Re: [PATCH net-next v3 2/3] xsk: use atomic operations around cached_prod for copy mode

[Paolo Abeni <pabeni@redhat.com>]

On 12/3/25 7:56 AM, Jason Xing wrote:
> On Sat, Nov 29, 2025 at 8:55 AM Jason Xing <kerneljasonxing@gmail.com> wrote:
>> On Fri, Nov 28, 2025 at 10:20 PM Paolo Abeni <pabeni@redhat.com> wrote:
>>> On 11/28/25 2:46 PM, Jason Xing wrote:
>>>> From: Jason Xing <kernelxing@tencent.com>
>>>>
>>>> Use atomic_try_cmpxchg operations to replace spin lock. Technically
>>>> CAS (Compare And Swap) is better than a coarse way like spin-lock
>>>> especially when we only need to perform a few simple operations.
>>>> Similar idea can also be found in the recent commit 100dfa74cad9
>>>> ("net: dev_queue_xmit() llist adoption") that implements the lockless
>>>> logic with the help of try_cmpxchg.
>>>>
>>>> Signed-off-by: Jason Xing <kernelxing@tencent.com>
>>>> ---
>>>> Paolo, sorry that I didn't try to move the lock to struct xsk_queue
>>>> because after investigation I reckon try_cmpxchg can add less overhead
>>>> when multiple xsks contend at this point. So I hope this approach
>>>> can be adopted.
>>>
>>> I still think that moving the lock would be preferable, because it makes
>>> sense also from a maintenance perspective.
>>
>> I can see your point here. Sure, moving the lock is relatively easier
>> to understand. But my take is that atomic changes here are not that
>> hard to read :) It has the same effect as spin lock because it will
>> atomically check, compare and set in try_cmpxchg().
>>
>>> Can you report the difference
>>> you measure atomics vs moving the spin lock?
>>
>> No problem, hopefully I will give a detailed report next week because
>> I'm going to apply it directly in production where we have multiple
>> xsk sharing the same umem.
> 
> I'm done with the test in production where a few applications rely on
> multiple xsks sharing the same pool to send UDP packets. Here are
> significant numbers from bcc tool that recorded the latency caused by
> these particular functions:
> 
> 1. use spin lock
> $ sudo ./funclatency xsk_cq_reserve_locked
> Tracing 1 functions for "xsk_cq_reserve_locked"... Hit Ctrl-C to end.
> ^C
>      nsecs               : count     distribution
>          0 -> 1          : 0        |                                        |
>          2 -> 3          : 0        |                                        |
>          4 -> 7          : 0        |                                        |
>          8 -> 15         : 0        |                                        |
>         16 -> 31         : 0        |                                        |
>         32 -> 63         : 0        |                                        |
>         64 -> 127        : 0        |                                        |
>        128 -> 255        : 25308114 |**                                      |
>        256 -> 511        : 283924647 |**********************                  |
>        512 -> 1023       : 501589652 |****************************************|
>       1024 -> 2047       : 93045664 |*******                                 |
>       2048 -> 4095       : 746395   |                                        |
>       4096 -> 8191       : 424053   |                                        |
>       8192 -> 16383      : 1041     |                                        |
>      16384 -> 32767      : 0        |                                        |
>      32768 -> 65535      : 0        |                                        |
>      65536 -> 131071     : 0        |                                        |
>     131072 -> 262143     : 0        |                                        |
>     262144 -> 524287     : 0        |                                        |
>     524288 -> 1048575    : 6        |                                        |
>    1048576 -> 2097151    : 2        |                                        |
> 
> avg = 664 nsecs, total: 601186432273 nsecs, count: 905039574
> 
> 2. use atomic
> $ sudo ./funclatency xsk_cq_cached_prod_reserve
> Tracing 1 functions for "xsk_cq_cached_prod_reserve"... Hit Ctrl-C to end.
> ^C
>      nsecs               : count     distribution
>          0 -> 1          : 0        |                                        |
>          2 -> 3          : 0        |                                        |
>          4 -> 7          : 0        |                                        |
>          8 -> 15         : 0        |                                        |
>         16 -> 31         : 0        |                                        |
>         32 -> 63         : 0        |                                        |
>         64 -> 127        : 0        |                                        |
>        128 -> 255        : 109815401 |*********                               |
>        256 -> 511        : 485028947 |****************************************|
>        512 -> 1023       : 320121627 |**************************              |
>       1024 -> 2047       : 38538584 |***                                     |
>       2048 -> 4095       : 377026   |                                        |
>       4096 -> 8191       : 340961   |                                        |
>       8192 -> 16383      : 549      |                                        |
>      16384 -> 32767      : 0        |                                        |
>      32768 -> 65535      : 0        |                                        |
>      65536 -> 131071     : 0        |                                        |
>     131072 -> 262143     : 0        |                                        |
>     262144 -> 524287     : 0        |                                        |
>     524288 -> 1048575    : 10       |                                        |
> 
> avg = 496 nsecs, total: 473682265261 nsecs, count: 954223105
> 
> And those numbers were verified over and over again which means they
> are quite stable.
> 
> You can see that when using atomic, the avg is smaller and the count
> of [128 -> 255] is larger, which shows better performance.
> 
> I will add the above numbers in the commit log after the merge window is open.

It's not just a matter of performance. Spinlock additionally give you
fairness and lockdep guarantees, beyond being easier to graps for
however is going to touch this code in the future, while raw atomic none
of them.

From a maintainability perspective spinlocks are much more preferable.

IMHO micro-benchmarking is not a strong enough argument to counter the
spinlock adavantages: at very _least_ large performance gain should be
observed in relevant test-cases and/or real live workloads.

>>> Have you tried moving cq_prod_lock, too?
>>
>> Not yet, thanks for reminding me. It should not affect the sending
>> rate but the tx completion time, I think.
> 
> I also tried moving this lock, but sadly I noticed that in completion
> time the lock was set which led to invalidation of the cache line of
> another thread sending packets. It can be obviously proved by perf
> cycles:ppp:
> 1. before
> 8.70% xsk_cq_cached_prod_reserve
> 
> 2. after
> 12.31% xsk_cq_cached_prod_reserve
> 
> So I decided not to bring such a modification. Anyway, thanks for your
> valuable suggestions and I learnt a lot from those interesting
> experiments.

The goal of such change would be reducing the number of touched
cachelines; when I suggested the above, I did not dive into the producer
specifics, I assumed the relevant producer data were inside the
xsk_queue struct.

It looks like the data is actually inside 'struct xdp_ring', so the
producer lock should be moved there, specifically:

struct xdp_ring {
	u32 producer ____cacheline_aligned_in_smp;
	spinlock_t producer_lock;
	// ...	

I'm a bit lost in the structs indirection, but I think the above would
be beneficial even for the ZC path.

/P