From 9ca8dbcc65cfc63d6f5ef3312a33184e1d726e00 Mon Sep 17 00:00:00 2001 From: Yunhong Jiang Date: Tue, 4 Aug 2015 12:17:53 -0700 Subject: Add the rt linux 4.1.3-rt3 as base Import the rt linux 4.1.3-rt3 as OPNFV kvm base. It's from git://git.kernel.org/pub/scm/linux/kernel/git/rt/linux-rt-devel.git linux-4.1.y-rt and the base is: commit 0917f823c59692d751951bf5ea699a2d1e2f26a2 Author: Sebastian Andrzej Siewior Date: Sat Jul 25 12:13:34 2015 +0200 Prepare v4.1.3-rt3 Signed-off-by: Sebastian Andrzej Siewior We lose all the git history this way and it's not good. We should apply another opnfv project repo in future. Change-Id: I87543d81c9df70d99c5001fbdf646b202c19f423 Signed-off-by: Yunhong Jiang --- kernel/Documentation/hwlat_detector.txt | 64 +++++++++++++++++++++++++++++++++ 1 file changed, 64 insertions(+) create mode 100644 kernel/Documentation/hwlat_detector.txt (limited to 'kernel/Documentation/hwlat_detector.txt') diff --git a/kernel/Documentation/hwlat_detector.txt b/kernel/Documentation/hwlat_detector.txt new file mode 100644 index 000000000..cb6151648 --- /dev/null +++ b/kernel/Documentation/hwlat_detector.txt @@ -0,0 +1,64 @@ +Introduction: +------------- + +The module hwlat_detector is a special purpose kernel module 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 usually not serviced by the Linux kernel, which typically does not +even know that they are occuring. SMIs are instead are 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 all of the cpus for configurable +amounts of time (by calling stop_machine()), 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 machine is stopped and +interrupts turned off the only thing that could do that would be an SMI. + +Note that the SMI 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: +------ + +Loading the module hwlat_detector passing the parameter "enabled=1" (or by +setting the "enable" entry in "hwlat_detector" debugfs toggled on) is the only +step required to start the hwlat_detector. It is possible to redefine the +threshold in microseconds (us) above which latency spikes will be taken +into account (parameter "threshold="). + +Example: + + # modprobe hwlat_detector enabled=1 threshold=100 + +After the module is loaded, it creates a directory named "hwlat_detector" under +the debugfs mountpoint, "/debug/hwlat_detector" for this text. It is necessary +to have debugfs mounted, which might be on /sys/debug on your system. + +The /debug/hwlat_detector interface contains the following files: + +count - number of latency spikes observed since last reset +enable - a global enable/disable toggle (0/1), resets count +max - maximum hardware latency actually observed (usecs) +sample - a pipe from which to read current raw sample data + in the format + (can be opened O_NONBLOCK for a single sample) +threshold - minimum latency value to be considered (usecs) +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 we will set width to 500,000 and window to 1,000,000, meaning that +we will sample every 1,000,000 usecs (1s) for 500,000 usecs (0.5s). If we +observe any latencies that exceed the threshold (initially 100 usecs), +then we write to a global sample ring buffer of 8K samples, which is +consumed by reading from the "sample" (pipe) debugfs file interface. -- cgit 1.2.3-korg