• Viktor Rosendahl (BMW)'s avatar
    ftrace: Implement fs notification for tracing_max_latency · 91edde2e
    Viktor Rosendahl (BMW) authored
    This patch implements the feature that the tracing_max_latency file,
    e.g. /sys/kernel/debug/tracing/tracing_max_latency will receive
    notifications through the fsnotify framework when a new latency is
    available.
    
    One particularly interesting use of this facility is when enabling
    threshold tracing, through /sys/kernel/debug/tracing/tracing_thresh,
    together with the preempt/irqsoff tracers. This makes it possible to
    implement a user space program that can, with equal probability,
    obtain traces of latencies that occur immediately after each other in
    spite of the fact that the preempt/irqsoff tracers operate in overwrite
    mode.
    
    This facility works with the hwlat, preempt/irqsoff, and wakeup
    tracers.
    
    The tracers may call the latency_fsnotify() from places such as
    __schedule() or do_idle(); this makes it impossible to call
    queue_work() directly without risking a deadlock. The same would
    happen with a softirq,  kernel thread or tasklet. For this reason we
    use the irq_work mechanism to call queue_work().
    
    This patch creates a new workqueue. The reason for doing this is that
    I wanted to use the WQ_UNBOUND and WQ_HIGHPRI flags.  My thinking was
    that WQ_UNBOUND might help with the latency in some important cases.
    
    If we use:
    
    queue_work(system_highpri_wq, &tr->fsnotify_work);
    
    then the work will (almost) always execute on the same CPU but if we are
    unlucky that CPU could be too busy while there could be another CPU in
    the system that would be able to process the work soon enough.
    
    queue_work_on() could be used to queue the work on another CPU but it
    seems difficult to select the right CPU.
    
    Link: http://lkml.kernel.org/r/20191008220824.7911-2-viktor.rosendahl@gmail.comReviewed-by: default avatarJoel Fernandes (Google) <joel@joelfernandes.org>
    Signed-off-by: default avatarViktor Rosendahl (BMW) <viktor.rosendahl@gmail.com>
    [ Added max() to have one compare for max latency ]
    Signed-off-by: default avatarSteven Rostedt (VMware) <rostedt@goodmis.org>
    91edde2e
trace.h 63.1 KB