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 <jain.sm@gmail.com>
Signed-off-by: David S. Miller <davem@davemloft.net>

0 files changed, 0 insertions, 0 deletions