• S
    ring-buffer: buffer record on/off switch · a3583244
    Steven Rostedt 提交于
    Impact: enable/disable ring buffer recording API added
    
    Several kernel developers have requested that there be a way to stop
    recording into the ring buffers with a simple switch that can also
    be enabled from userspace. This patch addes a new kernel API to the
    ring buffers called:
    
     tracing_on()
     tracing_off()
    
    When tracing_off() is called, all ring buffers will not be able to record
    into their buffers.
    
    tracing_on() will enable the ring buffers again.
    
    These two act like an on/off switch. That is, there is no counting of the
    number of times tracing_off or tracing_on has been called.
    
    A new file is added to the debugfs/tracing directory called
    
      tracing_on
    
    This allows for userspace applications to also flip the switch.
    
      echo 0 > debugfs/tracing/tracing_on
    
    disables the tracing.
    
      echo 1 > /debugfs/tracing/tracing_on
    
    enables it.
    
    Note, this does not disable or enable any tracers. It only sets or clears
    a flag that needs to be set in order for the ring buffers to write to
    their buffers. It is a global flag, and affects all ring buffers.
    
    The buffers start out with tracing_on enabled.
    
    There are now three flags that control recording into the buffers:
    
     tracing_on: which affects all ring buffer tracers.
    
     buffer->record_disabled: which affects an allocated buffer, which may be set
         if an anomaly is detected, and tracing is disabled.
    
     cpu_buffer->record_disabled: which is set by tracing_stop() or if an
         anomaly is detected. tracing_start can not reenable this if
         an anomaly occurred.
    
    The userspace debugfs/tracing/tracing_enabled is implemented with
    tracing_stop() but the user space code can not enable it if the kernel
    called tracing_stop().
    
    Userspace can enable the tracing_on even if the kernel disabled it.
    It is just a switch used to stop tracing if a condition was hit.
    tracing_on is not for protecting critical areas in the kernel nor is
    it for stopping tracing if an anomaly occurred. This is because userspace
    can reenable it at any time.
    
    Side effect: With this patch, I discovered a dead variable in ftrace.c
      called tracing_on. This patch removes it.
    Signed-off-by: NSteven Rostedt <srostedt@redhat.com>
    a3583244
ring_buffer.h 3.8 KB