On Tue, Jan 08, 2019 at 04:57:34PM -0500, Jeff Moyer wrote: > There was some confusion about what these functions did. Make it clear > that this is a hint for upper layers to pass to the block layer, and > that it does not guarantee that I/O will not be submitted between a > start and finish plug. > > Reported-by: "Darrick J. Wong" <[email protected]> > Signed-off-by: Jeff Moyer <[email protected]> > > diff --git a/block/blk-core.c b/block/blk-core.c > index c78042975737..f2732f106a2e 100644 > --- a/block/blk-core.c > +++ b/block/blk-core.c > @@ -1683,6 +1683,15 @@ EXPORT_SYMBOL(kblockd_mod_delayed_work_on); > * @plug: The &struct blk_plug that needs to be initialized > * > * Description: > + * blk_start_plug() indicates to the block layer an intent by the caller > + * to submit multiple I/O requests in a batch. The block layer may use > + * this hint to defer submitting I/Os from the caller until > blk_finish_plug() > + * is called. However, the block layer may choose to submit requests > + * before a call to blk_finish_plug() if the number of queued I/Os > + * exceeds %BLK_MAX_REQUEST_COUNT, or if the size of the I/O is larger than > + * %BLK_PLUG_FLUSH_SIZE. The queued I/Os may also be submitted early if > + * the task schedules (see below).
Makes sense to my poor brain, Reviewed-by: Darrick J. Wong <[email protected]> --D > * Tracking blk_plug inside the task_struct will help with auto-flushing > the > * pending I/O should the task end up blocking between blk_start_plug() and > * blk_finish_plug(). This is important from a performance perspective, but > @@ -1765,6 +1774,16 @@ void blk_flush_plug_list(struct blk_plug *plug, bool > from_schedule) > blk_mq_flush_plug_list(plug, from_schedule); > } > > +/** > + * blk_finish_plug - mark the end of a batch of submitted I/O > + * @plug: The &struct blk_plug passed to blk_start_plug() > + * > + * Description: > + * Indicate that a batch of I/O submissions is complete. This function > + * must be paired with an initial call to blk_start_plug(). The intent > + * is to allow the block layer to optimize I/O submission. See the > + * documentation for blk_start_plug() for more information. > + */ > void blk_finish_plug(struct blk_plug *plug) > { > if (plug != current->plug)
