Re: [PATCH v2 1/2] block: skip elevator initialization for flush requests
From: Vivek Goyal <vgoyal@redhat.com>
Date: 2011-02-02 21:52:09
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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
quoted hunk ↗ jump to hunk
Signed-off-by: Mike Snitzer <redacted> --- 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