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

["Jason A. Donenfeld" <Jason@zx2c4.com>]

Hi Sebastian,

On Wed, Sep 28, 2022 at 02:06:45PM +0200, Sebastian Andrzej Siewior wrote:
> 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.

Are you sure about that? So far as I can tell drivers/net/ethernet/
mellanox/mlx4 has plenty of napi_schedule/napi_enable and such. Or are
you looking at the infiniband driver instead? I don't really know how
these interact.

But yea, if we've got a driver not using NAPI at 40gbps that's obviously
going to be a problem.

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

Definitely || is preferable to &&.

> 
> > 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.

I've been doing various experiments, sending mini patches to Oracle and
having them test this in their rig. So far, it looks like the cost of
the body of the worker itself doesn't matter much, but rather the cost
of the enqueueing function is key. Still investigating though.

It's a bit frustrating, as all I have to work with are results from the
tests, and no perf analysis. It'd be great if an engineer at Oracle was
capable of tackling this interactively, but at the moment it's just me
sending them patches. So we'll see. Getting closer though, albeit very
slowly.

Jason