Re: [PATCH net 1/2] lib/dim: fix 32-bit overflow in dim_calc_stats() rates

[Leon Romanovsky]

On Wed, Sep 30, 2026 at 02:58:37PM +0530, shashank Jain wrote:
> > How does this 32-bit overflow differ from the overflow that can also occur on
> > 64-bit systems in `nbytes * USEC_PER_MSEC`?
> 
> On 64-bit the multiplication itself cannot overflow. nbytes (like npkts
> and ncomps) is a u32, and USEC_PER_MSEC is 1000L, so the product is
> computed in a 64-bit long and is at most (2^32 - 1) * 1000, about
> 4.3 * 10^12 (42 bits). DIV_ROUND_UP() adds at most delta_us - 1 to that,
> which also fits. So on 64-bit the product is always exact.
> 
> On 32-bit the same product is computed in a 32-bit long and wraps once
> nbytes exceeds 4294967, i.e. about 4.3 MB in one DIM window. That is
> reached at normal line rates, which is what the patch fixes. With the
> u64 cast and DIV_ROUND_UP_ULL() the 32-bit results are the same as on
> 64-bit.
> 
> There are two other limits, but they apply to 32-bit and 64-bit alike
> and the patch does not change them:
> 
> - The counters in struct dim_sample are u32 and BIT_GAP() takes the
>   difference modulo 2^32, so a window that carries 4 GiB or more is
>   already undercounted before the multiplication. With 64 events per
>   window that needs a very long window at very high rates (for example
>   about 86 ms at 400 Gbit/s).
> 
> - The rates are stored in int fields of struct dim_stats. bpms is bytes
>   per millisecond, so it only exceeds INT_MAX above 2^31 bytes/ms,
>   about 17 Tbit/s.
> 
> I can add a sentence to the changelog saying that the 64-bit product
> cannot overflow, if you think that helps.

There is no need.

Thanks

> 
> Thanks,
> Shashank
> 
> On Wed, Sep 30, 2026 at 2:19 PM Leon Romanovsky <leon@xxxxxxxxxx> wrote:
> >
> > On Sun, Sep 27, 2026 at 10:47:42AM +0530, Shashank Mohan Jain wrote:
> > > 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.
> >
> > How does this 32-bit overflow differ from the overflow that can also occur on
> > 64-bit systems in `nbytes * USEC_PER_MSEC`?
> >
> > Thanks