From nobody Thu Dec 25 21:48:25 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 89A3A53800 for ; Thu, 11 Jan 2024 16:17:30 +0000 (UTC) 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="Yl5sYijT" Received: by mail-wr1-f73.google.com with SMTP id ffacd0b85a97d-33769c5db4aso3309908f8f.0 for ; Thu, 11 Jan 2024 08:17:30 -0800 (PST) DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=google.com; s=20230601; t=1704989849; x=1705594649; 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=XCuyvwf8Wck/dCBmVNrAbagpZQESmdweCPz4Yt1A3mA=; b=Yl5sYijTJddYPE8A0XbzsIxeUXUxr0R4yCq8ZzoKqOZLWBwT59vScwLfcMHzGaEu6o 6db7yvLqTcwCj3kmHSKnsoA7tsyBhu3XnFD0jJgCUhT17S2t6Xg3M2GPq/JTCg3rSYh1 kgO+QW8caVlNLBRh+oPED2UD5vqxTKGaMYkqUu24QuTZbEEZlf7yclpmBbDHNg63nu7U 21SDh14jgxJFTHdQ4Klav2uQNq3erZlwpA+UGLAxlJ5lm1u+BBJ/kgkV83Y+XT130706 iRF1p64/p6IipwupCJHs34EWLxrJmLjOX2QgM7855UYsqrEVGkXGoPbCUNG1Fp/Cgc0+ Pu2g== X-Google-DKIM-Signature: v=1; a=rsa-sha256; c=relaxed/relaxed; d=1e100.net; s=20230601; t=1704989849; x=1705594649; 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=XCuyvwf8Wck/dCBmVNrAbagpZQESmdweCPz4Yt1A3mA=; b=oO3mqcogYJUj6mhDwo6aIjeLzJIr+pSfw9zi5llD6IocTu9lLGUQ405mI+8a/DspE1 YLKGscHbVr6u8SxCnAlMkTguSE7Osl/PKJnMGs/OoVpMdORK9xxJcPc3fWR0iGy5VBIR wjAD2Z70Kf+1QKogScP3t13k1HT2kQr/Iz0PxN8fTNtDZ+6/vUMiNJhKvGN7h6VqZH99 VrE0wAjLSJFrZjQl9ULLZfM44maZbRF8txmQYVbeFaiRN2P6A2Ewjmpx62DjLigRneNu z+KT7rdSn00ez24Bi2qn65+KrfL6bbi65KGciqqMx16tlFVeL8LJu/i+7L7XaYOO8Oic KhQg== X-Gm-Message-State: AOJu0YzhYMrzSuY05dcKNeGaufPpxuHVTDUoahF9ic3IIjBLPMvB8MsX 3A5KichaKYTrtC0JmrOk5xjyoeV0G1e6ZX2LoWc/D20= X-Google-Smtp-Source: AGHT+IEvnBMOdjovwRbSuDDixvEjJMk5Y+pp16eUzVYNCiCUKpLMzTRtwJWKqUEgoUcb13fZcHQcweiUd7Zg6Ybm X-Received: from vdonnefort.c.googlers.com ([fda3:e722:ac3:cc00:28:9cb1:c0a8:2eea]) (user=vdonnefort job=sendgmr) by 2002:a05:6000:618:b0:337:8f60:972e with SMTP id bn24-20020a056000061800b003378f60972emr1263wrb.3.1704989848845; Thu, 11 Jan 2024 08:17:28 -0800 (PST) Date: Thu, 11 Jan 2024 16:17:11 +0000 In-Reply-To: <20240111161712.1480333-1-vdonnefort@google.com> Precedence: bulk X-Mailing-List: linux-kernel@vger.kernel.org List-Id: List-Subscribe: List-Unsubscribe: Mime-Version: 1.0 References: <20240111161712.1480333-1-vdonnefort@google.com> X-Mailer: git-send-email 2.43.0.275.g3460e3d667-goog Message-ID: <20240111161712.1480333-5-vdonnefort@google.com> Subject: [PATCH v11 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..2ba7b5339178 --- /dev/null +++ b/Documentation/trace/ring-buffer-map.rst @@ -0,0 +1,105 @@ +.. 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 subbuf ID which can be safely= read +by the mapper (see ring-buffer-design.rst). + +The meta-page is followed by all the subbuf, 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); + 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: %lu\n", meta->entries); + printf("overrun: %lu\n", meta->overrun); + printf("read: %lu\n", meta->read); + printf("subbufs_touched:%lu\n", meta->subbufs_touched); + printf("subbufs_lost: %lu\n", meta->subbufs_lost); + printf("subbufs_read: %lu\n", meta->subbufs_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, d= ata_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; + + munmap(data, data_len); + munmap(meta, meta_len); + close (fd); + + return 0; + } --=20 2.43.0.275.g3460e3d667-goog