kernel: add HWLAT_TRACER kernel config option

This adds the HWLAT_TRACER kernel configuration option. The hwlat tracer
allows to detect hardware latencies.

Signed-off-by: Thomas Richard <thomas.richard@bootlin.com>
Link: https://github.com/openwrt/openwrt/pull/21413
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>

1 files changed, 35 insertions, 0 deletions

diff --git a/config/Config-kernel.in b/config/Config-kernel.in
index 870e53206a..86d99bd49b 100644
--- a/config/Config-kernel.in
+++ b/config/Config-kernel.in
@@ -413,6 +413,41 @@ config KERNEL_PREEMPT_TRACER
 	  enabled. This option and the irqs-off timing option can be
 	  used together or separately.)
 
+config KERNEL_HWLAT_TRACER
+	bool "Tracer to detect hardware latencies (like SMIs)"
+	depends on KERNEL_FTRACE
+	help
+	  This tracer, when enabled will create one or more kernel threads,
+	  depending on what the cpumask file is set to, which each thread
+	  spinning in a loop looking for interruptions caused by
+	  something other than the kernel. For example, if a
+	  System Management Interrupt (SMI) takes a noticeable amount of
+	  time, this tracer will detect it. This is useful for testing
+	  if a system is reliable for Real Time tasks.
+
+	  Some files are created in the tracing directory when this
+	  is enabled:
+
+	    hwlat_detector/width   - time in usecs for how long to spin for
+	    hwlat_detector/window  - time in usecs between the start of each
+				     iteration
+
+	  A kernel thread is created that will spin with interrupts disabled
+	  for "width" microseconds in every "window" cycle. It will not spin
+	  for "window - width" microseconds, where the system can
+	  continue to operate.
+
+	  The output will appear in the trace and trace_pipe files.
+
+	  When the tracer is not running, it has no affect on the system,
+	  but when it is running, it can cause the system to be
+	  periodically non responsive. Do not run this tracer on a
+	  production system.
+
+	  To enable this tracer, echo in "hwlat" into the current_tracer
+	  file. Every time a latency is greater than tracing_thresh, it will
+	  be recorded into the ring buffer.
+
 config KERNEL_HIST_TRIGGERS
 	bool "Histogram triggers"
 	depends on KERNEL_FTRACE