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From: Alban Crequy <alban.crequy@gmail.com>
To: Andrii Nakryiko <andriin@fb.com>
Cc: bpf@vger.kernel.org, netdev@vger.kernel.org, ast@fb.com,
	Daniel Borkmann <daniel@iogearbox.net>,
	andrii.nakryiko@gmail.com, kernel-team@fb.com,
	"Paul E . McKenney" <paulmck@kernel.org>,
	Jonathan Lemon <jonathan.lemon@gmail.com>,
	Stanislav Fomichev <sdf@google.com>,
	Alban Crequy <alban@kinvolk.io>,
	mauricio@kinvolk.io, kai@kinvolk.io
Subject: Re: [PATCH v2 bpf-next 7/7] docs/bpf: add BPF ring buffer design notes
Date: Mon, 25 May 2020 11:59:58 +0200
Message-ID: <CAMXgnP424S5s-mrwFB_nuZNSuqLyi1K8r519WKVkyMBPtv1PMQ@mail.gmail.com> (raw)
In-Reply-To: <20200517195727.279322-8-andriin@fb.com>

Hi,

Thanks. Both motivators look very interesting to me:

On Sun, 17 May 2020 at 21:58, Andrii Nakryiko <andriin@fb.com> wrote:
[...]
> +Motivation
> +----------
> +There are two distinctive motivators for this work, which are not satisfied by
> +existing perf buffer, which prompted creation of a new ring buffer
> +implementation.
> +  - more efficient memory utilization by sharing ring buffer across CPUs;

I have a use case with traceloop
(https://github.com/kinvolk/traceloop) where I use one
BPF_MAP_TYPE_PERF_EVENT_ARRAY per container, so when the number of
containers times the number of CPU is high, it can use a lot of
memory.

> +  - preserving ordering of events that happen sequentially in time, even
> +  across multiple CPUs (e.g., fork/exec/exit events for a task).

I had the problem to keep track of TCP connections and when
tcp-connect and tcp-close events can be on different CPUs, it makes it
difficult to get the correct order.

[...]
> +There are a bunch of similarities between perf buffer
> +(BPF_MAP_TYPE_PERF_EVENT_ARRAY) and new BPF ring buffer semantics:
> +  - variable-length records;
> +  - if there is no more space left in ring buffer, reservation fails, no
> +    blocking;
[...]

BPF_MAP_TYPE_PERF_EVENT_ARRAY can be set as both 'overwriteable' and
'backward': if there is no more space left in ring buffer, it would
then overwrite the old events. For that, the buffer needs to be
prepared with mmap(...PROT_READ) instead of mmap(...PROT_READ |
PROT_WRITE), and set the write_backward flag. See details in commit
9ecda41acb97 ("perf/core: Add ::write_backward attribute to perf
event"):

struct perf_event_attr attr = {0,};
attr.write_backward = 1; /* backward */
fd = perf_event_open_map(&attr, ...);
base = mmap(fd, 0, size, PROT_READ /* overwriteable */, MAP_SHARED);

I use overwriteable and backward ring buffers in traceloop: buffers
are continuously overwritten and are usually not read, except when a
user explicitly asks for it (e.g. to inspect the last few events of an
application after a crash). If BPF_MAP_TYPE_RINGBUF implements the
same features, then I would be able to switch and use less memory.

Do you think it will be possible to implement that in BPF_MAP_TYPE_RINGBUF?

Cheers,
Alban

  parent reply index

Thread overview: 33+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-05-17 19:57 [PATCH v2 bpf-next 0/7] BPF ring buffer Andrii Nakryiko
2020-05-17 19:57 ` [PATCH v2 bpf-next 1/7] bpf: implement BPF ring buffer and verifier support for it Andrii Nakryiko
2020-05-19 12:57   ` kbuild test robot
2020-05-19 23:53   ` kbuild test robot
2020-05-22  0:25   ` Paul E. McKenney
2020-05-22 18:46     ` Andrii Nakryiko
2020-05-25 16:01       ` Paul E. McKenney
2020-05-25 18:45         ` Andrii Nakryiko
2020-05-22  1:07   ` Alexei Starovoitov
2020-05-22 18:48     ` Andrii Nakryiko
2020-05-25 20:34     ` Andrii Nakryiko
2020-05-17 19:57 ` [PATCH v2 bpf-next 2/7] tools/memory-model: add BPF ringbuf MPSC litmus tests Andrii Nakryiko
2020-05-22  0:34   ` Paul E. McKenney
2020-05-22 18:51     ` Andrii Nakryiko
2020-05-25 23:33       ` Andrii Nakryiko
2020-05-26  3:05         ` Paul E. McKenney
2020-05-17 19:57 ` [PATCH v2 bpf-next 3/7] bpf: track reference type in verifier Andrii Nakryiko
2020-05-22  1:13   ` Alexei Starovoitov
2020-05-22 18:53     ` Andrii Nakryiko
2020-05-17 19:57 ` [PATCH v2 bpf-next 4/7] libbpf: add BPF ring buffer support Andrii Nakryiko
2020-05-22  1:15   ` Alexei Starovoitov
2020-05-22 18:56     ` Andrii Nakryiko
2020-05-17 19:57 ` [PATCH v2 bpf-next 5/7] selftests/bpf: add BPF ringbuf selftests Andrii Nakryiko
2020-05-22  1:20   ` Alexei Starovoitov
2020-05-22 18:58     ` Andrii Nakryiko
2020-05-17 19:57 ` [PATCH v2 bpf-next 6/7] bpf: add BPF ringbuf and perf buffer benchmarks Andrii Nakryiko
2020-05-22  1:21   ` Alexei Starovoitov
2020-05-22 19:07     ` Andrii Nakryiko
2020-05-17 19:57 ` [PATCH v2 bpf-next 7/7] docs/bpf: add BPF ring buffer design notes Andrii Nakryiko
2020-05-22  1:23   ` Alexei Starovoitov
2020-05-22 19:08     ` Andrii Nakryiko
2020-05-25  9:59   ` Alban Crequy [this message]
2020-05-25 19:12     ` Andrii Nakryiko

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