Re: [PATCH bpf-next 1/2] xdp: Move devmap bulk queue into struct net_device

[Eric Dumazet <eric.dumazet@gmail.com>]

On 1/10/20 2:34 PM, Toke Høiland-Jørgensen wrote:
> Jesper Dangaard Brouer <brouer@redhat.com> writes:
> 
>> On Fri, 10 Jan 2020 15:22:02 +0100
>> Toke Høiland-Jørgensen <toke@redhat.com> wrote:
>>
>>> diff --git a/include/linux/netdevice.h b/include/linux/netdevice.h
>>> index 2741aa35bec6..1b2bc2a7522e 100644
>>> --- a/include/linux/netdevice.h
>>> +++ b/include/linux/netdevice.h
>> [...]
>>> @@ -1993,6 +1994,8 @@ struct net_device {
>>>  	spinlock_t		tx_global_lock;
>>>  	int			watchdog_timeo;
>>>  
>>> +	struct xdp_dev_bulk_queue __percpu *xdp_bulkq;
>>> +
>>>  #ifdef CONFIG_XPS
>>>  	struct xps_dev_maps __rcu *xps_cpus_map;
>>>  	struct xps_dev_maps __rcu *xps_rxqs_map;
>>
>> We need to check that the cache-line for this location in struct
>> net_device is not getting updated (write operation) from different CPUs.
>>
>> The test you ran was a single queue single CPU test, which will not
>> show any regression for that case.
> 
> Well, pahole says:
> 
> 	/* --- cacheline 14 boundary (896 bytes) --- */
> 	struct netdev_queue *      _tx __attribute__((__aligned__(64))); /*   896     8 */
> 	unsigned int               num_tx_queues;        /*   904     4 */
> 	unsigned int               real_num_tx_queues;   /*   908     4 */
> 	struct Qdisc *             qdisc;                /*   912     8 */
> 	struct hlist_head  qdisc_hash[16];               /*   920   128 */
> 	/* --- cacheline 16 boundary (1024 bytes) was 24 bytes ago --- */
> 	unsigned int               tx_queue_len;         /*  1048     4 */
> 	spinlock_t                 tx_global_lock;       /*  1052     4 */
> 	int                        watchdog_timeo;       /*  1056     4 */
> 
> 	/* XXX 4 bytes hole, try to pack */
> 
> 	struct xdp_dev_bulk_queue * xdp_bulkq;           /*  1064     8 */
> 	struct xps_dev_maps *      xps_cpus_map;         /*  1072     8 */
> 	struct xps_dev_maps *      xps_rxqs_map;         /*  1080     8 */
> 	/* --- cacheline 17 boundary (1088 bytes) --- */
> 
> 
> of those, tx_queue_len is the max queue len (so only set on init),
> tx_global_lock is not used by multi-queue devices, watchdog_timeo also
> seems to be a static value thats set on init, and the xps* pointers also
> only seems to be set once on init. So I think we're fine?
> 
> I can run a multi-CPU test just to be sure, but I really don't see which
> of those fields might be updated on TX...
> 

Note that another interesting field is miniq_egress, your patch
moves it to another cache line.

We probably should move qdisc_hash array elsewhere.