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