Re: [PATCH v3] random: use expired per-cpu timer rather than wq for mixing fast pool

[Sebastian Andrzej Siewior <bigeasy@linutronix.de>]

On 2022-09-27 12:42:33 [+0200], Jason A. Donenfeld wrote:
…
> This is an ordinary pattern done all over the kernel. However, Sherry
> noticed a 10% performance regression in qperf TCP over a 40gbps
> InfiniBand card. Quoting her message:
> 
> > MT27500 Family [ConnectX-3] cards:
> > Infiniband device 'mlx4_0' port 1 status:
…

While looking at the mlx4 driver, it looks like they don't use any NAPI
handling in their interrupt handler which _might_ be the case that they
handle more than 1k interrupts a second. I'm still curious to get that
ACKed from Sherry's side.

Jason, from random's point of view: deferring until 1k interrupts + 1sec
delay is not desired due to low entropy, right?

> Rather than incur the scheduling latency from queue_work_on, we can
> instead switch to running on the next timer tick, on the same core. This
> also batches things a bit more -- once per jiffy -- which is okay now
> that mix_interrupt_randomness() can credit multiple bits at once.

Hmmm. Do you see higher contention on input_pool.lock? Just asking
because if more than once CPUs invokes this timer callback aligned, then
they block on the same lock.

Sebastian