net: devmem: convert binding refcount to percpu_ref

net: devmem: convert binding refcount to percpu_ref

Convert net_devmem_dmabuf_binding refcount from refcount_t to percpu_ref
to optimize common-case reference counting on the hot path.

The typical devmem workflow involves binding a dmabuf to a queue
(acquiring the initial reference on binding->ref), followed by
high-volume traffic where every skb fragment acquires a reference.
Eventually traffic stops and the unbind operation releases the initial
reference. Additionally, the high traffic hot path is often multi-core.
This access pattern is ideal for percpu_ref as the first and last
reference during bind/unbind normally book-ends activity in the hot
path.

__net_devmem_dmabuf_binding_free becomes the percpu_ref callback invoked
when the last reference is dropped.

kperf test:
- 4MB message sizes
- 60s of workload each run
- 5 runs
- 4 flows

Throughput:
	Before: 45.31 GB/s (+/- 3.17 GB/s)
	After: 48.67 GB/s (+/- 0.01 GB/s)

Picking throughput-matched kperf runs (both before and after matched at
~48 GB/s) for apples-to-apples comparison:

Summary (averaged across 4 workers):

  TX worker CPU idle %:
    Before: 34.44%
    After: 87.13%

  RX worker CPU idle %:
    Before: 5.38%
    After: 9.73%

kperf before:

client: == Source
client:   Tx 98.100 Gbps (735764807680 bytes in 60001149 usec)
client:   Tx102.798 Gbps (770996961280 bytes in 60001149 usec)
client:   Tx101.534 Gbps (761517834240 bytes in 60001149 usec)
client:   Tx 82.794 Gbps (620966707200 bytes in 60001149 usec)
client:   net CPU 56: usr: 0.01% sys: 0.12% idle:17.06% iow: 0.00% irq: 9.89% sirq:72.91%
client:   app CPU 60: usr: 0.08% sys:63.30% idle:36.24% iow: 0.00% irq: 0.30% sirq: 0.06%
client:   net CPU 57: usr: 0.03% sys: 0.08% idle:75.68% iow: 0.00% irq: 2.96% sirq:21.23%
client:   app CPU 61: usr: 0.06% sys:67.67% idle:31.94% iow: 0.00% irq: 0.28% sirq: 0.03%
client:   net CPU 58: usr: 0.01% sys: 0.06% idle:76.87% iow: 0.00% irq: 2.84% sirq:20.19%
client:   app CPU 62: usr: 0.06% sys:69.78% idle:29.79% iow: 0.00% irq: 0.30% sirq: 0.05%
client:   net CPU 59: usr: 0.06% sys: 0.16% idle:74.97% iow: 0.00% irq: 3.76% sirq:21.03%
client:   app CPU 63: usr: 0.06% sys:59.82% idle:39.80% iow: 0.00% irq: 0.25% sirq: 0.05%
client: == Target
client:   Rx 98.092 Gbps (735764807680 bytes in 60006084 usec)
client:   Rx102.785 Gbps (770962161664 bytes in 60006084 usec)
client:   Rx101.523 Gbps (761499566080 bytes in 60006084 usec)
client:   Rx 82.783 Gbps (620933136384 bytes in 60006084 usec)
client:   net CPU  2: usr: 0.00% sys: 0.01% idle:24.51% iow: 0.00% irq: 1.67% sirq:73.79%
client:   app CPU  6: usr: 1.51% sys:96.43% idle: 1.13% iow: 0.00% irq: 0.36% sirq: 0.55%
client:   net CPU  1: usr: 0.00% sys: 0.01% idle:25.18% iow: 0.00% irq: 1.99% sirq:72.80%
client:   app CPU  5: usr: 2.21% sys:94.54% idle: 2.54% iow: 0.00% irq: 0.38% sirq: 0.30%
client:   net CPU  3: usr: 0.00% sys: 0.01% idle:26.34% iow: 0.00% irq: 2.12% sirq:71.51%
client:   app CPU  7: usr: 2.22% sys:94.28% idle: 2.52% iow: 0.00% irq: 0.59% sirq: 0.37%
client:   net CPU  0: usr: 0.00% sys: 0.03% idle: 0.00% iow: 0.00% irq:10.44% sirq:89.51%
client:   app CPU  4: usr: 2.39% sys:81.46% idle:15.33% iow: 0.00% irq: 0.50% sirq: 0.30%

