netdev - Re: [PATCH net-next 1/3] perf, events: add non-linear data support for raw records

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Date:	Wed, 13 Jul 2016 11:24:13 +0200
From:	Daniel Borkmann <daniel@...earbox.net>
To:	Peter Zijlstra <peterz@...radead.org>
CC:	davem@...emloft.net, alexei.starovoitov@...il.com, tgraf@...g.ch,
	netdev@...r.kernel.org, linux-kernel@...r.kernel.org
Subject: Re: [PATCH net-next 1/3] perf, events: add non-linear data support
 for raw records

Hi Peter,

On 07/13/2016 09:52 AM, Peter Zijlstra wrote:
> On Wed, Jul 13, 2016 at 12:36:17AM +0200, Daniel Borkmann wrote:
>> This patch adds support for non-linear data on raw records. It means
>> that for such data, the newly introduced __output_custom() helper will
>> be used instead of __output_copy(). __output_custom() will invoke
>> whatever custom callback is passed in via struct perf_raw_record_frag
>> to extract the data into the ring buffer slot.
>>
>> To keep changes in perf_prepare_sample() and in perf_output_sample()
>> minimal, size/size_head split was added to perf_raw_record that call
>> sites fill out, so that two extra tests in fast-path can be avoided.
>>
>> The few users of raw records are adapted to initialize their size_head
>> and frag data; no change in behavior for them. Later patch will extend
>> BPF side with a first user and callback for this facility, future users
>> could be things like XDP BPF programs (that work on different context
>> though and would thus have a different callback), etc.
>
> Why? What problem are we solving?

I've tried to summarize it in patch 3/3, there are a number of improvements
this set would provide: the current BPF_FUNC_perf_event_output helper that
can be used from tc/networking programs has the limitation that if data from
the skb should be part of the sample, that data first needs to be copied to
eBPF stack with the bpf_skb_load_bytes() helper, and only then passed into
bpf_event_output().

This currently has 3 issues we'd like to resolve: i) We need two copies instead
of just a single one for the skb data. The data can be non-linear, see also
skb_copy_bits() as an example for walking/extracting it, ii) for static
verification reasons, the bpf_skb_load_bytes() helper needs to see a constant
size on the passed buffer to make sure BPF verifier can do its sanity checks
on it during verification time, so just passing in skb->len (or any other
non-constant value) wouldn't work, but changing bpf_skb_load_bytes() is also
not the real solution since we still have two copies we'd like to avoid as well
and iii) bpf_skb_load_bytes() is just for rather smaller buffers (e.g. headers)
since they need to sit on the limited eBPF stack anyway. The set would improve
the BPF helper to address all 3 at once.

Thanks,
Daniel