From nobody Fri Sep 12 09:18:51 2025 Received: from mail-wr1-f73.google.com (mail-wr1-f73.google.com [209.85.221.73]) (using TLSv1.2 with cipher ECDHE-RSA-AES128-GCM-SHA256 (128/128 bits)) (No client certificate requested) by smtp.subspace.kernel.org (Postfix) with ESMTPS id 712D8170846 for ; Fri, 10 May 2024 14:04:50 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.221.73 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1715349892; cv=none; b=JfPwHWn32xhDXAl8ytS8RHgtMsc3P+BhJNbB9WLAVIOCLZIJjGHpt6dl0eSGn9+BlnZy71CGhGFUx7k/vNaivSDwVJihQYzagvpe5Z+OKppVIp9//MGtxljkT8jG+wbCvU82nVeuvu5oBEdKsi180d2bgtmaPyuyGthUOncXOg4= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1715349892; c=relaxed/simple; bh=E4U44deHsribGfpVVd25C/wzlfyjpvU+ho5dACdmFzw=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=LLCDzoGMelLd04Vw/ajR3lWj3yCT5ozB/nLFsGdcAPn2U8YkB4KTarr/00125zCChZ4Sd69HuB/eGPvYPVNCZJxy/GSjlVONHNOoqLZOn3VJzgXQcXfvsswXJzJ/76O8IMBevfiv06w0L2pb8Xu8u9n5r8tdmXn6d0kaPD0Ut7M= ARC-Authentication-Results: i=1; smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com; spf=pass smtp.mailfrom=flex--vdonnefort.bounces.google.com; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b=MicMeQmq; arc=none smtp.client-ip=209.85.221.73 Authentication-Results: smtp.subspace.kernel.org; dmarc=pass (p=reject dis=none) header.from=google.com Authentication-Results: smtp.subspace.kernel.org; spf=pass smtp.mailfrom=flex--vdonnefort.bounces.google.com Authentication-Results: smtp.subspace.kernel.org; dkim=pass (2048-bit key) header.d=google.com header.i=@google.com header.b="MicMeQmq" Received: by mail-wr1-f73.google.com with SMTP id ffacd0b85a97d-34da03e19beso1223393f8f.1 for ; Fri, 10 May 2024 07:04:50 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1715349889; x=1715954689; darn=vger.kernel.org; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:from:to:cc:subject:date:message-id:reply-to; bh=CL/txfNgiuuPvPzsKQhjy3/tSvqVyiCfEzCcmoskOAs=; b=MicMeQmqsH6YiGJtLQho1/uvLfQJTdGbjJSZM7vqXwHLxR4Q/Vzh3Xv//q+fMIhac9 CLU0W3HRDtSxtwZrXLKarZrDLYiKPEmyDrerhoB7LqJ3oxqUOv2Gt5U6EKooQf1U70yi rVIar6kW4krj7cPTXYV5wcENlnv3zyi/mRRHA0D6HSIuSHRc+cBLtGztqaMJVZDH7oLi +ZHtIWFWvcuJSzBRjWT0GnYGp2M3ToMXTYebj6Jdfrq9jh90eI8tRCRXF7WUhtI30REz TxD8AZW5TxoxiaWCXlhwoA6ZliTqi0ZQt4nf935EnTw9JbhyWppI9hgHhjh7D2nb4+W8 DY+Q== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1715349889; x=1715954689; h=cc:to:from:subject:message-id:references:mime-version:in-reply-to :date:x-gm-message-state:from:to:cc:subject:date:message-id:reply-to; bh=CL/txfNgiuuPvPzsKQhjy3/tSvqVyiCfEzCcmoskOAs=; b=ioBmlbCrBdOFRc6QKrCmAz1bh5QO1ZtYsJQT9dJtSTaGJk+cTio+dVYtJXFWLsvS+5 91RARQj5t03seJKULFFZCJDIJmfd4XGTX/4+QG1UkL9jGwo/Zm1h2O/uGtZXjFOr5g+y p2r76fcpw5NaoBYNnplTSjKA4vx7L5ZXpnsZYgNQxgjdRzZHdsqdnn8CIl9LXfhPqcOd jG7TzkdDIfqRQjpgAVIhHcOLJrixqEB45FYp+8lhQin8oQRGJ0TRuCd4tJgiOt/jsuFL Eni8Nw9/5Fhr6OWTCn+Au7p3hFvBPE1vs3XkWoGZk5uzu6ndiHisSvGTZ4fHuuw2QBlI 0KJw== X-Forwarded-Encrypted: i=1; AJvYcCWb3SS/A+7BeK7YaGLNZONytrcbFyvpfaOU7B3wDONvWSfqM9FMUoWvT0t9+AA/647rrEovy2MTvKWo4uzNiv9r6zR8q+qDM2lv6tcn X-Gm-Message-State: AOJu0Yw6SBGjUiyqlNis+DQV7bV+Ckwf7FYG7NECKRlFDi+Q4/VMJRKv 0OtlemrAnA/l2WZbx4BOpezHGRUCRDMLo2UjMJfudsuR/Tb5KutXmHF4UVyUoXczHqFtf2z0RXz Yl3LOh9tZKJuCUoAxvA== X-Google-Smtp-Source: AGHT+IHh/kQBnPFQXEdNhHJ2omU/SZm3QBjI91vUOd11c8oIXUCoUudPXv9RKB/U/PJtgOPyCP8KiDNz8TUZAtDF X-Received: from vdonnefort.c.googlers.com ([fda3:e722:ac3:cc00:28:9cb1:c0a8:2eea]) (user=vdonnefort job=sendgmr) by 2002:a5d:55c2:0:b0:34e:23d7:3ad with SMTP id ffacd0b85a97d-3504aa62e82mr5154f8f.13.1715349889010; Fri, 10 May 2024 07:04:49 -0700 (PDT) Date: Fri, 10 May 2024 15:04:33 +0100 In-Reply-To: <20240510140435.3550353-1-vdonnefort@google.