[dm-devel] [PATCH] dm: do not call dm_sync_table() when creating new devices

[Mikulas Patocka]

On Wed, 5 Nov 2014, Hannes Reinecke wrote:

> When creating new devices dm_sync_table() calls
> synchronize_rcu_expedited(), causing _all_ pending
> RCU pointers to be flushed. This causes a latency
> overhead especially noticeable when creating lots
> of devices.
> And all of this is pointless as there are no old
> maps to be disconnected, and hence no stale pointers
> which would need to be cleared up.
> 
> Signed-off-by: Hannes Reinecke <hare at suse.de>

Reviewed-by: Mikulas Patocka <mpatocka at redhat.com>

> ---
>  drivers/md/dm.c | 9 ++++++---
>  1 file changed, 6 insertions(+), 3 deletions(-)
> 
> diff --git a/drivers/md/dm.c b/drivers/md/dm.c
> index 58f3927..2c2c6cf 100644
> --- a/drivers/md/dm.c
> +++ b/drivers/md/dm.c
> @@ -2341,7 +2341,8 @@ static struct dm_table *__bind(struct mapped_device *md, struct dm_table *t,
>  		set_bit(DMF_MERGE_IS_OPTIONAL, &md->flags);
>  	else
>  		clear_bit(DMF_MERGE_IS_OPTIONAL, &md->flags);
> -	dm_sync_table(md);
> +	if (old_map)
> +		dm_sync_table(md);
>  
>  	return old_map;
>  }
> @@ -2782,7 +2783,8 @@ int dm_suspend(struct mapped_device *md, unsigned suspend_flags)
>  	 * flush_workqueue(md->wq).
>  	 */
>  	set_bit(DMF_BLOCK_IO_FOR_SUSPEND, &md->flags);
> -	synchronize_srcu(&md->io_barrier);
> +	if (map)
> +		synchronize_srcu(&md->io_barrier);
>  
>  	/*
>  	 * Stop md->queue before flushing md->wq in case request-based
> @@ -2802,7 +2804,8 @@ int dm_suspend(struct mapped_device *md, unsigned suspend_flags)
>  
>  	if (noflush)
>  		clear_bit(DMF_NOFLUSH_SUSPENDING, &md->flags);
> -	synchronize_srcu(&md->io_barrier);
> +	if (map)
> +		synchronize_srcu(&md->io_barrier);
>  
>  	/* were we interrupted ? */
>  	if (r < 0) {
> -- 
> 1.8.5.2
> 
> --
> dm-devel mailing list
> dm-devel at redhat.com
> https://www.redhat.com/mailman/listinfo/dm-devel
>