From mboxrd@z Thu Jan 1 00:00:00 1970 Return-Path: Received: (majordomo@vger.kernel.org) by vger.kernel.org via listexpand id S1758780AbcHDPGP (ORCPT ); Thu, 4 Aug 2016 11:06:15 -0400 Received: from mail.kernel.org ([198.145.29.136]:35728 "EHLO mail.kernel.org" rhost-flags-OK-OK-OK-OK) by vger.kernel.org with ESMTP id S1758719AbcHDPGK (ORCPT ); Thu, 4 Aug 2016 11:06:10 -0400 Message-Id: <20160804145708.158968389@goodmis.org> User-Agent: quilt/0.61-1 Date: Thu, 04 Aug 2016 10:57:08 -0400 From: Steven Rostedt To: linux-kernel@vger.kernel.org, linux-rt-users@vger.kernel.org Cc: Ingo Molnar , Andrew Morton , Clark Williams , Thomas Gleixner , Jon Masters , Daniel Wagner , Carsten Emde , Sebastian Andrzej Siewior Subject: [RFC][PATCH 0/3] tracing: Add Hardware Latency detector tracer Sender: linux-kernel-owner@vger.kernel.org List-ID: X-Mailing-List: linux-kernel@vger.kernel.org This adds the PREEMPT_RT hwlat detector as a Linux tracer in mainline. In the PREEMPT_RT patch set, it is a separate entity that is controlled by the debugfs file system. I found that it is better suited as a latency tracer in the tracing directory, as it follows pretty much the same paradigm as the other latency tracers that already exist. All I had to add was a hwlat_detector directory that contained a window and width for the period and duration respectively of the test. But the samples would just write to the tracing ring buffer and the max latency would be stored in tracing_max_latency, and the threshold can be set by the existing tracing_threshold. The last patch also adds a new feature that would have the kthread migrate after each period to another CPU specified by tracing_cpumask. Here's the documention that is added to hwlat_detector.txt in the Documentation directory: Introduction: ------------- The tracer hwlat_detector is a special purpose tracer that is used to detect large system latencies induced by the behavior of certain underlying hardware or firmware, independent of Linux itself. The code was developed originally to detect SMIs (System Management Interrupts) on x86 systems, however there is nothing x86 specific about this patchset. It was originally written for use by the "RT" patch since the Real Time kernel is highly latency sensitive. SMIs are not serviced by the Linux kernel, which means that it does not even know that they are occuring. SMIs are instead set up by BIOS code and are serviced by BIOS code, usually for "critical" events such as management of thermal sensors and fans. Sometimes though, SMIs are used for other tasks and those tasks can spend an inordinate amount of time in the handler (sometimes measured in milliseconds). Obviously this is a problem if you are trying to keep event service latencies down in the microsecond range. The hardware latency detector works by hogging one of the cpus for configurable amounts of time (with interrupts disabled), polling the CPU Time Stamp Counter for some period, then looking for gaps in the TSC data. Any gap indicates a time when the polling was interrupted and since the interrupts are disabled, the only thing that could do that would be an SMI or other hardware hiccup (or an NMI, but those can be tracked). Note that the hwlat detector should *NEVER* be used in a production environment. It is intended to be run manually to determine if the hardware platform has a problem with long system firmware service routines. Usage: ------ Write the ASCII text "hwlat" into the current_tracer file of the tracing system (mounted at /sys/kernel/tracing or /sys/kernel/tracing). It is possible to redefine the threshold in microseconds (us) above which latency spikes will be taken into account. Example: # echo hwlat > /sys/kernel/tracing/current_tracer # echo 100 > /sys/kernel/tracing/tracing_thresh The /sys/kernel/tracing/hwlat_detector interface contains the following files: width - time period to sample with CPUs held (usecs) must be less than the total window size (enforced) window - total period of sampling, width being inside (usecs) By default the width is set to 500,000 and window to 1,000,000, meaning that for every 1,000,000 usecs (1s) the hwlat detector will spin for 500,000 usecs (0.5s). If tracing_thresh contains zero when hwlat tracer is enabled, it will change to a default of 10 usecs. If any latencies that exceed the threshold is observed then the data will be written to the tracing ring buffer. The minimum sleep time between periods is 1 millisecond. Even if width is less than 1 millisecond apart from window, to allow the system to not be totally starved. If tracing_thresh was zero when hwlat detector was started, it will be set back to zero if another tracer is loaded. Note, the last value in tracing_thresh that hwlat detector had will be saved and this value will be restored in tracing_thresh if it is still zero when hwlat detector is started again. The following tracing directory files are used by the hwlat_detector: in /sys/kernel/tracing: tracing_threshold - minimum latency value to be considered (usecs) tracing_max_latency - maximum hardware latency actually observed (usecs) hwlat_detector/width - specified amount of time to spin within window (usecs) hwlat_detector/window - amount of time between (width) runs (usecs) -- Steve Jon Masters (1): tracing: Add documentation for hwlat_detector tracer Steven Rostedt (Red Hat) (2): tracing: Added hardware latency tracer tracing: Have hwlat trace migrate across tracing_cpumask CPUs ---- Documentation/trace/hwlat_detector.txt | 79 +++++ kernel/trace/Kconfig | 35 ++ kernel/trace/Makefile | 1 + kernel/trace/trace.c | 2 +- kernel/trace/trace.h | 3 + kernel/trace/trace_entries.h | 23 ++ kernel/trace/trace_hwlat.c | 582 +++++++++++++++++++++++++++++++++ kernel/trace/trace_output.c | 52 +++ 8 files changed, 776 insertions(+), 1 deletion(-) create mode 100644 Documentation/trace/hwlat_detector.txt create mode 100644 kernel/trace/trace_hwlat.c