From nobody Mon Feb 9 19:05:16 2026 Received: from mail-yb1-f201.google.com (mail-yb1-f201.google.com [209.85.219.201]) (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 937B06BFA9 for ; Tue, 26 Mar 2024 10:04:36 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.219.201 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1711447478; cv=none; b=iBDA01vi4NS6Tgn1GeWlddL4bvnQeY0lNNZrQV+Cj17aq+r/IamtwNPA0X/Xv4F6LAmJMZMqyH3c1CWn62u+DgTIfTj2W/d7lp212eHdyFVpfgMa+MqTK8t73m/XOwOUfe/eDAg2iqXlalmHjEBKbJgN1zEouLoDPBxNtSH6J4w= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1711447478; c=relaxed/simple; bh=f5mervW7JwMqtuacV5u33z0c+ZkKALYygYqRjTTFdEY=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=YbiWdITHw9e36RRGyg2d9yNE17TIlRLudIVR0/msZQJhhamENPOXkbaQez3QyrnVyc/3wHZNuVX9LvZ6gOuWnx0SuYvhAWMG6GJ6uh3FWhZKZT9Vc3Nk9g9maXe8c3OK6JVWPt8snaA7aZnN2HvC869EgRMOHO5L0DzOq85T83c= 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=I9P26f9W; arc=none smtp.client-ip=209.85.219.201 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="I9P26f9W" Received: by mail-yb1-f201.google.com with SMTP id 3f1490d57ef6-dc6b2682870so8589795276.0 for ; Tue, 26 Mar 2024 03:04:36 -0700 (PDT) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1711447475; x=1712052275; 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=VyUYUDwDC6lCJYWgpyLnykCyTck5nvq4IGoYsSlman4=; b=I9P26f9W42M0Q9qCUHOi+9Ef9/bSl7r6+9Tucr3kmK4y8/1daL9U8nRr8R7cXbJdSF UGE6q140BC/F2Mw+cYfJKh7T7m7Ox/eUTaEgB8hqyHv1MXojtfgjVqYcePEKta/rTKzU g8uE74lznnzvWH9V/SnLRWt7OPYmWPaihb9YUmElsRCUaXvKgrFUFqCKGMvNr9f6KAPC vxYKqG1ccRgD3va2YkUeLTo/oxuRZ5ulHQF0XxAKfL/DnguGeV/+LrSfRF6pkycRneNa ti/2K1OMHYGhGvCx5tHhzAFCWWKgfUFDG6QKMjKLp/s2OJRtMP9+0l1csfUhyKajZmcr qEew== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1711447475; x=1712052275; 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=VyUYUDwDC6lCJYWgpyLnykCyTck5nvq4IGoYsSlman4=; b=OP0AfBbhgB1HkRdTJy85Z48cRS1hZ/SU7oxEO4jsREKghm1AOqQT9cI8NDY0u2tZj0 3WNe39XfJvbOQVpwJL9wsktSwW7Rjp8XeBSjI+WBigXtgQJblxXkhFO+MXxh6/nqtWqa poiexF1FtWrKMQpNUUV1AXNMlrXCVv+CI7f4aTjB/2wnCWpwQlHwiE9XS1w+IfM/aCyf 10YMRRdHlyS3jGYdryyIUEGULbn2T9zQ5wtMK87OSpxSmXt+92F4TxI+qMnydWXnC+sd rC3LwitzYF8rC3uOI/z3IEXZ4jTCtHWPwY3OZS7pc0x6OUjatMvlpsbsCd+CmhuFXF23 vQdw== X-Forwarded-Encrypted: i=1; AJvYcCX1loEHXPDVvUR690/UQyCi9dtCrnTbxwl43d5iNJ/mbvjdNCa5CFwPaQoE6nYJar8fubtaEEGp2J8wq4R0HnfzASeEBu3/xTG/He5i X-Gm-Message-State: AOJu0Yw4D6vCCmrufjkMygzbFrG6FS85FR1NDtvGC+yIlRegta/JKb8F GanPA/iy49TtPCisQJTeBCtcKOHKBCsKZ2JPpWF0z7w/r7EDygQFBNsbH+ktkwcrDue1DGRS78o 7E1x+uBwFc/wVhMkIEQ== X-Google-Smtp-Source: AGHT+IHgLUq0j1df9+PiiUajP5L4I97m2lIkGUW6cHJevFr7Mnz8hYtJ9sxrR9ygJqZl3MwodQNWLN7LOMP6I6jv X-Received: from vdonnefort.c.googlers.com ([fda3:e722:ac3:cc00:28:9cb1:c0a8:2eea]) (user=vdonnefort job=sendgmr) by 2002:a05:6902:2503:b0:dc7:5c0d:f177 with SMTP id dt3-20020a056902250300b00dc75c0df177mr2934431ybb.6.1711447475702; Tue, 26 Mar 2024 03:04:35 -0700 (PDT) Date: Tue, 26 Mar 2024 10:03:29 +0000 In-Reply-To: <20240326100330.1321861-1-vdonnefort@google.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20240326100330.1321861-1-vdonnefort@google.com> X-Mailer: git-send-email 2.44.0.396.g6e790dbe36-goog Message-ID: <20240326100330.1321861-5-vdonnefort@google.com> Subject: [PATCH v19 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, 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..0426ab4bcf3d --- /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 as the meta-page. One of the most important fi= eld of +the meta-page is the reader. It contains the sub-buffer ID which can be sa= fely +read by the mapper (see ring-buffer-design.rst). + +The meta-page is followed by all the sub-buffers, ordered by ascendant 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 2.44.0.396.g6e790dbe36-goog