From 0843b23890642a02ff85c36406224e8874e30664 Mon Sep 17 00:00:00 2001 From: Shashank Mohan Jain Date: Sun, 27 Sep 2026 10:47:42 +0530 Subject: [PATCH 1/2] lib/dim: fix 32-bit overflow in dim_calc_stats() rates dim_calc_stats() computes the per-millisecond rates as DIV_ROUND_UP(nbytes * USEC_PER_MSEC, delta_us) where nbytes is a u32 and USEC_PER_MSEC is 1000L. On 64-bit the product is done in 64-bit long arithmetic, but on 32-bit architectures long is 32 bits wide and the product wraps as soon as a measurement window carries more than 4294967 bytes (about 4.3 MB). The same applies to the packet and completion counts, although those need more than 4.29 million packets or completions per window. A DIM window spans DIM_NEVENTS (64) events. Drivers count events per interrupt or per NAPI poll, so under sustained load a window can easily carry more than 4.3 MB: 64 full NAPI polls of 64 MTU-sized frames are already 6.2 MB, and drivers such as mtk_eth_soc count one event per interrupt while NAPI keeps polling with the interrupt masked. On 32-bit users of the library (for example mtk_eth_soc on MT7621, bcmgenet and bcmsysport on 32-bit ARM, or virtio_net in a 32-bit guest) bpms then becomes the product modulo 2^32 divided by the window length, and net_dim_stats_compare() makes its BETTER/WORSE decisions on a value that has little to do with the real throughput. For example, a 1 Gbit/s link at line rate that moves 5 MB in a 40 ms window gives bpms = 125000 on 64-bit but 17626 on 32-bit, and 5 million packets in 2 s gives ppms = 353 instead of 2500. Widen the products to 64 bits and divide with DIV_ROUND_UP_ULL(). The results are unchanged on 64-bit. Fixes: cb3c7fd4f839 ("net/mlx5e: Support adaptive RX coalescing") Fixes: 4c4dbb4a7363 ("net/mlx5e: Move dynamic interrupt coalescing code to include/linux") Assisted-by: LLM Signed-off-by: Shashank Mohan Jain Signed-off-by: David S. Miller --- lib/dim/dim.c | 10 +++++++--- 1 file changed, 7 insertions(+), 3 deletions(-) diff --git a/lib/dim/dim.c b/lib/dim/dim.c index 97c3d084ebf0..7f3eebae73cb 100644 --- a/lib/dim/dim.c +++ b/lib/dim/dim.c @@ -69,11 +69,15 @@ bool dim_calc_stats(const struct dim_sample *start, if (!delta_us) return false; - curr_stats->ppms = DIV_ROUND_UP(npkts * USEC_PER_MSEC, delta_us); - curr_stats->bpms = DIV_ROUND_UP(nbytes * USEC_PER_MSEC, delta_us); + /* u32 * USEC_PER_MSEC overflows a 32-bit long */ + curr_stats->ppms = DIV_ROUND_UP_ULL((u64)npkts * USEC_PER_MSEC, + delta_us); + curr_stats->bpms = DIV_ROUND_UP_ULL((u64)nbytes * USEC_PER_MSEC, + delta_us); curr_stats->epms = DIV_ROUND_UP(DIM_NEVENTS * USEC_PER_MSEC, delta_us); - curr_stats->cpms = DIV_ROUND_UP(ncomps * USEC_PER_MSEC, delta_us); + curr_stats->cpms = DIV_ROUND_UP_ULL((u64)ncomps * USEC_PER_MSEC, + delta_us); if (curr_stats->epms != 0) curr_stats->cpe_ratio = DIV_ROUND_DOWN_ULL( curr_stats->cpms * 100, curr_stats->epms); From 636035157a814f289f293fd0edea1a367a637797 Mon Sep 17 00:00:00 2001 From: Shashank Mohan Jain Date: Sun, 27 Sep 2026 10:47:43 +0530 Subject: [PATCH 2/2] lib/dim: add KUnit test for dim_calc_stats() Add a KUnit suite for the DIM library that checks the packet, byte, event and completion rates computed by dim_calc_stats(), including counter wraparound and windows whose byte or packet count times USEC_PER_MSEC does not fit in 32 bits. The latter cases fail on 32-bit architectures without the previous commit. Assisted-by: LLM Signed-off-by: Shashank Mohan Jain Signed-off-by: David S. Miller --- lib/Kconfig.debug | 15 ++++++ lib/dim/Makefile | 2 + lib/dim/dim_kunit.c | 126 ++++++++++++++++++++++++++++++++++++++++++++ 3 files changed, 143 insertions(+) create mode 100644 lib/dim/dim_kunit.c diff --git a/lib/Kconfig.debug b/lib/Kconfig.debug index 134b15a44625..6b24a2c89b84 100644 --- a/lib/Kconfig.debug +++ b/lib/Kconfig.debug @@ -2906,6 +2906,21 @@ config LIVEUPDATE_TEST If unsure, say N +config DIMLIB_KUNIT_TEST + tristate "KUnit test for the DIM library" if !KUNIT_ALL_TESTS + depends on KUNIT && NET + select DIMLIB + default KUNIT_ALL_TESTS + help + This builds the KUnit test for the Dynamic Interrupt Moderation + library (lib/dim/), which checks the rate statistics computed by + dim_calc_stats(). + + For more