• Steven Rostedt (Red Hat)'s avatar
    tracing: Added hardware latency tracer · e7c15cd8
    Steven Rostedt (Red Hat) authored
    The hardware latency tracer has been in the PREEMPT_RT patch for some time.
    It is used to detect possible SMIs or any other hardware interruptions that
    the kernel is unaware of. Note, NMIs may also be detected, but that may be
    good to note as well.
    
    The logic is pretty simple. It simply creates a thread that spins on a
    single CPU for a specified amount of time (width) within a periodic window
    (window). These numbers may be adjusted by their cooresponding names in
    
       /sys/kernel/tracing/hwlat_detector/
    
    The defaults are window = 1000000 us (1 second)
                     width  =  500000 us (1/2 second)
    
    The loop consists of:
    
    	t1 = trace_clock_local();
    	t2 = trace_clock_local();
    
    Where trace_clock_local() is a variant of sched_clock().
    
    The difference of t2 - t1 is recorded as the "inner" timestamp and also the
    timestamp  t1 - prev_t2 is recorded as the "outer" timestamp. If either of
    these differences are greater than the time denoted in
    /sys/kernel/tracing/tracing_thresh then it records the event.
    
    When this tracer is started, and tracing_thresh is zero, it changes to the
    default threshold of 10 us.
    
    The hwlat tracer in the PREEMPT_RT patch was originally written by
    Jon Masters. I have modified it quite a bit and turned it into a
    tracer.
    Based-on-code-by: default avatarJon Masters <jcm@redhat.com>
    Signed-off-by: default avatarSteven Rostedt <rostedt@goodmis.org>
    e7c15cd8
trace_hwlat.c 14.2 KB