Date:	Sun, 05 Jul 2009 12:27:31 +0300
From:	Boaz Harrosh <bharrosh@...asas.com>
To:	Tejun Heo <tj@...nel.org>
CC:	Jens Axboe <jens.axboe@...cle.com>,
	Linux Kernel <linux-kernel@...r.kernel.org>,
	James Bottomley <James.Bottomley@...senPartnership.com>,
	linux-scsi <linux-scsi@...r.kernel.org>,
	Niel Lambrechts <niel.lambrechts@...il.com>,
	FUJITA Tomonori <fujita.tomonori@....ntt.co.jp>,
	Jens Axboe <axboe@...nel.dk>
Subject: Re: [PATCH 3/4] block: implement mixed merge of different failfast
 requests

On 07/03/2009 11:48 AM, Tejun Heo wrote:
> Failfast has characteristics from other attributes.  When issuing,
> executing and successuflly completing requests, failfast doesn't make
> any difference.  It only affects how a request is handled on failure.
> Allowing requests with different failfast settings to be merged cause
> normal IOs to fail prematurely while not allowing has performance
> penalties as failfast is used for read aheads which are likely to be
> located near in-flight or to-be-issued normal IOs.
> 
> This patch introduces the concept of 'mixed merge'.  A request is a
> mixed merge if it is merge of segments which require different
> handling on failure.  Currently the only mixable attributes are
> failfast ones (or lack thereof).
> 
> When a bio with different failfast settings is added to an existing
> request or requests of different failfast settings are merged, the
> merged request is marked mixed.  Each bio carries failfast settings
> and the request always tracks failfast state of the first bio.  When
> the request fails, blk_rq_err_bytes() can be used to determine how
> many bytes can be safely failed without crossing into an area which
> requires further retrials.
> 
> This allows request merging regardless of failfast settings while
> keeping the failure handling correct.
> 
> This patch only implements mixed merge but doesn't enable it.  The
> next one will update SCSI to make use of mixed merge.
> 
> Signed-off-by: Tejun Heo <tj@...nel.org>
> Cc: Jens Axboe <axboe@...nel.dk>
> Cc: Niel Lambrechts <niel.lambrechts@...il.com>
> ---
>  block/blk-core.c       |   99 ++++++++++++++++++++++++++++++++++++++++++++++++
>  block/blk-merge.c      |   43 +++++++++++++++++++++
>  block/blk.h            |    1 +
>  include/linux/blkdev.h |   23 +++++++++--
>  4 files changed, 161 insertions(+), 5 deletions(-)
> 
> diff --git a/block/blk-core.c b/block/blk-core.c
> index cd3b265..0214837 100644
> --- a/block/blk-core.c
> +++ b/block/blk-core.c
> @@ -1165,6 +1165,7 @@ static int __make_request(struct request_queue *q, struct bio *bio)
>  	const unsigned short prio = bio_prio(bio);
>  	const int sync = bio_sync(bio);
>  	const int unplug = bio_unplug(bio);
> +	const unsigned int ff = bio->bi_rw & REQ_FAILFAST_MASK;

Perhaps a bio_fail_fast(bio)
and also an blk_failfast(rq).

