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

From: Toke Høiland-Jørgensen <[email-masked]>
Date: Sat, 11 Jan 2020 00:16:12 +0100

Eric Dumazet <[email-masked]> writes:

> On 1/10/20 2:34 PM, Toke Høiland-Jørgensen wrote:
>> Jesper Dangaard Brouer <[email-masked]> writes:
>> 
>>> On Fri, 10 Jan 2020 15:22:02 +0100
>>> Toke Høiland-Jørgensen <[email-masked]> 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.

Hmm, since there's that 4-byte hole, I gust we could just move
watchdog_timeo down to fix that. Any reason that's a bad idea?

> We probably should move qdisc_hash array elsewhere.

You certainly won't hear me object to that :)

-Toke