From: Vivek Goyal Subject: Re: [PATCH v2 1/2] block: skip elevator initialization for flush requests Date: Wed, 2 Feb 2011 16:51:52 -0500 Message-ID: <20110202215152.GC12559@redhat.com> References: <20110201185225.GT14211@htj.dyndns.org> <1296600373-6906-1-git-send-email-snitzer@redhat.com> Mime-Version: 1.0 Content-Type: text/plain; charset=us-ascii Cc: Tejun Heo , Jens Axboe , tytso@mit.edu, djwong@us.ibm.com, shli@kernel.org, neilb@suse.de, adilger.kernel@dilger.ca, jack@suse.cz, linux-kernel@vger.kernel.org, kmannth@us.ibm.com, cmm@us.ibm.com, linux-ext4@vger.kernel.org, rwheeler@redhat.com, hch@lst.de, josef@redhat.com, jmoyer@redhat.com To: Mike Snitzer Return-path: Content-Disposition: inline In-Reply-To: <1296600373-6906-1-git-send-email-snitzer@redhat.com> Sender: linux-kernel-owner@vger.kernel.org List-Id: linux-ext4.vger.kernel.org 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 Vivek > > Signed-off-by: Mike Snitzer > --- > 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