Re: [PATCH v3] drbd: fix throttling on newly created DM backing devices

[Lars <lars.ellenberg@linbit.com>]

On Fri, Sep 05, 2014 at 08:41:18PM +0200, Imre Palik wrote:
> From: "Palik, Imre" <imrep@amazon.de>
> 
> If the drbd backing device is a new device mapper device (e.g., a
> dm-linear mapping of an existing block device that contains data), the
> counters are initially 0 even though the device contains useful
> data. This causes throttling until something accesses the drbd device
> or the backing device.

What was wrong with my previous proposal?

How does changing the signedness help with
rs_last_events not being properly initialized?

Are you sure you have also considered all wrap-around cases?

Maybe you are too focused on your particular corner case
(disk_stats starting with 0).
Maybe I'm just thick right now, so please explain.

	Lars

> The patch disables throttling, as long as only resync is responsible
> for disk activity on a freshly created device.
> 
> Reported-by: Mikhail Sugakov <msugakov@amazon.de>
> Cc: Matt Wilson <msw@amazon.com>
> Signed-off-by: Imre Palik <imrep@amazon.de>
> ---
>  drivers/block/drbd/drbd_int.h      |    4 ++--
>  drivers/block/drbd/drbd_receiver.c |   10 +++++-----
>  2 files changed, 7 insertions(+), 7 deletions(-)
> 
> diff --git a/drivers/block/drbd/drbd_int.h b/drivers/block/drbd/drbd_int.h
> index 1a00001..298b1dc 100644
> --- a/drivers/block/drbd/drbd_int.h
> +++ b/drivers/block/drbd/drbd_int.h
> @@ -960,8 +960,8 @@ struct drbd_device {
>  	atomic_t rs_sect_in; /* for incoming resync data rate, SyncTarget */
>  	atomic_t rs_sect_ev; /* for submitted resync data rate, both */
>  	int rs_last_sect_ev; /* counter to compare with */
> -	int rs_last_events;  /* counter of read or write "events" (unit sectors)
> -			      * on the lower level device when we last looked. */
> +	unsigned int rs_last_events;  /* counter of read or write "events" (unit sectors)
> +				       * on the lower level device when we last looked. */
>  	int c_sync_rate; /* current resync rate after syncer throttle magic */
>  	struct fifo_buffer *rs_plan_s; /* correction values of resync planer (RCU, connection->conn_update) */
>  	int rs_in_flight; /* resync sectors in flight (to proxy, in proxy and from proxy) */
> diff --git a/drivers/block/drbd/drbd_receiver.c b/drivers/block/drbd/drbd_receiver.c
> index 9342b8d..147c917 100644
> --- a/drivers/block/drbd/drbd_receiver.c
> +++ b/drivers/block/drbd/drbd_receiver.c
> @@ -2467,7 +2467,7 @@ bool drbd_rs_c_min_rate_throttle(struct drbd_device *device)
>  	struct gendisk *disk = device->ldev->backing_bdev->bd_contains->bd_disk;
>  	unsigned long db, dt, dbdt;
>  	unsigned int c_min_rate;
> -	int curr_events;
> +	unsigned int curr_events;
>  
>  	rcu_read_lock();
>  	c_min_rate = rcu_dereference(device->ldev->disk_conf)->c_min_rate;
> @@ -2477,12 +2477,12 @@ bool drbd_rs_c_min_rate_throttle(struct drbd_device *device)
>  	if (c_min_rate == 0)
>  		return false;
>  
> -	curr_events = (int)part_stat_read(&disk->part0, sectors[0]) +
> -		      (int)part_stat_read(&disk->part0, sectors[1]) -
> -			atomic_read(&device->rs_sect_ev);
> +	curr_events = (unsigned int)part_stat_read(&disk->part0, sectors[0]) +
> +		      (unsigned int)part_stat_read(&disk->part0, sectors[1]) -
> +		(unsigned int)atomic_read(&device->rs_sect_ev);
>  
>  	if (atomic_read(&device->ap_actlog_cnt)
> -	    || !device->rs_last_events || curr_events - device->rs_last_events > 64) {
> +		|| curr_events - device->rs_last_events > 64) {
>  		unsigned long rs_left;
>  		int i;
>  
> -- 
> 1.7.9.5
>