kperf after:

client: == Source
client:   Tx 99.257 Gbps (744447016960 bytes in 60001303 usec)
client:   Tx101.013 Gbps (757617131520 bytes in 60001303 usec)
client:   Tx 88.179 Gbps (661357854720 bytes in 60001303 usec)
client:   Tx101.002 Gbps (757533245440 bytes in 60001303 usec)
client:   net CPU 56: usr: 0.00% sys: 0.01% idle: 6.22% iow: 0.00% irq: 8.68% sirq:85.06%
client:   app CPU 60: usr: 0.08% sys:12.56% idle:87.21% iow: 0.00% irq: 0.08% sirq: 0.05%
client:   net CPU 57: usr: 0.00% sys: 0.05% idle:69.53% iow: 0.00% irq: 2.02% sirq:28.38%
client:   app CPU 61: usr: 0.11% sys:13.40% idle:86.36% iow: 0.00% irq: 0.08% sirq: 0.03%
client:   net CPU 58: usr: 0.00% sys: 0.03% idle:70.04% iow: 0.00% irq: 3.38% sirq:26.53%
client:   app CPU 62: usr: 0.10% sys:11.46% idle:88.31% iow: 0.00% irq: 0.08% sirq: 0.03%
client:   net CPU 59: usr: 0.01% sys: 0.06% idle:71.18% iow: 0.00% irq: 1.97% sirq:26.75%
client:   app CPU 63: usr: 0.10% sys:13.10% idle:86.64% iow: 0.00% irq: 0.10% sirq: 0.05%
client: == Target
client:   Rx 99.250 Gbps (744415182848 bytes in 60003297 usec)
client:   Rx101.006 Gbps (757589737472 bytes in 60003297 usec)
client:   Rx 88.171 Gbps (661319475200 bytes in 60003297 usec)
client:   Rx100.996 Gbps (757514792960 bytes in 60003297 usec)
client:   net CPU  2: usr: 0.00% sys: 0.01% idle:28.02% iow: 0.00% irq: 1.95% sirq:70.00%
client:   app CPU  6: usr: 2.03% sys:87.20% idle:10.04% iow: 0.00% irq: 0.37% sirq: 0.33%
client:   net CPU  3: usr: 0.00% sys: 0.00% idle:27.63% iow: 0.00% irq: 1.90% sirq:70.45%
client:   app CPU  7: usr: 1.78% sys:89.70% idle: 7.79% iow: 0.00% irq: 0.37% sirq: 0.34%
client:   net CPU  0: usr: 0.00% sys: 0.01% idle: 0.00% iow: 0.00% irq: 9.96% sirq:90.01%
client:   app CPU  4: usr: 2.33% sys:83.51% idle:13.24% iow: 0.00% irq: 0.64% sirq: 0.26%
client:   net CPU  1: usr: 0.00% sys: 0.01% idle:27.60% iow: 0.00% irq: 1.94% sirq:70.43%
client:   app CPU  5: usr: 1.88% sys:89.61% idle: 7.86% iow: 0.00% irq: 0.35% sirq: 0.27%

Signed-off-by: Bobby Eshleman <bobbyeshleman@meta.com>
Acked-by: Stanislav Fomichev <sdf@fomichev.me>
Link: https://patch.msgid.link/20260107-upstream-precpu-ref-v2-v2-1-a709f098b3dc@meta.com
Signed-off-by: Jakub Kicinski <kuba@kernel.org>