Re: [Xen-devel] [PATCH v4 10/10] xen/blkback: make pool of persistent grants and free pages per-queue

[Konrad Rzeszutek Wilk <konrad.wilk@xxxxxxxxxx>]

On Mon, Nov 02, 2015 at 12:21:46PM +0800, Bob Liu wrote:
> Make pool of persistent grants and free pages per-queue/ring instead of
> per-device to get better scalability.

How much better scalability do we get?

.. snip .. 
>  
>  
>  /*
> - * pers_gnts_lock must be used around all the persistent grant helpers
> - * because blkback may use multi-thread/queue for each backend.
> + * We don't need locking around the persistent grant helpers
> + * because blkback uses a single-thread for each backed, so we

s/backed/backend/
> + * can be sure that this functions will never be called recursively.
> + *
> + * The only exception to that is put_persistent_grant, that can be called
> + * from interrupt context (by xen_blkbk_unmap), so we have to use atomic
> + * bit operations to modify the flags of a persistent grant and to count
> + * the number of used grants.
>   */

..snip..
> --- a/drivers/block/xen-blkback/common.h
> +++ b/drivers/block/xen-blkback/common.h
> @@ -282,6 +282,22 @@ struct xen_blkif_ring {
>       spinlock_t              pending_free_lock;
>       wait_queue_head_t       pending_free_wq;
>  
> +     /* buffer of free pages to map grant refs */

Full stop.
> +     spinlock_t              free_pages_lock;
> +     int                     free_pages_num;
> +     struct list_head        free_pages;
> +
> +     /* tree to store persistent grants */

Full stop.
> +     spinlock_t              pers_gnts_lock;
> +     struct rb_root          persistent_gnts;
> +     unsigned int            persistent_gnt_c;
> +     atomic_t                persistent_gnt_in_use;
> +     unsigned long           next_lru;
> +
> +     /* used by the kworker that offload work from the persistent purge */

Full stop.
> +     struct list_head        persistent_purge_list;
> +     struct work_struct      persistent_purge_work;
> +
>       /* statistics */
>       unsigned long           st_print;
>       unsigned long long                      st_rd_req;

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