Re: [PATCH v2 1/2] block: skip elevator initialization for flush requests

Vivek Goyal
2011-02-02T21:52:09Z

On Tue, Feb 01, 2011 at 05:46:12PM -0500, Mike Snitzer wrote:
> Skip elevator initialization during request allocation if REQ_SORTED
> is not set in the @rw_flags passed to the request allocator.
>
> Set REQ_SORTED for all requests that may be put on IO scheduler. Flush
> requests are not put on IO scheduler so REQ_SORTED is not set for
> them.

So we are doing all this so that elevator_private and flush data can
share the space through union and we can avoid increasing the size
of struct rq by 1 pointer (4 or 8 bytes depneding on arch)?

Looks good to me. One minor comment inline.

Acked-by: Vivek Goyal <vgoyal@redhat.com>

Vivek

>
> Signed-off-by: Mike Snitzer <snitzer@redhat.com>
> ---
> block/blk-core.c | 24 +++++++++++++++++++-----
> 1 files changed, 19 insertions(+), 5 deletions(-)
>
> diff --git a/block/blk-core.c b/block/blk-core.c
> index 72dd23b..f6fcc64 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -764,7 +764,7 @@ static struct request *get_request(struct request_queue *q, int rw_flags,
> struct request_list *rl = &q->rq;
> struct io_context *ioc = NULL;
> const bool is_sync = rw_is_sync(rw_flags) != 0;
> - int may_queue, priv;
> + int may_queue, priv = 0;
>
> may_queue = elv_may_queue(q, rw_flags);
> if (may_queue == ELV_MQUEUE_NO)
> @@ -808,9 +808,14 @@ static struct request *get_request(struct request_queue *q, int rw_flags,
> rl->count[is_sync]++;
> rl->starved[is_sync] = 0;
>
> - priv = !test_bit(QUEUE_FLAG_ELVSWITCH, &q->queue_flags);
> - if (priv)
> - rl->elvpriv++;
> + /*
> + * Only initialize elevator data if REQ_SORTED is set.
> + */
> + if (rw_flags & REQ_SORTED) {
> + priv = !test_bit(QUEUE_FLAG_ELVSWITCH, &q->queue_flags);
> + if (priv)
> + rl->elvpriv++;
> + }
>
> if (blk_queue_io_stat(q))
> rw_flags |= REQ_IO_STAT;
> @@ -1197,6 +1202,7 @@ static int __make_request(struct request_queue *q, struct bio *bio)
> const unsigned short prio = bio_prio(bio);
> const bool sync = !!(bio->bi_rw & REQ_SYNC);
> const bool unplug = !!(bio->bi_rw & REQ_UNPLUG);
> + const bool flush = !!(bio->bi_rw & (REQ_FLUSH | REQ_FUA));
> const unsigned long ff = bio->bi_rw & REQ_FAILFAST_MASK;
> int where = ELEVATOR_INSERT_SORT;
> int rw_flags;
> @@ -1210,7 +1216,7 @@ static int __make_request(struct request_queue *q, struct bio *bio)
>
> spin_lock_irq(q->queue_lock);
>
> - if (bio->bi_rw & (REQ_FLUSH | REQ_FUA)) {
> + if (flush) {
> where = ELEVATOR_INSERT_FLUSH;
> goto get_rq;
> }
> @@ -1293,6 +1299,14 @@ get_rq:
> rw_flags |= REQ_SYNC;
>
> /*
> + * Set REQ_SORTED for all requests that may be put on IO scheduler.
> + * The request allocator's IO scheduler initialization will be skipped
> + * if REQ_SORTED is not set.
> + */

Do you want to mention here that why do we want to avoid IO scheduler
initialization. Specifically mention that set_request() is avoided so
that elevator_private[*] are not initialized and that space can be
used by flush request data.

> + if (!flush)
> + rw_flags |= REQ_SORTED;
> +
> + /*
> * Grab a free request. This is might sleep but can not fail.
> * Returns with the queue unlocked.
> */
> --
> 1.7.3.4