Re: dm: optimize flushes

[Mike Snitzer]

On Thu, May 16, 2024 at 10:49:55PM +0200, Mikulas Patocka wrote:
> Device mapper sends flush bios to all the targets and the targets send it
> to the underlying device. That may be inefficient, for example if a table
> contains 10 linear targets pointing to the same physical device, then
> device mapper would send 10 flush bios to that device - despite the fact
> that only one bio would be sufficient.
> 
> This commit optimizes the flush behavior. It introduces a per-target
> variable flush_pass_around - it is set when the target supports flush
> optimization - currently, the dm-linear and dm-stripe targets support it.
> When all the targets in a table have flush_pass_around, flush_pass_around
> on the table is set. __send_empty_flush tests if the table has
> flush_pass_around - and if it has, no flush bios are sent to the targets
> and the list dm_table->devices is iterated and the flush bios are sent to
> each member of the list.

What does "pass around" mean?  Seems like an awkward name for this.
(Naming can be hard, I don't have better suggestions at the moment.)

> Signed-off-by: Mikulas Patocka <mpatocka@xxxxxxxxxx>
> Reported-by: Yang Yang <yang.yang@xxxxxxxx>
> 
> ---
>  drivers/md/dm-core.h          |    4 ++-
>  drivers/md/dm-linear.c        |    1 
>  drivers/md/dm-stripe.c        |    1 
>  drivers/md/dm-table.c         |    4 +++
>  drivers/md/dm.c               |   47 +++++++++++++++++++++++++++++-------------
>  include/linux/device-mapper.h |    5 ++++
>  6 files changed, 47 insertions(+), 15 deletions(-)
> 
> Index: linux-2.6/drivers/md/dm-core.h
> ===================================================================
> --- linux-2.6.orig/drivers/md/dm-core.h	2024-05-15 16:56:49.000000000 +0200
> +++ linux-2.6/drivers/md/dm-core.h	2024-05-15 16:56:49.000000000 +0200
> @@ -206,7 +206,9 @@ struct dm_table {
>  
>  	bool integrity_supported:1;
>  	bool singleton:1;
> -	unsigned integrity_added:1;
> +	bool integrity_added:1;
> +	/* set if all the targets in the table have "flush_pass_around" set */
> +	bool flush_pass_around:1;
>  
>  	/*
>  	 * Indicates the rw permissions for the new logical device.  This
> Index: linux-2.6/drivers/md/dm-linear.c
> ===================================================================
> --- linux-2.6.orig/drivers/md/dm-linear.c	2024-05-15 16:56:49.000000000 +0200
> +++ linux-2.6/drivers/md/dm-linear.c	2024-05-15 16:56:49.000000000 +0200
> @@ -62,6 +62,7 @@ static int linear_ctr(struct dm_target *
>  	ti->num_discard_bios = 1;
>  	ti->num_secure_erase_bios = 1;
>  	ti->num_write_zeroes_bios = 1;
> +	ti->flush_pass_around = true;
>  	ti->private = lc;
>  	return 0;
>  
> Index: linux-2.6/drivers/md/dm-stripe.c
> ===================================================================
> --- linux-2.6.orig/drivers/md/dm-stripe.c	2024-05-15 16:56:49.000000000 +0200
> +++ linux-2.6/drivers/md/dm-stripe.c	2024-05-15 16:56:49.000000000 +0200
> @@ -157,6 +157,7 @@ static int stripe_ctr(struct dm_target *
>  	ti->num_discard_bios = stripes;
>  	ti->num_secure_erase_bios = stripes;
>  	ti->num_write_zeroes_bios = stripes;
> +	ti->flush_pass_around = true;
>  
>  	sc->chunk_size = chunk_size;
>  	if (chunk_size & (chunk_size - 1))
> Index: linux-2.6/drivers/md/dm-table.c
> ===================================================================
> --- linux-2.6.orig/drivers/md/dm-table.c	2024-05-15 16:56:49.000000000 +0200
> +++ linux-2.6/drivers/md/dm-table.c	2024-05-15 16:56:49.000000000 +0200
> @@ -160,6 +160,7 @@ int dm_table_create(struct dm_table **re
>  	t->type = DM_TYPE_NONE;
>  	t->mode = mode;
>  	t->md = md;
> +	t->flush_pass_around = 1;
>  	*result = t;
>  	return 0;
>  }

Should be: t->flush_pass_around = true;

> @@ -738,6 +739,9 @@ int dm_table_add_target(struct dm_table
>  	if (ti->limit_swap_bios && !static_key_enabled(&swap_bios_enabled.key))
>  		static_branch_enable(&swap_bios_enabled);
>  
> +	if (!ti->flush_pass_around)
> +		t->flush_pass_around = false;
> +
>  	return 0;
>  
>   bad:
> Index: linux-2.6/include/linux/device-mapper.h
> ===================================================================
> --- linux-2.6.orig/include/linux/device-mapper.h	2024-05-15 16:56:49.000000000 +0200
> +++ linux-2.6/include/linux/device-mapper.h	2024-05-15 16:56:49.000000000 +0200
> @@ -397,6 +397,11 @@ struct dm_target {
>  	 * bio_set_dev(). NOTE: ideally a target should _not_ need this.
>  	 */
>  	bool needs_bio_set_dev:1;
> +
> +	/*
> +	 * Set if the target supports flush optimization
> +	 */
> +	bool flush_pass_around:1;
>  };

How does a developer _know_ if a target can set this flag?  Please
elaborate on the requirements in this code comment.