Also blk_noretry_request() could see some love now

>  	int rw_flags;
>  
>  	if (bio_barrier(bio) && bio_has_data(bio) &&
> @@ -1194,6 +1195,9 @@ static int __make_request(struct request_queue *q, struct bio *bio)
>  
>  		trace_block_bio_backmerge(q, bio);
>  
> +		if ((req->cmd_flags & REQ_FAILFAST_MASK) != ff)
> +			blk_rq_set_mixed_merge(req);
> +
>  		req->biotail->bi_next = bio;
>  		req->biotail = bio;
>  		req->__data_len += bytes;
> @@ -1213,6 +1217,12 @@ static int __make_request(struct request_queue *q, struct bio *bio)
>  
>  		trace_block_bio_frontmerge(q, bio);
>  
> +		if ((req->cmd_flags & REQ_FAILFAST_MASK) != ff) {
> +			blk_rq_set_mixed_merge(req);
> +			req->cmd_flags &= ~REQ_FAILFAST_MASK;
> +			req->cmd_flags |= ff;
> +		}
> +
>  		bio->bi_next = req->bio;
>  		req->bio = bio;
>  
> @@ -1657,6 +1667,50 @@ int blk_insert_cloned_request(struct request_queue *q, struct request *rq)
>  }
>  EXPORT_SYMBOL_GPL(blk_insert_cloned_request);
>  
> +/**
> + * blk_rq_err_bytes - determine number of bytes till the next failure boundary
> + * @rq: request to examine
> + *
> + * Description:
> + *     A request could be merge of IOs which require different failure
> + *     handling.  This function determines the number of bytes which
> + *     can be failed from the beginning of the request without
> + *     crossing into area which need to be retried further.
> + *
> + * Return:
> + *     The number of bytes to fail.
> + *
> + * Context:
> + *     queue_lock must be held.
> + */
> +unsigned int blk_rq_err_bytes(const struct request *rq)
> +{
> +	unsigned int ff = rq->cmd_flags & REQ_FAILFAST_MASK;
> +	unsigned int bytes = 0;
> +	struct bio *bio;
> +
> +	if (!(rq->cmd_flags & REQ_MIXED_MERGE))
> +		return blk_rq_bytes(rq);
> +
> +	/*
> +	 * Currently the only 'mixing' which can happen is between
> +	 * different fastfail types.  We can safely fail portions
> +	 * which have all the failfast bits that the first one has -
> +	 * the ones which are at least as eager to fail as the first
> +	 * one.
> +	 */
> +	for (bio = rq->bio; bio; bio = bio->bi_next) {
> +		if ((bio->bi_rw & ff) != ff)
> +			break;
> +		bytes += bio->bi_size;
> +	}
> +
> +	/* this could lead to infinite loop */
> +	BUG_ON(blk_rq_bytes(rq) && !bytes);
> +	return bytes;
> +}
> +EXPORT_SYMBOL_GPL(blk_rq_err_bytes);
> +
>  static void blk_account_io_completion(struct request *req, unsigned int bytes)
>  {
>  	if (blk_do_io_stat(req)) {
> @@ -2003,6 +2057,12 @@ bool blk_update_request(struct request *req, int error, unsigned int nr_bytes)
>  	if (blk_fs_request(req) || blk_discard_rq(req))
>  		req->__sector += total_bytes >> 9;
>  
> +	/* mixed attributes always follow the first bio */
> +	if (req->cmd_flags & REQ_MIXED_MERGE) {
> +		req->cmd_flags &= ~REQ_FAILFAST_MASK;
> +		req->cmd_flags |= req->bio->bi_rw & REQ_FAILFAST_MASK;
> +	}
> +
>  	/*
>  	 * If total number of sectors is less than the first segment
>  	 * size, something has gone terribly wrong.
> @@ -2182,6 +2242,25 @@ bool blk_end_request_cur(struct request *rq, int error)
>  EXPORT_SYMBOL_GPL(blk_end_request_cur);
>  
>  /**
> + * blk_end_request_err - Finish a request till the next failure boundary.
> + * @rq: the request to finish till the next failure boundary for
> + * @error: must be negative errno
> + *
> + * Description:
> + *     Complete @rq till the next failure boundary.
> + *
> + * Return:
> + *     %false - we are done with this request
> + *     %true  - still buffers pending for this request
> + */
> +bool blk_end_request_err(struct request *rq, int error)
> +{
> +	WARN_ON(error >= 0);
> +	return blk_end_request(rq, error, blk_rq_err_bytes(rq));
> +}
> +EXPORT_SYMBOL_GPL(blk_end_request_err);
> +
> +/**
>   * __blk_end_request - Helper function for drivers to complete the request.
