Re: [PATCH 1/2] blkdev: add flush generation counter

[Dmitry Monakhov]

On Mon, 15 Apr 2013 16:14:39 +0200, Jan Kara <jack@xxxxxxx> wrote:
> On Sun 14-04-13 23:34:27, Dmitry Monakhov wrote:
> > Callers may use this counter to optimize flushes
> > 
> > Signed-off-by: Dmitry Monakhov <dmonakhov@xxxxxxxxxx>
> > ---
> >  block/blk-core.c       |    1 +
> >  block/blk-flush.c      |    3 ++-
> >  include/linux/blkdev.h |    1 +
> >  3 files changed, 4 insertions(+), 1 deletions(-)
> > 
> > diff --git a/block/blk-core.c b/block/blk-core.c
> > index 074b758..afb5a4b 100644
> > --- a/block/blk-core.c
> > +++ b/block/blk-core.c
> > @@ -537,6 +537,7 @@ void blk_cleanup_queue(struct request_queue *q)
> >  	spin_unlock_irq(lock);
> >  	mutex_unlock(&q->sysfs_lock);
> >  
> > +	atomic_set(&q->flush_tag, 0);
> >  	/*
> >  	 * Drain all requests queued before DYING marking. Set DEAD flag to
> >  	 * prevent that q->request_fn() gets invoked after draining finished.
> > diff --git a/block/blk-flush.c b/block/blk-flush.c
> > index cc2b827..b1adc75 100644
> > --- a/block/blk-flush.c
> > +++ b/block/blk-flush.c
> > @@ -203,7 +203,7 @@ static void flush_end_io(struct request *flush_rq, int error)
> >  	/* account completion of the flush request */
> >  	q->flush_running_idx ^= 1;
> >  	elv_completed_request(q, flush_rq);
> > -
> > +	atomic_inc(&q->flush_tag);
> >  	/* and push the waiting requests to the next stage */
> >  	list_for_each_entry_safe(rq, n, running, flush.list) {
> >  		unsigned int seq = blk_flush_cur_seq(rq);
> > @@ -268,6 +268,7 @@ static bool blk_kick_flush(struct request_queue *q)
> >  	q->flush_rq.end_io = flush_end_io;
> >  
> >  	q->flush_pending_idx ^= 1;
> > +	atomic_inc(&q->flush_tag);
> >  	list_add_tail(&q->flush_rq.queuelist, &q->queue_head);
> >  	return true;
> >  }
>   But this doesn't quite work, does it? When fs reads flush_tag counter,
> CACHE_FLUSH command can be already issued so you are not sure how the IO
> you are issuing relates to the cache flush in flight. If you want to make
> this work, you really need two counters - one for flush submissions and one
> for flush completions. Fs would need to read the submission counter before
> issuing the IO and the completion counter when it wants to make sure date is
> on the stable storage. But it all gets somewhat complex and I'm not sure
> it's worth it given the block layer tries to merge flush requests by
> itself. But you can try for correct implementation and measure the
> performance impact of course...

Probably I've missed good comments, but I've tried to make things correct
Here are an flush implementation assumptions from block/blk-flush.c:

Currently, the following conditions are used to determine when to e flush.

C1. At any given time, only one flush shall be in progress.  This is
    double buffering sufficient.
C2. Flush is deferred if any request is executing DATA of its ence.
    This avoids issuing separate POSTFLUSHes for requests which ed
    PREFLUSH.
C3. The second condition is ignored if there is a request which has
    waited longer than FLUSH_PENDING_TIMEOUT.  This is to avoid
    starvation in the unlikely case where there are continuous am of
    FUA (without FLUSH) requests.

So if we will increment flush_tag counter in two places:
blk_kick_flush: the place where flush request is issued
flush_end_io : the place where flush is completed
And by using rule (C1) we can guarantee that

if (flush_tag & 0x1 == 1) then flush_tag  is in progress
if (flush_tag & 0x1 == 0) then (flush_tag & ~(0x1)) completed
In other words we can define it as:

FLUSH_TAG_IDX    = (flush_tag +1) & ~0x1
FLUSH_TAG_STATE  = flush_tag & 0x1 ? IN_PROGRESS : COMPLETED

After that we can define rules for flush optimization:
We can be sure that our data was flushed only if:
1) data's bio was completed before flush request was QUEUED
   and COMPLETED
So in terms of formulas we can write it like follows:
is_data_flushed = (blkdev->flush_tag & ~(0x1)) >
                  ((data->flush_tag + 0x1) & (~0x1))

> 
> 								Honza
> > diff --git a/include/linux/blkdev.h b/include/linux/blkdev.h
> > index 78feda9..e079fbd 100644
> > --- a/include/linux/blkdev.h
> > +++ b/include/linux/blkdev.h
> > @@ -416,6 +416,7 @@ struct request_queue {
> >  	unsigned int		flush_queue_delayed:1;
> >  	unsigned int		flush_pending_idx:1;
> >  	unsigned int		flush_running_idx:1;
> > +	atomic_t		flush_tag;
> >  	unsigned long		flush_pending_since;
> >  	struct list_head	flush_queue[2];
> >  	struct list_head	flush_data_in_flight;
> > -- 
> > 1.7.1
> > 
> -- 
> Jan Kara <jack@xxxxxxx>
> SUSE Labs, CR
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