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Message-ID: <871f34ca-e417-4e46-8593-b3e10b64b8b9@redhat.com> Date: Tue, 2 Dec 2025 11:52:06 +0100 From: Paolo Abeni <pabeni@...hat.com> To: Stanislav Fomichev <stfomichev@...il.com>, Bobby Eshleman <bobbyeshleman@...il.com> Cc: "David S. Miller" <davem@...emloft.net>, Eric Dumazet <edumazet@...gle.com>, Jakub Kicinski <kuba@...nel.org>, Simon Horman <horms@...nel.org>, netdev@...r.kernel.org, linux-kernel@...r.kernel.org, Mina Almasry <almasrymina@...gle.com>, Stanislav Fomichev <sdf@...ichev.me>, asml.silence@...il.com, Bobby Eshleman <bobbyeshleman@...a.com> Subject: Re: [PATCH net-next] net: devmem: convert binding refcount to percpu_ref On 12/1/25 6:12 PM, Stanislav Fomichev wrote: > On 11/26, Bobby Eshleman wrote: >> From: Bobby Eshleman <bobbyeshleman@...a.com> >> >> 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 and 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@...a.com> >> --- >> net/core/devmem.c | 38 +++++++++++++++++++++++++++++++++----- >> net/core/devmem.h | 18 ++++++++++-------- >> 2 files changed, 43 insertions(+), 13 deletions(-) >> >> diff --git a/net/core/devmem.c b/net/core/devmem.c >> index 1d04754bc756..83989cf4a987 100644 >> --- a/net/core/devmem.c >> +++ b/net/core/devmem.c >> @@ -54,10 +54,26 @@ static dma_addr_t net_devmem_get_dma_addr(const struct net_iov *niov) >> ((dma_addr_t)net_iov_idx(niov) << PAGE_SHIFT); >> } >> >> -void __net_devmem_dmabuf_binding_free(struct work_struct *wq) >> +/* >> + * percpu_ref release callback invoked when the last reference to the binding >> + * is dropped. Schedules the actual cleanup in a workqueue because >> + * ref->release() cb is not allowed to sleep as it may be called in RCU >> + * callback context. >> + */ > > Can we drop this and the rest of the comments? I feel like they mostly > explain how percpu_ref works, nothing devmem specific. I agree with Stan, the code looks good, but the comments are a bit distracting. It should be assumed that people touching this code has read/studied percpu_ref documentation. Please strip them, thanks! Paolo
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