>   * @rq:       the request being processed
>   * @error:    %0 for success, < %0 for error
> @@ -2240,6 +2319,26 @@ bool __blk_end_request_cur(struct request *rq, int error)
>  }
>  EXPORT_SYMBOL_GPL(__blk_end_request_cur);
>  
> +/**
> + * __blk_end_request_err - Finish a request till the next failure boundary.
> + * @rq: the request to finish till the next failure boundary for
> + * @error: must be negative errno
> + *
> + * Description:
> + *     Complete @rq till the next failure boundary.  Must be called
> + *     with queue lock held.
> + *
> + * Return:
> + *     %false - we are done with this request
> + *     %true  - still buffers pending for this request
> + */
> +bool __blk_end_request_err(struct request *rq, int error)
> +{
> +	WARN_ON(error >= 0);
> +	return __blk_end_request(rq, error, blk_rq_err_bytes(rq));
> +}
> +EXPORT_SYMBOL_GPL(__blk_end_request_err);
> +
>  void blk_rq_bio_prep(struct request_queue *q, struct request *rq,
>  		     struct bio *bio)
>  {
> diff --git a/block/blk-merge.c b/block/blk-merge.c
> index e199967..7c9ca01 100644
> --- a/block/blk-merge.c
> +++ b/block/blk-merge.c
> @@ -311,6 +311,36 @@ static int ll_merge_requests_fn(struct request_queue *q, struct request *req,
>  	return 1;
>  }
>  
> +/**
> + * blk_rq_set_mixed_merge - mark a request as mixed merge
> + * @rq: request to mark as mixed merge
> + *
> + * Description:
> + *     @rq is about to be mixed merged.  Make sure the attributes
> + *     which can be mixed are set in each bio and mark @rq as mixed
> + *     merged.
> + */
> +void blk_rq_set_mixed_merge(struct request *rq)
> +{
> +	unsigned int ff = rq->cmd_flags & REQ_FAILFAST_MASK;
> +	struct bio *bio;
> +
> +	if (rq->cmd_flags & REQ_MIXED_MERGE)
> +		return;
> +
> +	/*
> +	 * @rq will no longer represent mixable attributes for all the
> +	 * contained bios.  It will just track those of the first one.
> +	 * Distributes the attributs to each bio.
> +	 */
> +	for (bio = rq->bio; bio; bio = bio->bi_next) {
> +		WARN_ON_ONCE((bio->bi_rw & REQ_FAILFAST_MASK) &&
> +			     (bio->bi_rw & REQ_FAILFAST_MASK) != ff);
> +		bio->bi_rw |= ff;
> +	}
> +	rq->cmd_flags |= REQ_MIXED_MERGE;
> +}
> +
>  static void blk_account_io_merge(struct request *req)
>  {
>  	if (blk_do_io_stat(req)) {
> @@ -366,6 +396,19 @@ static int attempt_merge(struct request_queue *q, struct request *req,
>  		return 0;
>  
>  	/*
> +	 * If failfast settings disagree or any of the two is already
> +	 * a mixed merge, mark both as mixed before proceeding.  This
> +	 * makes sure that all involved bios have mixable attributes
> +	 * set properly.
