• Omar Sandoval's avatar
    blk-stat: convert to callback-based statistics reporting · 34dbad5d
    Omar Sandoval authored
    Currently, statistics are gathered in ~0.13s windows, and users grab the
    statistics whenever they need them. This is not ideal for both in-tree
    1. Writeback throttling wants its own dynamically sized window of
       statistics. Since the blk-stats statistics are reset after every
       window and the wbt windows don't line up with the blk-stats windows,
       wbt doesn't see every I/O.
    2. Polling currently grabs the statistics on every I/O. Again, depending
       on how the window lines up, we may miss some I/Os. It's also
       unnecessary overhead to get the statistics on every I/O; the hybrid
       polling heuristic would be just as happy with the statistics from the
       previous full window.
    This reworks the blk-stats infrastructure to be callback-based: users
    register a callback that they want called at a given time with all of
    the statistics from the window during which the callback was active.
    Users can dynamically bucketize the statistics. wbt and polling both
    currently use read vs. write, but polling can be extended to further
    subdivide based on request size.
    The callbacks are kept on an RCU list, and each callback has percpu
    stats buffers. There will only be a few users, so the overhead on the
    I/O completion side is low. The stats flushing is also simplified
    considerably: since the timer function is responsible for clearing the
    statistics, we don't have to worry about stale statistics.
    wbt is a trivial conversion. After the conversion, the windowing problem
    mentioned above is fixed.
    For polling, we register an extra callback that caches the previous
    window's statistics in the struct request_queue for the hybrid polling
    heuristic to use.
    Since we no longer have a single stats buffer for the request queue,
    this also removes the sysfs and debugfs stats entries. To replace those,
    we add a debugfs entry for the poll statistics.
    Signed-off-by: default avatarOmar Sandoval <osandov@fb.com>
    Signed-off-by: default avatarJens Axboe <axboe@fb.com>
blk-sysfs.c 22.7 KB