linux-ext4 - Re: [PATCH v2 1/2] block: skip elevator initialization for flush requests

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Date:	Wed, 2 Feb 2011 17:06:49 -0500
From:	Mike Snitzer <snitzer@...hat.com>
To:	Vivek Goyal <vgoyal@...hat.com>
Cc:	Tejun Heo <tj@...nel.org>, Jens Axboe <jaxboe@...ionio.com>,
	tytso@....edu, djwong@...ibm.com, shli@...nel.org, neilb@...e.de,
	adilger.kernel@...ger.ca, jack@...e.cz,
	linux-kernel@...r.kernel.org, kmannth@...ibm.com, cmm@...ibm.com,
	linux-ext4@...r.kernel.org, rwheeler@...hat.com, hch@....de,
	josef@...hat.com, jmoyer@...hat.com
Subject: Re: [PATCH v2 1/2] block: skip elevator initialization for flush
 requests

On Wed, Feb 02 2011 at  4:51pm -0500,
Vivek Goyal <vgoyal@...hat.com> wrote:

> 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)? 

Correct.

> > 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
> > @@ -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.

Sure, I'll post a v3 for this patch with that edit and your Acked-by.

Thanks,
Mike
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