> +	 */
> +	if ((req->cmd_flags | next->cmd_flags) & REQ_MIXED_MERGE ||
> +	    (req->cmd_flags & REQ_FAILFAST_MASK) !=
> +	    (next->cmd_flags & REQ_FAILFAST_MASK)) {
> +		blk_rq_set_mixed_merge(req);
> +		blk_rq_set_mixed_merge(next);
> +	}
> +
> +	/*
>  	 * At this point we have either done a back merge
>  	 * or front merge. We need the smaller start_time of
>  	 * the merged requests to be the current request
> diff --git a/block/blk.h b/block/blk.h
> index 3fae6ad..5ee3d7e 100644
> --- a/block/blk.h
> +++ b/block/blk.h
> @@ -104,6 +104,7 @@ int ll_front_merge_fn(struct request_queue *q, struct request *req,
>  int attempt_back_merge(struct request_queue *q, struct request *rq);
>  int attempt_front_merge(struct request_queue *q, struct request *rq);
>  void blk_recalc_rq_segments(struct request *rq);
> +void blk_rq_set_mixed_merge(struct request *rq);
>  
>  void blk_queue_congestion_threshold(struct request_queue *q);
>  
> diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> index a0e5ce1..e58079f 100644
> --- a/include/linux/blkdev.h
> +++ b/include/linux/blkdev.h
> @@ -120,6 +120,7 @@ enum rq_flag_bits {
>  	__REQ_INTEGRITY,	/* integrity metadata has been remapped */
>  	__REQ_NOIDLE,		/* Don't anticipate more IO after this one */
>  	__REQ_IO_STAT,		/* account I/O stat */
> +	__REQ_MIXED_MERGE,	/* merge of different types, fail separately */
>  	__REQ_NR_BITS,		/* stops here */
>  };
>  
> @@ -148,6 +149,7 @@ enum rq_flag_bits {
>  #define REQ_INTEGRITY	(1 << __REQ_INTEGRITY)
>  #define REQ_NOIDLE	(1 << __REQ_NOIDLE)
>  #define REQ_IO_STAT	(1 << __REQ_IO_STAT)
> +#define REQ_MIXED_MERGE	(1 << __REQ_MIXED_MERGE)
>  
>  #define REQ_FAILFAST_MASK	(REQ_FAILFAST_DEV | REQ_FAILFAST_TRANSPORT | \
>  				 REQ_FAILFAST_DRIVER)
> @@ -836,11 +838,13 @@ static inline void blk_run_address_space(struct address_space *mapping)
>  }
>  
>  /*
> - * blk_rq_pos()		: the current sector
> - * blk_rq_bytes()	: bytes left in the entire request
> - * blk_rq_cur_bytes()	: bytes left in the current segment
> - * blk_rq_sectors()	: sectors left in the entire request
> - * blk_rq_cur_sectors()	: sectors left in the current segment
> + * blk_rq_pos()			: the current sector
> + * blk_rq_bytes()		: bytes left in the entire request
> + * blk_rq_cur_bytes()		: bytes left in the current segment
> + * blk_rq_err_bytes()		: bytes left till the next error boundary
> + * blk_rq_sectors()		: sectors left in the entire request
> + * blk_rq_cur_sectors()		: sectors left in the current segment
> + * blk_rq_err_sectors()		: sectors left till the next error boundary
>   */
>  static inline sector_t blk_rq_pos(const struct request *rq)
>  {
> @@ -857,6 +861,8 @@ static inline int blk_rq_cur_bytes(const struct request *rq)
>  	return rq->bio ? bio_cur_bytes(rq->bio) : 0;
>  }
>  
> +extern unsigned int blk_rq_err_bytes(const struct request *rq);
> +
>  static inline unsigned int blk_rq_sectors(const struct request *rq)
>  {
>  	return blk_rq_bytes(rq) >> 9;
> @@ -867,6 +873,11 @@ static inline unsigned int blk_rq_cur_sectors(const struct request *rq)
>  	return blk_rq_cur_bytes(rq) >> 9;
>  }
>  
> +static inline unsigned int blk_rq_err_sectors(const struct request *rq)
> +{
> +	return blk_rq_err_bytes(rq) >> 9;
> +}
> +
>  /*
>   * Request issue related functions.
>   */
> @@ -893,10 +904,12 @@ extern bool blk_end_request(struct request *rq, int error,
>  			    unsigned int nr_bytes);
>  extern void blk_end_request_all(struct request *rq, int error);
>  extern bool blk_end_request_cur(struct request *rq, int error);
> +extern bool blk_end_request_err(struct request *rq, int error);
>  extern bool __blk_end_request(struct request *rq, int error,
>  			      unsigned int nr_bytes);
>  extern void __blk_end_request_all(struct request *rq, int error);
>  extern bool __blk_end_request_cur(struct request *rq, int error);
> +extern bool __blk_end_request_err(struct request *rq, int error);
>  
>  extern void blk_complete_request(struct request *);
>  extern void __blk_complete_request(struct request *);

Boaz
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