From nobody Wed Dec 24 03:34:46 2025 Received: from mail-wm1-f73.google.com (mail-wm1-f73.google.com [209.85.128.73]) (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 C858312BF03 for ; Mon, 29 Jan 2024 14:28:23 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.128.73 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1706538505; cv=none; b=djmaQMLTYWbcK5+AXBV8m8i7lPI6IlF09E7HYXlq0f4Iuk6FlXA6NQBu0l4Iyvvfv288Snig1PTI+//3H04YB7GO6uYNmrEjd9lhaTw8WH5W4RJnYqG/emOAcgCy8H691u8Dg9G03hBNRjz5CK6P01xv03hTU688A7JUQuhM0+c= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1706538505; c=relaxed/simple; bh=YblHfJ3M/JrJf9iYDXRUo2YdNQnwLH+/GIs3bb4ORz8=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=Hv2m9k6nfKlPMq2Bbf3920enRbWscG3AL/a8Df1Q3tPDJwINzXVEv3voLcYRIwih/rBYAzOQxwMnw07zgbqfpA7pT8OtJVEMySczpXV6qyLJmhMplpZql/DTq1P0WH0pJjP13XkmliRh4+V3QUP4Pz4WhWwkZVeWisSKR+gXY/8= 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=2lm6NGiP; arc=none smtp.client-ip=209.85.128.73 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="2lm6NGiP" Received: by mail-wm1-f73.google.com with SMTP id 5b1f17b1804b1-40e412c1e65so24718865e9.2 for ; Mon, 29 Jan 2024 06:28:23 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1706538502; x=1707143302; 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=JjOgbZd7O2uNB62/0XeQH/S8Gkl55RM7l7WUxXBvFwY=; b=2lm6NGiPCM13+AFsH9VVJF5xhx8d0cEVTjEQAI5riITQsb/NhpeGsaDX4JP3em1V5W KuSb3Z80jfIF1UOoL5r5Xl/hYspJlZB7d9Iq43nZMpWs9suCogWkkeyMHmaxozF4rC9F ZcpMHIts/CWMkf1Ed9mO2GeCbr7WgQjK4J9UUcYeoddnTzUiI3EseRLuQg7a4cUwlxsp 0y5/ujrJqBoZfXc806GuRwXg9JLzxFf4M7Kr9tBzJ4znw1BtdRV3+vdwstunamo4OwMd P1nQy38vQcUBh/mgfoWB7lkV+c+lVJk/kTV8r+W5ZCoZ3dfryElGlx3lLc+vBzBWTeVN b3QA== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1706538502; x=1707143302; 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=JjOgbZd7O2uNB62/0XeQH/S8Gkl55RM7l7WUxXBvFwY=; b=uVuN+S0z9ZWfjT8oXv06rYmArsHqfYM7oF7YkKoSr7ZZNEJ8Tgll91DuY1wA/rIE/t PZA/5yFettwc6lPgnGOfQPow2ScVJs94wQDEkVmjo4v9oEhQsoZKHJ9J1DMVnifqNFOk 7bDC7AkGVZyRUl+X6a9nlh9DGWfk+RYfn+2aAhjGnbmAWz/KWwyr30eosqKvcXo1JquH lZOzXTWq/8BEZn7zINU3DQWmXyA/j4S9w/OSX7cROqhWpro61WEVg6iLDtmVlcFlnMP6 9zeXAo4vf91Uuq6PM14v3tDAT5x7gHXdy2ss4hiLkPEcJsv6YtlVfsifHMmaBaKGR5/p n51A== X-Gm-Message-State: AOJu0Yxz0v/LqMWG6Vy4XNnP4Rmc1Q/YbbsM1+iDDEzQKvHNfu5ub+f+ bsS+vagJ+X+BUmMtA3NmeUkkNYZzqEgp4vgJfmnbGMb0AuMBVIQx8m5lYwyTiVjOytMU+Lsn8C6 mvGhNqFNeF+F0dEdq1w== X-Google-Smtp-Source: AGHT+IGt5kmwQnjlcwjJQrN6BwzRxHTZ56mdRpquTuZas1fv6cw9q59q8AGqGw9UdLm/FBc0k5uTfl9zGj28OWNo X-Received: from vdonnefort.c.googlers.com ([fda3:e722:ac3:cc00:28:9cb1:c0a8:2eea]) (user=vdonnefort job=sendgmr) by 2002:a05:600c:3d10:b0:40e:e994:eb79 with SMTP id bh16-20020a05600c3d1000b0040ee994eb79mr26477wmb.0.1706538501926; Mon, 29 Jan 2024 06:28:21 -0800 (PST) Date: Mon, 29 Jan 2024 14:28:01 +0000 In-Reply-To: <20240129142802.2145305-1-vdonnefort@google.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20240129142802.2145305-1-vdonnefort@google.com> X-Mailer: git-send-email 2.43.0.429.g432eaa2c6b-goog Message-ID: <20240129142802.2145305-6-vdonnefort@google.com> Subject: [PATCH v13 5/6] 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..628254e63830 --- /dev/null +++ b/Documentation/trace/ring-buffer-map.rst @@ -0,0 +1,104 @@ +.. 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 or splice. + +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. + +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 2.43.0.429.g432eaa2c6b-goog