Re: [PATCH] net/af_packet: cache align Rx/Tx structs

[Honnappa Nagarahalli <Honnappa.Nagarahalli@arm.com>]

> On Apr 23, 2024, at 3:56 PM, Mattias Rönnblom <hofors@lysator.liu.se> wrote:
> 
> On 2024-04-23 13:15, Ferruh Yigit wrote:
>> On 4/23/2024 10:08 AM, Mattias Rönnblom wrote:
>>> Cache align Rx and Tx queue struct to avoid false sharing.
>>> 
>>> RX struct happens to be 64 bytes on x86_64 already, so cache alignment
>>> makes no change there, but it does on 32-bit ISAs.
>>> 
>>> TX struct is 56 bytes on x86_64.
>>> 
>> Hi Mattias,
>> No objection to the patch. Is the improvement theoretical or do you
>> measure any improvement practically, if so how much is the improvement?
> 
> I didn't run any benchmarks.
> 
> Two cores storing to a (falsely) shared cache line on a per-packet basis is going to be very expensive, at least for "light touch" applications.
> 
>>> Both structs keep counters, and in the RX case they are updated even
>>> for empty polls.
>>> 
>> Do you think does it help if move 'rx_pkts' & 'rx_bytes' update within
>> the loop?
> 
> No, why? Wouldn't that be worse? Especially since rx_pkts and rx_bytes are declared volatile, so you are forcing a load-modify-store cycle for every increment.
> 
> I would drop "volatile", or replace it with an atomic (although *not* use an atomic add for incrementing, but rather atomic load + <n> non-atomic adds + atomic store).
(Slightly unrelated discussion)
Does the atomic load + increment + atomic store help in a non-contended case like this? Some platforms have optimizations for atomic-increments as well which would be missed.

> 
>>> Signed-off-by: Mattias Rönnblom <mattias.ronnblom@ericsson.com>
>>> ---
>>>  drivers/net/af_packet/rte_eth_af_packet.c | 5 +++--
>>>  1 file changed, 3 insertions(+), 2 deletions(-)
>>> 
>>> diff --git a/drivers/net/af_packet/rte_eth_af_packet.c b/drivers/net/af_packet/rte_eth_af_packet.c
>>> index 397a32db58..28aeb7d08e 100644
>>> --- a/drivers/net/af_packet/rte_eth_af_packet.c
>>> +++ b/drivers/net/af_packet/rte_eth_af_packet.c
>>> @@ -6,6 +6,7 @@
>>>   * All rights reserved.
>>>   */
>>>  +#include <rte_common.h>
>>>  #include <rte_string_fns.h>
>>>  #include <rte_mbuf.h>
>>>  #include <ethdev_driver.h>
>>> @@ -53,7 +54,7 @@ struct pkt_rx_queue {
>>>     volatile unsigned long rx_pkts;
>>>   volatile unsigned long rx_bytes;
>>> -};
>>> +} __rte_cache_aligned;
>>>  
>> Latest location for '__rte_cache_aligned' tag is at the beginning of the
>> struct [1], so something like:
>> `struct __rte_cache_aligned pkt_rx_queue {`
>> [1]
>> https://patchwork.dpdk.org/project/dpdk/list/?series=31746&state=%2A&archive=both