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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>

authored by

Bobby Eshleman and committed by
Jakub Kicinski 5c024716 218f8dc9

+22 -10
+19 -3
net/core/devmem.c
··· 54 54 ((dma_addr_t)net_iov_idx(niov) << PAGE_SHIFT); 55 55 } 56 56 57 + static void net_devmem_dmabuf_binding_release(struct percpu_ref *ref) 58 + { 59 + struct net_devmem_dmabuf_binding *binding = 60 + container_of(ref, struct net_devmem_dmabuf_binding, ref); 61 + 62 + INIT_WORK(&binding->unbind_w, __net_devmem_dmabuf_binding_free); 63 + schedule_work(&binding->unbind_w); 64 + } 65 + 57 66 void __net_devmem_dmabuf_binding_free(struct work_struct *wq) 58 67 { 59 68 struct net_devmem_dmabuf_binding *binding = container_of(wq, typeof(*binding), unbind_w); ··· 84 75 dma_buf_detach(binding->dmabuf, binding->attachment); 85 76 dma_buf_put(binding->dmabuf); 86 77 xa_destroy(&binding->bound_rxqs); 78 + percpu_ref_exit(&binding->ref); 87 79 kvfree(binding->tx_vec); 88 80 kfree(binding); 89 81 } ··· 153 143 __net_mp_close_rxq(binding->dev, rxq_idx, &mp_params); 154 144 } 155 145 156 - net_devmem_dmabuf_binding_put(binding); 146 + percpu_ref_kill(&binding->ref); 157 147 } 158 148 159 149 int net_devmem_bind_dmabuf_to_queue(struct net_device *dev, u32 rxq_idx, ··· 219 209 binding->dev = dev; 220 210 xa_init_flags(&binding->bound_rxqs, XA_FLAGS_ALLOC); 221 211 222 - refcount_set(&binding->ref, 1); 212 + err = percpu_ref_init(&binding->ref, 213 + net_devmem_dmabuf_binding_release, 214 + 0, GFP_KERNEL); 215 + if (err < 0) 216 + goto err_free_binding; 223 217 224 218 mutex_init(&binding->lock); 225 219 ··· 234 220 if (IS_ERR(binding->attachment)) { 235 221 err = PTR_ERR(binding->attachment); 236 222 NL_SET_ERR_MSG(extack, "Failed to bind dmabuf to device"); 237 - goto err_free_binding; 223 + goto err_exit_ref; 238 224 } 239 225 240 226 binding->sgt = dma_buf_map_attachment_unlocked(binding->attachment, ··· 336 322 direction); 337 323 err_detach: 338 324 dma_buf_detach(dmabuf, binding->attachment); 325 + err_exit_ref: 326 + percpu_ref_exit(&binding->ref); 339 327 err_free_binding: 340 328 kfree(binding); 341 329 err_put_dmabuf:
+3 -7
net/core/devmem.h
··· 41 41 * retransmits) hold a reference to the binding until the skb holding 42 42 * them is freed. 43 43 */ 44 - refcount_t ref; 44 + struct percpu_ref ref; 45 45 46 46 /* The list of bindings currently active. Used for netlink to notify us 47 47 * of the user dropping the bind. ··· 125 125 static inline bool 126 126 net_devmem_dmabuf_binding_get(struct net_devmem_dmabuf_binding *binding) 127 127 { 128 - return refcount_inc_not_zero(&binding->ref); 128 + return percpu_ref_tryget(&binding->ref); 129 129 } 130 130 131 131 static inline void 132 132 net_devmem_dmabuf_binding_put(struct net_devmem_dmabuf_binding *binding) 133 133 { 134 - if (!refcount_dec_and_test(&binding->ref)) 135 - return; 136 - 137 - INIT_WORK(&binding->unbind_w, __net_devmem_dmabuf_binding_free); 138 - schedule_work(&binding->unbind_w); 134 + percpu_ref_put(&binding->ref); 139 135 } 140 136 141 137 void net_devmem_get_net_iov(struct net_iov *niov);