Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S932763AbcJULjz (ORCPT ); Fri, 21 Oct 2016 07:39:55 -0400 Received: from szxga03-in.huawei.com ([119.145.14.66]:17488 "EHLO szxga03-in.huawei.com" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1752307AbcJULjy (ORCPT ); Fri, 21 Oct 2016 07:39:54 -0400 From: Wang Nan To: CC: , , , , , Subject: [PATCH 2/2] perf_event_open.2: Document write_backward Date: Fri, 21 Oct 2016 11:38:13 +0000 Message-ID: <1477049893-143199-2-git-send-email-wangnan0@huawei.com> X-Mailer: git-send-email 1.8.3.4 In-Reply-To: <1477049893-143199-1-git-send-email-wangnan0@huawei.com> References: <1477049893-143199-1-git-send-email-wangnan0@huawei.com> MIME-Version: 1.0 Content-Type: text/plain X-Originating-IP: [10.107.193.248] X-CFilter-Loop: Reflected Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org Content-Length: 3889 Lines: 106 Linux 4.7 (9ecda41acb971ebd07c8fb35faf24005c0baea12) introduces write_backward attribute to perf_event_attr. Document this feature. Signed-off-by: Wang Nan Cc: Michael Kerrisk --- man2/perf_event_open.2 | 56 +++++++++++++++++++++++++++++++++++++++++++++++--- 1 file changed, 53 insertions(+), 3 deletions(-) diff --git a/man2/perf_event_open.2 b/man2/perf_event_open.2 index 2d3acad..e5fdfec 100644 --- a/man2/perf_event_open.2 +++ b/man2/perf_event_open.2 @@ -244,8 +244,8 @@ struct perf_event_attr { due to exec */ use_clockid : 1, /* use clockid for time fields */ context_switch : 1, /* context switch data */ - - __reserved_1 : 37; + write_backward : 1, /* Write ring buffer from end to beginning */ + __reserved_1 : 36; union { __u32 wakeup_events; /* wakeup every n events */ @@ -1127,6 +1127,29 @@ The advantage of this method is that it will give full information even with strict .I perf_event_paranoid settings. +.IR "write_backward" " (since Linux 4.6)" +.\" commit 9ecda41acb971ebd07c8fb35faf24005c0baea12 +This makes the resuling event use a backward ring-buffer, which +writes samples from the end of the ring-buffer. + +It is not allowed to connect events with backward and forward +ring-buffer settings together using +.B PERF_EVENT_IOC_SET_OUTPUT. + +Backward ring-buffer is useful when the ring-buffer is overwritable +(created by readonly +.BR mmap (2) +). In this case, +.IR data_tail +is useless, +.IR data_head +points to the head of the most recent sample in a backward +ring-buffer. It is easy to iterate over the whole ring-buffer by reading +samples one by one because size of a sample can be found from decoding +its header. In contract, in a forward overwritable ring-buffer, the only +information is the end of the most recent sample which is pointed by +.IR data_head, +but the size of a sample can't be determined from the end of it. .TP .IR "wakeup_events" ", " "wakeup_watermark" This union sets how many samples @@ -1671,7 +1694,9 @@ And vice versa: .TP .I data_head This points to the head of the data section. -The value continuously increases, it does not wrap. +The value continuously increases (or decrease if +.IR write_backward +is set), it does not wrap. The value needs to be manually wrapped by the size of the mmap buffer before accessing the samples. @@ -2727,6 +2752,24 @@ Starting with Linux 3.18, .B POLL_HUP is indicated if the event being monitored is attached to a different process and that process exits. +.SS Reading from overwritable ring-buffer +Reader is unable to update +.IR data_tail +if the mapping is not +.BR PROT_WRITE . +In this case, kernel will overwrite data without considering whether +they are read or not, so ring-buffer is overwritable and +behaves like a flight recorder. To read from an overwritable +ring-buffer, setting +.IR write_backward +is suggested, or it would be hard to find a proper position to start +decoding. In addition, ring-buffer should be paused before reading +through +.BR ioctl (2) +with +.B PERF_EVENT_IOC_PAUSE_OUTPUT +to avoid racing between kernel and reader. Ring-buffer should be resumed +after finish reading. .SS rdpmc instruction Starting with Linux 3.4 on x86, you can use the .\" commit c7206205d00ab375839bd6c7ddb247d600693c09 @@ -2839,6 +2882,13 @@ The file descriptors must all be on the same CPU. The argument specifies the desired file descriptor, or \-1 if output should be ignored. + +Two events with different +.IR write_backward +settings are not allowed to be connected together using +.B PERF_EVENT_IOC_SET_OUTPUT. +.B EINVAL +is returned in this case. .TP .BR PERF_EVENT_IOC_SET_FILTER " (since Linux 2.6.33)" .\" commit 6fb2915df7f0747d9044da9dbff5b46dc2e20830 -- 2.10.1