Re: [PATCH v2 1/3] md/bitmap: Add chunk-threshold unplugging

[Jonathan Derrick <jonathan.derrick@linux.dev>]

On 10/13/2022 4:41 PM, Jonathan Derrick wrote:
> Add a mechanism to allow bitmap unplugging and flushing to wait until it
> has surpassed a defined threshold of dirty chunks. This allows certain
> high I/O write workloads to make good forward progress between bitmap
> updates or provide reliable bitmap consistency. The default behavior is
> previous behavior of always unplugging when called.
> 
> Signed-off-by: Jonathan Derrick <jonathan.derrick@linux.dev>
> ---
>  drivers/md/md-bitmap.c | 35 +++++++++++++++++++++++++++++++----
>  drivers/md/md-bitmap.h |  1 +
>  drivers/md/md.h        |  1 +
>  3 files changed, 33 insertions(+), 4 deletions(-)
> 
> diff --git a/drivers/md/md-bitmap.c b/drivers/md/md-bitmap.c
> index bf6dffadbe6f..c5c77f8371a8 100644
> --- a/drivers/md/md-bitmap.c
> +++ b/drivers/md/md-bitmap.c
> @@ -1004,7 +1004,7 @@ static int md_bitmap_file_test_bit(struct bitmap *bitmap, sector_t block)
>  /* this gets called when the md device is ready to unplug its underlying
>   * (slave) device queues -- before we let any writes go down, we need to
>   * sync the dirty pages of the bitmap file to disk */
> -void md_bitmap_unplug(struct bitmap *bitmap)
> +static void __md_bitmap_unplug(struct bitmap *bitmap)
>  {
>  	unsigned long i;
>  	int dirty, need_write;
> @@ -1038,6 +1038,33 @@ void md_bitmap_unplug(struct bitmap *bitmap)
>  	if (test_bit(BITMAP_WRITE_ERROR, &bitmap->flags))
>  		md_bitmap_file_kick(bitmap);
>  }
> +
> +/*
> + * Conditional unplug based on user-defined parameter
> + * Defaults to unconditional behavior
> + */
> +void md_bitmap_unplug(struct bitmap *bitmap)
> +{
> +	unsigned int flush_threshold = bitmap->mddev->bitmap_info.flush_threshold;
> +
> +	if (!flush_threshold) {
> +		__md_bitmap_unplug(bitmap);
> +	} else {
> +		struct bitmap_page *bp = bitmap->counts.bp;
> +		unsigned long pages = bitmap->counts.pages;
> +		unsigned long k, count = 0;
> +
> +		for (k = 0; k < pages; k++)
> +			if (bp[k].map && !bp[k].hijacked)
> +				count += bp[k].count;
> +
> +		if (count - bitmap->unplugged_count > flush_threshold) {
> +			bitmap->unplugged_count = count;
> +			md_bitmap_daemon_work(&bitmap->mddev->daemon_timer);
I just noticed I call daemon_timer before adding it in 3/3
I'll fix that in v3

> +			__md_bitmap_unplug(bitmap);
> +		}
> +	}
> +}
>  EXPORT_SYMBOL(md_bitmap_unplug);
>  
>  static void md_bitmap_set_memory_bits(struct bitmap *bitmap, sector_t offset, int needed);
> @@ -2012,9 +2039,9 @@ int md_bitmap_copy_from_slot(struct mddev *mddev, int slot,
>  		for (i = 0; i < bitmap->storage.file_pages; i++)
>  			if (test_page_attr(bitmap, i, BITMAP_PAGE_PENDING))
>  				set_page_attr(bitmap, i, BITMAP_PAGE_NEEDWRITE);
> -		md_bitmap_unplug(bitmap);
> +		__md_bitmap_unplug(bitmap);
>  	}
> -	md_bitmap_unplug(mddev->bitmap);
> +	__md_bitmap_unplug(mddev->bitmap);
>  	*low = lo;
>  	*high = hi;
>  	md_bitmap_free(bitmap);
> @@ -2246,7 +2273,7 @@ int md_bitmap_resize(struct bitmap *bitmap, sector_t blocks,
>  	spin_unlock_irq(&bitmap->counts.lock);
>  
>  	if (!init) {
> -		md_bitmap_unplug(bitmap);
> +		__md_bitmap_unplug(bitmap);
>  		bitmap->mddev->pers->quiesce(bitmap->mddev, 0);
>  	}
>  	ret = 0;
> diff --git a/drivers/md/md-bitmap.h b/drivers/md/md-bitmap.h
> index cfd7395de8fd..49a93d8ff307 100644
> --- a/drivers/md/md-bitmap.h
> +++ b/drivers/md/md-bitmap.h
> @@ -223,6 +223,7 @@ struct bitmap {
>  	unsigned long daemon_lastrun; /* jiffies of last run */
>  	unsigned long last_end_sync; /* when we lasted called end_sync to
>  				      * update bitmap with resync progress */
> +	unsigned long unplugged_count; /* last dirty count from md_bitmap_unplug */
>  
>  	atomic_t pending_writes; /* pending writes to the bitmap file */
>  	wait_queue_head_t write_wait;
> diff --git a/drivers/md/md.h b/drivers/md/md.h
> index b4e2d8b87b61..1a558cb18bd4 100644
> --- a/drivers/md/md.h
> +++ b/drivers/md/md.h
> @@ -501,6 +501,7 @@ struct mddev {
>  		int			external;
>  		int			nodes; /* Maximum number of nodes in the cluster */
>  		char                    cluster_name[64]; /* Name of the cluster */
> +		unsigned int		flush_threshold; /* how many dirty chunks between updates */
>  	} bitmap_info;
>  
>  	atomic_t			max_corr_read_errors; /* max read retries */