struct bpf_perf_counter_time *, buf, u32, size)
> >+{
> >+	struct perf_event *pe;
> >+	u64 now;
> >+	int err;
> >+
> >+	if (unlikely(size != sizeof(struct bpf_perf_counter_time)))
> >+		return -EINVAL;
> >+	err = get_map_perf_counter(map, flags, &buf->counter, &pe);
> >+	if (err)
> >+		return err;
> >+
> >+	calc_timer_values(pe, &now, &buf->time.enabled, &buf->time.running);
> >+	return 0;
> >+}
> 
> Peter,
> I believe we're doing it correctly above.
> It's a copy paste of the same logic as in total_time_enabled/running.
> We cannot expose total_time_enabled/running to bpf, since they are
> different counters. The above two are specific to bpf usage.
> See commit log.

No, the patch is atrocious and the usage is wrong.

Exporting a function called 'calc_timer_values' is a horrible violation
of the namespace.

And its wrong because it should be done in conjunction with
perf_event_read_local(). You cannot afterwards call this because you
don't know if the event was active when you read it and you don't have
temporal guarantees; that is, reading these timestamps long after or
before the read is wrong, and this interface allows it.

So no, sorry this is just fail.