• Robert Richter's avatar
    oprofile: port to the new ring_buffer · 6dad828b
    Robert Richter authored
    This patch replaces the current oprofile cpu buffer implementation
    with the ring buffer provided by the tracing framework. The motivation
    here is to leave the pain of implementing ring buffers to others. Oh,
    no, there are more advantages. Main reason is the support of different
    sample sizes that could be stored in the buffer. Use cases for this
    are IBS and Cell spu profiling. Using the new ring buffer ensures
    valid and complete samples and allows copying the cpu buffer stateless
    without knowing its content. Second it will use generic kernel API and
    also reduce code size. And hopefully, there are less bugs.
    
    Since the new tracing ring buffer implementation uses spin locks to
    protect the buffer during read/write access, it is difficult to use
    the buffer in an NMI handler. In this case, writing to the buffer by
    the NMI handler (x86) could occur also during critical sections when
    reading the buffer. To avoid this, there are 2 buffers for independent
    read and write access. Read access is in process context only, write
    access only in the NMI handler. If the read buffer runs empty, both
    buffers are swapped atomically. There is potentially a small window
    during swapping where the buffers are disabled and samples could be
    lost.
    
    Using 2 buffers is a little bit overhead, but the solution is clear
    and does not require changes in the ring buffer implementation. It can
    be changed to a single buffer solution when the ring buffer access is
    implemented as non-locking atomic code.
    
    The new buffer requires more size to store the same amount of samples
    because each sample includes an u32 header. Also, there is more code
    to execute for buffer access. Nonetheless, the buffer implementation
    is proven in the ftrace environment and worth to use also in oprofile.
    
    Patches that changes the internal IBS buffer usage will follow.
    
    Cc: Steven Rostedt <rostedt@goodmis.org>
    Signed-off-by: default avatarRobert Richter <robert.richter@amd.com>
    6dad828b
cpu_buffer.c 9.21 KB