>  
>  void *dm_per_bio_data(struct bio *bio, size_t data_size);
> Index: linux-2.6/drivers/md/dm.c
> ===================================================================
> --- linux-2.6.orig/drivers/md/dm.c	2024-05-15 16:56:49.000000000 +0200
> +++ linux-2.6/drivers/md/dm.c	2024-05-16 20:06:32.000000000 +0200
> @@ -645,7 +645,7 @@ static struct bio *alloc_tio(struct clon
>  
>  	/* Set default bdev, but target must bio_set_dev() before issuing IO */
>  	clone->bi_bdev = md->disk->part0;
> -	if (unlikely(ti->needs_bio_set_dev))
> +	if (likely(ti != NULL) && unlikely(ti->needs_bio_set_dev))
>  		bio_set_dev(clone, md->disk->part0);
>  
>  	if (len) {
> @@ -1107,7 +1107,7 @@ static void clone_endio(struct bio *bio)
>  	blk_status_t error = bio->bi_status;
>  	struct dm_target_io *tio = clone_to_tio(bio);
>  	struct dm_target *ti = tio->ti;
> -	dm_endio_fn endio = ti->type->end_io;
> +	dm_endio_fn endio = likely(ti != NULL) ? ti->type->end_io : NULL;
>  	struct dm_io *io = tio->io;
>  	struct mapped_device *md = io->md;
>  
> @@ -1154,7 +1154,7 @@ static void clone_endio(struct bio *bio)
>  	}
>  
>  	if (static_branch_unlikely(&swap_bios_enabled) &&
> -	    unlikely(swap_bios_limit(ti, bio)))
> +	    likely(ti != NULL) && unlikely(swap_bios_limit(ti, bio)))
>  		up(&md->swap_bios_semaphore);
>  
>  	free_tio(bio);

What is it about this commit that makes it important to verify ti
isn't NULL in the above 3 hunks?

Should these NULL checks be factored out as a separate fix?

Or can these hunks be dropped?

> @@ -1566,17 +1566,36 @@ static void __send_empty_flush(struct cl
>  	ci->sector_count = 0;
>  	ci->io->tio.clone.bi_iter.bi_size = 0;
>  
> -	for (unsigned int i = 0; i < t->num_targets; i++) {
> -		unsigned int bios;
> -		struct dm_target *ti = dm_table_get_target(t, i);
> -
> -		if (unlikely(ti->num_flush_bios == 0))
> -			continue;
> -
> -		atomic_add(ti->num_flush_bios, &ci->io->io_count);
> -		bios = __send_duplicate_bios(ci, ti, ti->num_flush_bios,
> -					     NULL, GFP_NOWAIT);
> -		atomic_sub(ti->num_flush_bios - bios, &ci->io->io_count);
> +	if (!t->flush_pass_around) {
> +		for (unsigned int i = 0; i < t->num_targets; i++) {
> +			unsigned int bios;
> +			struct dm_target *ti = dm_table_get_target(t, i);
> +
> +			if (unlikely(ti->num_flush_bios == 0))
> +				continue;
> +
> +			atomic_add(ti->num_flush_bios, &ci->io->io_count);
> +			bios = __send_duplicate_bios(ci, ti, ti->num_flush_bios,
> +						     NULL, GFP_NOWAIT);
> +			atomic_sub(ti->num_flush_bios - bios, &ci->io->io_count);
> +		}
> +	} else {
> +		/*
> +		 * Note that there's no need to grab t->devices_lock here
> +		 * because the targets that support flush pass-around don't
> +		 * modify the list of devices.
> +		 */
> +		struct list_head *devices = dm_table_get_devices(t);
> +		unsigned int len = 0;
> +		struct dm_dev_internal *dd;
> +		list_for_each_entry(dd, devices, list) {
> +			struct bio *clone;
> +			clone = alloc_tio(ci, NULL, 0, &len, GFP_NOIO);
> +			atomic_add(1, &ci->io->io_count);
> +			bio_set_dev(clone, dd->dm_dev->bdev);
> +			clone->bi_end_io = clone_endio;
> +			dm_submit_bio_remap(clone, NULL);
> +		}
>  	}
>  
>  	/*
> 
> 

Still missing what "pass-around" is meant to convey given that you
aren't passing around the same flush... you're cloning a new flush and
issuing one per device.  Probably worth explaining that's what you
mean by "flush_pass_around" (both in commit header and elaborate in
code)?

Also, you're issuing a flush to _all_ devices in a table. Not just
the data devices.  .iterate_devices returns only the data devices.
If/when there is a need to extend this feature to targets that have
metadata devices (e.g. dm-thin, cache, etc): would it make sense to
filter out non-data devices (by stepping through each target in the
table and using iterate_devices)?

Mike