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20240510140435.3550353-1-vdonnefort@google.com> X-Mailer: git-send-email 2.45.0.118.g7fe29c98d7-goog Message-ID: <20240510140435.3550353-5-vdonnefort@google.com> Subject: [PATCH v23 4/5] Documentation: tracing: Add ring-buffer mapping From: Vincent Donnefort To: rostedt@goodmis.org, mhiramat@kernel.org, linux-kernel@vger.kernel.org, linux-trace-kernel@vger.kernel.org Cc: mathieu.desnoyers@efficios.com, kernel-team@android.com, rdunlap@infradead.org, rppt@kernel.org, david@redhat.com, Vincent Donnefort Content-Transfer-Encoding: quoted-printable Content-Type: text/plain; charset="utf-8" It is now possible to mmap() a ring-buffer to stream its content. Add some documentation and a code example. Signed-off-by: Vincent Donnefort diff --git a/Documentation/trace/index.rst b/Documentation/trace/index.rst index 5092d6c13af5..0b300901fd75 100644 --- a/Documentation/trace/index.rst +++ b/Documentation/trace/index.rst @@ -29,6 +29,7 @@ Linux Tracing Technologies timerlat-tracer intel_th ring-buffer-design + ring-buffer-map stm sys-t coresight/index diff --git a/Documentation/trace/ring-buffer-map.rst b/Documentation/trace/= ring-buffer-map.rst new file mode 100644 index 000000000000..8e296bcc0d7f --- /dev/null +++ b/Documentation/trace/ring-buffer-map.rst @@ -0,0 +1,106 @@ +.. SPDX-License-Identifier: GPL-2.0 + +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D +Tracefs ring-buffer memory mapping +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D= =3D=3D=3D=3D=3D=3D=3D=3D=3D=3D + +:Author: Vincent Donnefort + +Overview +=3D=3D=3D=3D=3D=3D=3D=3D +Tracefs ring-buffer memory map provides an efficient method to stream data +as no memory copy is necessary. The application mapping the ring-buffer be= comes +then a consumer for that ring-buffer, in a similar fashion to trace_pipe. + +Memory mapping setup +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D +The mapping works with a mmap() of the trace_pipe_raw interface. + +The first system page of the mapping contains ring-buffer statistics and +description. It is referred to as the meta-page. One of the most important +fields of the meta-page is the reader. It contains the sub-buffer ID which= can +be safely read by the mapper (see ring-buffer-design.rst). + +The meta-page is followed by all the sub-buffers, ordered by ascending ID.= It is +therefore effortless to know where the reader starts in the mapping: + +.. code-block:: c + + reader_id =3D meta->reader->id; + reader_offset =3D meta->meta_page_size + reader_id * meta->subbuf_= size; + +When the application is done with the current reader, it can get a new one= using +the trace_pipe_raw ioctl() TRACE_MMAP_IOCTL_GET_READER. This ioctl also up= dates +the meta-page fields. + +Limitations +=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D=3D +When a mapping is in place on a Tracefs ring-buffer, it is not possible to +either resize it (either by increasing the entire size of the ring-buffer = or +each subbuf). It is also not possible to use snapshot and causes splice to= copy +the ring buffer data instead of using the copyless swap from the ring buff= er. + +Concurrent readers (either another application mapping that ring-buffer or= the +kernel with trace_pipe) are allowed but not recommended. They will compete= for +the ring-buffer and the output is unpredictable, just like concurrent read= ers on +trace_pipe would be. + +Example +=3D=3D=3D=3D=3D=3D=3D + +.. code-block:: c + + #include + #include + #include + #include + + #include + + #include + #include + + #define TRACE_PIPE_RAW "/sys/kernel/tracing/per_cpu/cpu0/trace_pip= e_raw" + + int main(void) + { + int page_size =3D getpagesize(), fd, reader_id; + unsigned long meta_len, data_len; + struct trace_buffer_meta *meta; + void *map, *reader, *data; + + fd =3D open(TRACE_PIPE_RAW, O_RDONLY | O_NONBLOCK); + if (fd < 0) + exit(EXIT_FAILURE); + + map =3D mmap(NULL, page_size, PROT_READ, MAP_SHARED, fd, 0= ); + if (map =3D=3D MAP_FAILED) + exit(EXIT_FAILURE); + + meta =3D (struct trace_buffer_meta *)map; + meta_len =3D meta->meta_page_size; + + printf("entries: %llu\n", meta->entries); + printf("overrun: %llu\n", meta->overrun); + printf("read: %llu\n", meta->read); + printf("nr_subbufs: %u\n", meta->nr_subbufs); + + data_len =3D meta->subbuf_size * meta->nr_subbufs; + data =3D mmap(NULL, data_len, PROT_READ, MAP_SHARED, fd, m= eta_len); + if (data =3D=3D MAP_FAILED) + exit(EXIT_FAILURE); + + if (ioctl(fd, TRACE_MMAP_IOCTL_GET_READER) < 0) + exit(EXIT_FAILURE); + + reader_id =3D meta->reader.id; + reader =3D data + meta->subbuf_size * reader_id; + + printf("Current reader address: %p\n", reader); + + munmap(data, data_len); + munmap(meta, meta_len); + close (fd); + + return 0; + } --=20 2.45.0.118.g7fe29c98d7-goog