Re: [patch 7/11] net: Use bigrefs for net_device.refcount

[Eric Dumazet]

Ravikiran G Thirumalai a écrit :
> The net_device has a refcnt used to keep track of it's uses.
> This is used at the time of unregistering the network device
> (module unloading ..) (see netdev_wait_allrefs) .
> For loopback_dev , this refcnt increment/decrement  is causing
> unnecessary traffic on the interlink for NUMA system
> affecting it's performance.  This patch improves tbench numbers by 6% on a
> 8way x86 Xeon (x445).
 ===================================================================
> --- alloc_percpu-2.6.13.orig/include/linux/netdevice.h	2005-08-28 16:41:01.000000000 -0700
> +++ alloc_percpu-2.6.13/include/linux/netdevice.h	2005-09-12 11:54:21.000000000 -0700
> @@ -37,6 +37,7 @@
>  #include <linux/config.h>
>  #include <linux/device.h>
>  #include <linux/percpu.h>
> +#include <linux/bigref.h>
>  
>  struct divert_blk;
>  struct vlan_group;
> @@ -377,7 +378,7 @@
>  	/* device queue lock */
>  	spinlock_t		queue_lock;
>  	/* Number of references to this device */
> -	atomic_t		refcnt;
> +	struct bigref	        netdev_refcnt;	
>  	/* delayed register/unregister */
>  	struct list_head	todo_list;
>  	/* device name hash chain */
> @@ -677,11 +678,11 @@

Hum...

Did you tried to place refcnt/netdev_refcnt in a separate cache line than 
queue_lock ? I got good results too...

>  	/* device queue lock */
>  	spinlock_t		queue_lock;
>  	/* Number of references to this device */
> -	atomic_t		refcnt;
> +	struct bigref	        netdev_refcnt ____cacheline_aligned_in_smp ;	
>  	/* delayed register/unregister */
>  	struct list_head	todo_list;
>  	/* device name hash chain */

Every time a cpu take the queue_lock spinlock, it exclusively gets one cache 
line. If another cpu try to access netdev_refcnt, it has to grab this cache 
line (even if properely per_cpu designed, there is still one shared field). In 
fact the whole struct net_device should be re-ordered for SMP/NUMA performance.

Eric
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