Re: [PATCH 06/12] block: lift setting the readahead size into the block layer

Jan Kara
2020-09-17T10:37:50Z

On Thu 10-09-20 16:48:26, Christoph Hellwig wrote:
> Drivers shouldn't really mess with the readahead size, as that is a VM
> concept. Instead set it based on the optimal I/O size by lifting the
> algorithm from the md driver when registering the disk. Also set
> bdi->io_pages there as well by applying the same scheme based on
> max_sectors.
>
> Signed-off-by: Christoph Hellwig <hch@lst.de>
> ---
> block/blk-settings.c | 5 ++---
> block/blk-sysfs.c | 10 +++++++++-
> block/genhd.c | 5 +++--
> drivers/block/aoe/aoeblk.c | 2 --
> drivers/block/drbd/drbd_nl.c | 12 +-----------
> drivers/md/bcache/super.c | 4 ----
> drivers/md/dm-table.c | 3 ---
> drivers/md/raid0.c | 16 ----------------
> drivers/md/raid10.c | 24 +-----------------------
> drivers/md/raid5.c | 13 +------------
> 10 files changed, 17 insertions(+), 77 deletions(-)
>
> diff --git a/block/blk-settings.c b/block/blk-settings.c
> index 76a7e03bcd6cac..01049e9b998f1d 100644
> --- a/block/blk-settings.c
> +++ b/block/blk-settings.c
> @@ -452,6 +452,8 @@ EXPORT_SYMBOL(blk_limits_io_opt);
> void blk_queue_io_opt(struct request_queue *q, unsigned int opt)
> {
> blk_limits_io_opt(&q->limits, opt);
> + q->backing_dev_info->ra_pages =
> + max(queue_io_opt(q) * 2 / PAGE_SIZE, VM_READAHEAD_PAGES);
> }
> EXPORT_SYMBOL(blk_queue_io_opt);
>
> @@ -628,9 +630,6 @@ void disk_stack_limits(struct gendisk *disk, struct block_device *bdev,
> printk(KERN_NOTICE "%s: Warning: Device %s is misaligned\n",
> top, bottom);
> }
> -
> - t->backing_dev_info->io_pages =
> - t->limits.max_sectors >> (PAGE_SHIFT - 9);
> }
> EXPORT_SYMBOL(disk_stack_limits);
>
> diff --git a/block/blk-sysfs.c b/block/blk-sysfs.c
> index 81722cdcf0cb21..95eb35324e1a61 100644
> --- a/block/blk-sysfs.c
> +++ b/block/blk-sysfs.c
> @@ -245,7 +245,6 @@ queue_max_sectors_store(struct request_queue *q, const char *page, size_t count)
>
> spin_lock_irq(&q->queue_lock);
> q->limits.max_sectors = max_sectors_kb << 1;
> - q->backing_dev_info->io_pages = max_sectors_kb >> (PAGE_SHIFT - 10);
> spin_unlock_irq(&q->queue_lock);

So do I get it right that readahead won't now be limited if you store lower
value to max_sectors? Why? I'd consider io_pages a "cached value" of
max_sectors and thus expect it to change together with max_sectors...

> @@ -854,6 +853,15 @@ int blk_register_queue(struct gendisk *disk)
> percpu_ref_switch_to_percpu(&q->q_usage_counter);
> }
>
> + /*
> + * For read-ahead of large files to be effective, we need to read ahead
> + * at least twice the optimal I/O size.
> + */
> + q->backing_dev_info->ra_pages =
> + max(queue_io_opt(q) * 2 / PAGE_SIZE, VM_READAHEAD_PAGES);
> + q->backing_dev_info->io_pages =
> + queue_max_sectors(q) >> (PAGE_SHIFT - 9);
> +
> ret = blk_trace_init_sysfs(dev);
> if (ret)
> return ret;
> diff --git a/block/genhd.c b/block/genhd.c
> index 081f1039d9367f..db311a14ddc71a 100644
> --- a/block/genhd.c
> +++ b/block/genhd.c
> @@ -772,6 +772,7 @@ static void __device_add_disk(struct device *parent, struct gendisk *disk,
> const struct attribute_group **groups,
> bool register_queue)
> {
> + struct request_queue *q = disk->queue;
> dev_t devt;
> int retval;
>
> @@ -782,7 +783,7 @@ static void __device_add_disk(struct device *parent, struct gendisk *disk,
> * registration.
> */
> if (register_queue)
> - elevator_init_mq(disk->queue);
> + elevator_init_mq(q);
>
> /* minors == 0 indicates to use ext devt from part0 and should
> * be accompanied with EXT_DEVT flag. Make sure all
> @@ -812,7 +813,7 @@ static void __device_add_disk(struct device *parent, struct gendisk *disk,
> disk->flags |= GENHD_FL_SUPPRESS_PARTITION_INFO;
> disk->flags |= GENHD_FL_NO_PART_SCAN;
> } else {
> - struct backing_dev_info *bdi = disk->queue->backing_dev_info;
> + struct backing_dev_info *bdi = q->backing_dev_info;
> struct device *dev = disk_to_dev(disk);
> int ret;

Not sure how/why these changes got here... Not that I care too much :)

>
> diff --git a/drivers/block/aoe/aoeblk.c b/drivers/block/aoe/aoeblk.c
> index 5ca7216e9e01f3..89b33b402b4e52 100644
> --- a/drivers/block/aoe/aoeblk.c
> +++ b/drivers/block/aoe/aoeblk.c
> @@ -347,7 +347,6 @@ aoeblk_gdalloc(void *vp)
> mempool_t *mp;
> struct request_queue *q;
> struct blk_mq_tag_set *set;
> - enum { KB = 1024, MB = KB * KB, READ_AHEAD = 2 * MB, };
> ulong flags;
> int late = 0;
> int err;
> @@ -407,7 +406,6 @@ aoeblk_gdalloc(void *vp)
> WARN_ON(d->gd);
> WARN_ON(d->flags & DEVFL_UP);
> blk_queue_max_hw_sectors(q, BLK_DEF_MAX_SECTORS);
> - q->backing_dev_info->ra_pages = READ_AHEAD / PAGE_SIZE;
> d->bufpool = mp;
> d->blkq = gd->queue = q;
> q->queuedata = d;

Shouldn't AOE set 2MB optimal IO size so that readahead is equivalent to
previous behavior?

> diff --git a/drivers/md/bcache/super.c b/drivers/md/bcache/super.c
> index 1bbdc410ee3c51..ff2101d56cd7f1 100644
> --- a/drivers/md/bcache/super.c
> +++ b/drivers/md/bcache/super.c
> @@ -1427,10 +1427,6 @@ static int cached_dev_init(struct cached_dev *dc, unsigned int block_size)
> if (ret)
> return ret;
>
> - dc->disk.disk->queue->backing_dev_info->ra_pages =
> - max(dc->disk.disk->queue->backing_dev_info->ra_pages,
> - q->backing_dev_info->ra_pages);
> -

So bcache is basically stacking readahead here on top of underlying cache
device. I don't see this being replicated by your patch so it is lost now?
Probably this should be replaced by properly inheriting optimal IO size?

Honza
--
Jan Kara <jack@suse.com>
SUSE Labs, CR