• Steven Rostedt (VMware)'s avatar
    tracing: Create a sparse bitmask for pid filtering · 8d6e9098
    Steven Rostedt (VMware) authored
    When the trace_pid_list was created, the default pid max was 32768.
    Creating a bitmask that can hold one bit for all 32768 took up 4096 (one
    page). Having a one page bitmask was not much of a problem, and that was
    used for mapping pids. But today, systems are bigger and can run more
    tasks, and now the default pid_max is usually set to 4194304. Which means
    to handle that many pids requires 524288 bytes. Worse yet, the pid_max can
    be set to 2^30 (1073741824 or 1G) which would take 134217728 (128M) of
    memory to store this array.
    
    Since the pid_list array is very sparsely populated, it is a huge waste of
    memory to store all possible bits for each pid when most will not be set.
    
    Instead, use a page table scheme to store the array, and allow this to
    handle up to 30 bit pids.
    
    The pid_mask will start out with 256 entries for the first 8 MSB bits.
    This will cost 1K for 32 bit architectures and 2K for 64 bit. Each of
    these will have a 256 array to store the next 8 bits of the pid (another
    1 or 2K). These will hold an 2K byte bitmask (which will cover the LSB
    14 bits or 16384 pids).
    
    When the trace_pid_list is allocated, it will have the 1/2K upper bits
    allocated, and then it will allocate a cache for the next upper chunks and
    the lower chunks (default 6 of each). Then when a bit is "set", these
    chunks will be pulled from the free list and added to the array. If the
    free list gets down to a lever (default 2), it will trigger an irqwork
    that will refill the cache back up.
    
    On clearing a bit, if the clear causes the bitmask to be zero, that chunk
    will then be placed back into the free cache for later use, keeping the
    need to allocate more down to a minimum.
    Signed-off-by: default avatarSteven Rostedt (VMware) <rostedt@goodmis.org>
    8d6e9098
pid_list.c 11.9 KB