• NeilBrown's avatar
    block: trace completion of all bios. · fbbaf700
    NeilBrown authored
    Currently only dm and md/raid5 bios trigger
    trace_block_bio_complete().  Now that we have bio_chain() and
    bio_inc_remaining(), it is not possible, in general, for a driver to
    know when the bio is really complete.  Only bio_endio() knows that.
    
    So move the trace_block_bio_complete() call to bio_endio().
    
    Now trace_block_bio_complete() pairs with trace_block_bio_queue().
    Any bio for which a 'queue' event is traced, will subsequently
    generate a 'complete' event.
    
    There are a few cases where completion tracing is not wanted.
    1/ If blk_update_request() has already generated a completion
       trace event at the 'request' level, there is no point generating
       one at the bio level too.  In this case the bi_sector and bi_size
       will have changed, so the bio level event would be wrong
    
    2/ If the bio hasn't actually been queued yet, but is being aborted
       early, then a trace event could be confusing.  Some filesystems
       call bio_endio() but do not want tracing.
    
    3/ The bio_integrity code interposes itself by replacing bi_end_io,
       then restoring it and calling bio_endio() again.  This would produce
       two identical trace events if left like that.
    
    To handle these, we introduce a flag BIO_TRACE_COMPLETION and only
    produce the trace event when this is set.
    We address point 1 above by clearing the flag in blk_update_request().
    We address point 2 above by only setting the flag when
    generic_make_request() is called.
    We address point 3 above by clearing the flag after generating a
    completion event.
    
    When bio_split() is used on a bio, particularly in blk_queue_split(),
    there is an extra complication.  A new bio is split off the front, and
    may be handle directly without going through generic_make_request().
    The old bio, which has been advanced, is passed to
    generic_make_request(), so it will trigger a trace event a second
    time.
    Probably the best result when a split happens is to see a single
    'queue' event for the whole bio, then multiple 'complete' events - one
    for each component.  To achieve this was can:
    - copy the BIO_TRACE_COMPLETION flag to the new bio in bio_split()
    - avoid generating a 'queue' event if BIO_TRACE_COMPLETION is already set.
    This way, the split-off bio won't create a queue event, the original
    won't either even if it re-submitted to generic_make_request(),
    but both will produce completion events, each for their own range.
    
    So if generic_make_request() is called (which generates a QUEUED
    event), then bi_endio() will create a single COMPLETE event for each
    range that the bio is split into, unless the driver has explicitly
    requested it not to.
    Signed-off-by: default avatarNeilBrown <neilb@suse.com>
    Signed-off-by: default avatarJens Axboe <axboe@fb.com>
    fbbaf700
dm.c 63.1 KB