information on KUnit and unit tests in general please refer + to the KUnit documentation in Documentation/dev-tools/kunit/. + + If unsure, say N. + config CMDLINE_KUNIT_TEST tristate "KUnit test for cmdline API" if !KUNIT_ALL_TESTS depends on KUNIT diff --git a/lib/dim/Makefile b/lib/dim/Makefile index 5b9bfaac7ac1..47e1c36230e4 100644 --- a/lib/dim/Makefile +++ b/lib/dim/Makefile @@ -5,3 +5,5 @@ obj-$(CONFIG_DIMLIB) += dimlib.o dimlib-y := dim.o net_dim.o rdma_dim.o + +obj-$(CONFIG_DIMLIB_KUNIT_TEST) += dim_kunit.o diff --git a/lib/dim/dim_kunit.c b/lib/dim/dim_kunit.c new file mode 100644 index 000000000000..2e984f6b9fe4 --- /dev/null +++ b/lib/dim/dim_kunit.c @@ -0,0 +1,126 @@ +// SPDX-License-Identifier: GPL-2.0 +/* + * KUnit tests for the Dynamic Interrupt Moderation library + */ + +#include +#include +#include +#include + +struct dim_calc_stats_case { + const char *name; + u32 delta_us; + u32 start_pkts, end_pkts; + u32 start_bytes, end_bytes; + u32 start_comps, end_comps; + int ppms, bpms, cpms; +}; + +static const struct dim_calc_stats_case dim_calc_stats_cases[] = { + { + .name = "small", + .delta_us = 1000, + .end_pkts = 640, .end_bytes = 640 * 1500, .end_comps = 64, + .ppms = 640, .bpms = 960000, .cpms = 64, + }, + { + .name = "round_up", + .delta_us = 3000, + .end_pkts = 10, .end_bytes = 10, .end_comps = 1, + .ppms = 4, .bpms = 4, .cpms = 1, + }, + { + .name = "counter_wrap", + .delta_us = 1000, + .start_pkts = 0xffffff00, .end_pkts = 0x100, + .start_bytes = 0xfffff000, .end_bytes = 0x1000, + .start_comps = 0xfffffff0, .end_comps = 0x10, + .ppms = 0x200, .bpms = 0x2000, .cpms = 0x20, + }, + { + /* 30 MB in 10 ms (24 Gbit/s): nbytes * 1000 exceeds 32 bits */ + .name = "many_bytes", + .delta_us = 10000, + .end_pkts = 20000, .end_bytes = 30000000, .end_comps = 64, + .ppms = 2000, .bpms = 3000000, .cpms = 7, + }, + { + /* 4.3 MB in 16 ms (2.15 Gbit/s), just above the 32-bit limit */ + .name = "bytes_32bit_limit", + .delta_us = 16000, + .end_pkts = 2900, .end_bytes = 4300000, .end_comps = 64, + .ppms = 182, .bpms = 268750, .cpms = 4, + }, + { + /* 5 MB in 40 ms: a 1 Gbit/s link at line rate */ + .name = "gigabit", + .delta_us = 40000, + .end_pkts = 3300, .end_bytes = 5000000, .end_comps = 64, + .ppms = 83, .bpms = 125000, .cpms = 2, + }, + { + /* 5 million packets and completions in 2 s */ + .name = "many_packets", + .delta_us = 2000000, + .end_pkts = 5000000, .end_bytes = 5000000, .end_comps = 5000000, + .ppms = 2500, .bpms = 2500, .cpms = 2500, + }, +}; + +static void dim_calc_stats_case_desc(const struct dim_calc_stats_case *t, + char *desc) +{ + strscpy(desc, t->name, KUNIT_PARAM_DESC_SIZE); +} + +KUNIT_ARRAY_PARAM(dim_calc_stats, dim_calc_stats_cases, + dim_calc_stats_case_desc); + +static void dim_calc_stats_test(struct kunit *test) +{ + const struct dim_calc_stats_case *t = test->param_value; + struct dim_sample start = {}, end = {}; + struct dim_stats stats = {}; + + dim_update_sample_with_comps(0, t->start_pkts, t->start_bytes, + t->start_comps, &start); + dim_update_sample_with_comps(DIM_NEVENTS, t->end_pkts, t->end_bytes, + t->end_comps, &end); + /* dim_update_sample() stamps ktime_get(); use fixed times instead */ + start.time = ktime_set(1000, 0); + end.time = ktime_add_us(start.time, t->delta_us); + + KUNIT_ASSERT_TRUE(test, dim_calc_stats(&start, &end, &stats)); + KUNIT_EXPECT_EQ(test, stats.ppms, t->ppms); + KUNIT_EXPECT_EQ(test, stats.bpms, t->bpms); + KUNIT_EXPECT_EQ(test, stats.cpms, t->cpms); + KUNIT_EXPECT_EQ(test, stats.epms, + (int)DIV_ROUND_UP(DIM_NEVENTS * USEC_PER_MSEC, + t->delta_us)); +} + +static void dim_calc_stats_no_time_test(struct kunit *test) +{ + struct dim_sample sample = {}; + struct dim_stats stats = {}; + + dim_update_sample_with_comps(0, 100, 1000, 10, &sample); + KUNIT_EXPECT_FALSE(test, dim_calc_stats(&sample, &sample, &stats)); +} + +static struct kunit_case dim_test_cases[] = { + KUNIT_CASE_PARAM(dim_calc_stats_test, dim_calc_stats_gen_params), + KUNIT_CASE(dim_calc_stats_no_time_test), + {} +}; + +static struct kunit_suite dim_test_suite = { + .name = "dim", + .test_cases = dim_test_cases, +}; + +kunit_test_suite(dim_test_suite); + +MODULE_DESCRIPTION("KUnit tests for the DIM library"); +MODULE_LICENSE("GPL");