From patchwork Fri Feb 9 10:22:01 2024 Content-Type: text/plain; charset="utf-8" MIME-Version: 1.0 Content-Transfer-Encoding: 7bit X-Patchwork-Submitter: Vincent Donnefort X-Patchwork-Id: 13551029 Received: from mail-yw1-f202.google.com (mail-yw1-f202.google.com [209.85.128.202]) (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 377AF6A8BF for ; Fri, 9 Feb 2024 10:22:24 +0000 (UTC) Authentication-Results: smtp.subspace.kernel.org; arc=none smtp.client-ip=209.85.128.202 ARC-Seal: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1707474145; cv=none; b=LnMk63EZutu3dQmjb5sf8uaZkiUhqY6QHOiS0aTPSmsQTmDGvUw/OC0UmeeEJzMV+pI2S6/U48wEUVGC5N6FcAarE/y+LwH9lueLEAGkRX4vUgfCUx1an7KlK8WOjhAbnip5mvP0EfRsKfOtROgHWn+bC6xlVvtbv8FZppx/y/w= ARC-Message-Signature: i=1; a=rsa-sha256; d=subspace.kernel.org; s=arc-20240116; t=1707474145; c=relaxed/simple; bh=iPiTM2h7tTVjI8Bap8UHkeVYN1GEEKgHI982louZIPs=; h=Date:In-Reply-To:Mime-Version:References:Message-ID:Subject:From: To:Cc:Content-Type; b=hg2XmOm5G1eAj6/XRfgemh6Vf+486gDqklapMfFuI7Cfi2ZyTN1T4J4FJht6qNnfqOCmKMfbeYhK9knsBT8c8VxTPY3Jj1a19C341hO0dlLO2yVXn78fi9ydFdMb8O98MST7ag132tlcPl/MzSKIA/lhbMc4A1g4xihSM6a39f8= 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=tBWat87h; arc=none smtp.client-ip=209.85.128.202 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="tBWat87h" Received: by mail-yw1-f202.google.com with SMTP id 00721157ae682-604ad18981eso18249217b3.2 for ; Fri, 09 Feb 2024 02:22:24 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1707474143; x=1708078943; 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=QgkRDZd+PHk3bo7bl2fMfaI4+BE0z0Hk4GDGmCozprQ=; b=tBWat87hkNnH5xslKZXl0zV+dOOXPYB3JLZcZlutDQ2hCgQWBAjt5k6a8IvVL/PNmv 1tF5owEVNy0DVEVUFM9prOp5ZPWC2CYMdydy04bKfdg/rGSJEbnuaE+zXYAL/FkUjdSl C3LSNIFP9I+idO6O8bDG02JunEtfLCTtNKlE7bWS8aFgTMd45RjgWN4TLNHVoQ0j4QI4 fXAp9g75pCI4vgEH+9k6RF+zsPdZ0wEaqD4ZivrcaW9LqAMZvKw6eJUDgJd9Bwl0oQXg 5ii4feaV8B7gZ8VyrGJ2UqsNtJdAhuM9WBAB9KhzPnCPGGH6MlCcXqH2Pr0PX3HvHXZP iFag== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1707474143; x=1708078943; 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=QgkRDZd+PHk3bo7bl2fMfaI4+BE0z0Hk4GDGmCozprQ=; b=m5JjsqXG96D6XZ00vUERIgLeOVi9pDbfFh37HBKBOfkC17cYp8YX+iKeXJKiIiSjlM Ghv116XNGg34Gl2zIOIZFh6IWn/9QKdMUD1P3woa6GTo0yOlXrOfM6K2nxX07DSutLvL jKico7i8DwTmHcX/WdosU2kS+wwe4rEjS3WIj+J3XDuPWtfhpj9pqiMf8tsftdeiBPYl 70GUOEUinaBIGBq+KK/d40b/YKDEZGYHql7832FjWzKGrJhEGs3VrVH6C0iESKmGHF65 B/MwPcsINb73d8NTvsTY7WJmPTt9YziIdfdHfcT0x8FiRKaAjgWcilU8fq6CsElDcYHH H/zw== X-Gm-Message-State: AOJu0YxjDMGZWj6/Q4aw8tyt5oLZTGqkLKJPLZiNsZsLhegMVQV9rF3n oMLKSqwALgO+odg7Bdy7L7RGShMmxrpRUwxQtdesBqULOCpDxdhxFX9LD2wUdzYQiH8Lo/JJgpA TkA7M8gYHjT+Wzr4XSw== X-Google-Smtp-Source: AGHT+IGMS/FEXVJSaP5ycPnLKBn0cDzbJwah//e10Y95e1xJ+MuEsR37x1mSyHmJmnid1bxct4QemhKZc9MzLa1Q X-Received: from vdonnefort.c.googlers.com ([fda3:e722:ac3:cc00:28:9cb1:c0a8:2eea]) (user=vdonnefort job=sendgmr) by 2002:a05:690c:3687:b0:604:9cad:8c36 with SMTP id fu7-20020a05690c368700b006049cad8c36mr275510ywb.4.1707474143306; Fri, 09 Feb 2024 02:22:23 -0800 (PST) Date: Fri, 9 Feb 2024 10:22:01 +0000 In-Reply-To: <20240209102202.649049-1-vdonnefort@google.com> Precedence: bulk X-Mailing-List: linux-trace-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20240209102202.649049-1-vdonnefort@google.com> X-Mailer: git-send-email 2.43.0.687.g38aa6559b0-goog Message-ID: <20240209102202.649049-6-vdonnefort@google.com> Subject: [PATCH v15 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 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 + +================================== +Tracefs ring-buffer memory mapping +================================== + +:Author: Vincent Donnefort + +Overview +======== +Tracefs ring-buffer memory map provides an efficient method to stream data +as no memory copy is necessary. The application mapping the ring-buffer becomes +then a consumer for that ring-buffer, in a similar fashion to trace_pipe. + +Memory mapping setup +==================== +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 field 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 ascendant ID. It is +therefore effortless to know where the reader starts in the mapping: + +.. code-block:: c + + reader_id = meta->reader->id; + reader_offset = 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 updates +the meta-page fields. + +Limitations +=========== +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 +======= + +.. code-block:: c + + #include + #include + #include + #include + + #include + + #include + #include + + #define TRACE_PIPE_RAW "/sys/kernel/tracing/per_cpu/cpu0/trace_pipe_raw" + + int main(void) + { + int page_size = getpagesize(), fd, reader_id; + unsigned long meta_len, data_len; + struct trace_buffer_meta *meta; + void *map, *reader, *data; + + fd = open(TRACE_PIPE_RAW, O_RDONLY | O_NONBLOCK); + if (fd < 0) + exit(EXIT_FAILURE); + + map = mmap(NULL, page_size, PROT_READ, MAP_SHARED, fd, 0); + if (map == MAP_FAILED) + exit(EXIT_FAILURE); + + meta = (struct trace_buffer_meta *)map; + meta_len = 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 = meta->subbuf_size * meta->nr_subbufs; + data = mmap(NULL, data_len, PROT_READ, MAP_SHARED, fd, meta_len); + if (data == MAP_FAILED) + exit(EXIT_FAILURE); + + if (ioctl(fd, TRACE_MMAP_IOCTL_GET_READER) < 0) + exit(EXIT_FAILURE); + + reader_id = meta->reader.id; + reader = 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; + }