On 1/10/20 2:34 PM, Toke Høiland-Jørgensen wrote:
> Jesper Dangaard Brouer <brouer@xxxxxxxxxx> writes:
>
>> On Fri, 10 Jan 2020 15:22:02 +0100
>> Toke Høiland-Jørgensen <toke@xxxxxxxxxx> 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.
