• David Jeffery's avatar
    sbitmap: only queue kyber's wait callback if not already active · df034c93
    David Jeffery authored
    Under heavy loads where the kyber I/O scheduler hits the token limits for
    its scheduling domains, kyber can become stuck.  When active requests
    complete, kyber may not be woken up leaving the I/O requests in kyber
    stuck.
    
    This stuck state is due to a race condition with kyber and the sbitmap
    functions it uses to run a callback when enough requests have completed.
    The running of a sbt_wait callback can race with the attempt to insert the
    sbt_wait.  Since sbitmap_del_wait_queue removes the sbt_wait from the list
    first then sets the sbq field to NULL, kyber can see the item as not on a
    list but the call to sbitmap_add_wait_queue will see sbq as non-NULL. This
    results in the sbt_wait being inserted onto the wait list but ws_active
    doesn't get incremented.  So the sbitmap queue does not know there is a
    waiter on a wait list.
    
    Since sbitmap doesn't think there is a waiter, kyber may never be
    informed that there are domain tokens available and the I/O never advances.
    With the sbt_wait on a wait list, kyber believes it has an active waiter
    so cannot insert a new waiter when reaching the domain's full state.
    
    This race can be fixed by only adding the sbt_wait to the queue if the
    sbq field is NULL.  If sbq is not NULL, there is already an action active
    which will trigger the re-running of kyber.  Let it run and add the
    sbt_wait to the wait list if still needing to wait.
    Reviewed-by: default avatarOmar Sandoval <osandov@fb.com>
    Signed-off-by: default avatarDavid Jeffery <djeffery@redhat.com>
    Reported-by: default avatarJohn Pittman <jpittman@redhat.com>
    Tested-by: default avatarJohn Pittman <jpittman@redhat.com>
    Signed-off-by: default avatarJens Axboe <axboe@kernel.dk>
    df034c93
sbitmap.c 16.3 KB