Eric Dumazet <eric.dumazet@xxxxxxxxx> writes:
> 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.
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
