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...
-Toke
