On Tue, Sep 27, 2022 at 07:41:52AM +0000, David Laight wrote:
From: Jason A. Donenfeld
Sent: 26 September 2022 23:05
Previously, the fast pool was dumped into the main pool peroidically in the fast pool's hard IRQ handler. This worked fine and there weren't problems with it, until RT came around. Since RT converts spinlocks into sleeping locks, problems cropped up. Rather than switching to raw spinlocks, the RT developers preferred we make the transformation from originally doing:
do_some_stuff() spin_lock() do_some_other_stuff() spin_unlock()to doing:
do_some_stuff() queue_work_on(some_other_stuff_worker)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: default gid: fe80:0000:0000:0000:0010:e000:0178:9eb1 base lid: 0x6 sm lid: 0x1 state: 4: ACTIVE phys state: 5: LinkUp rate: 40 Gb/sec (4X QDR) link_layer: InfiniBand
Cards are configured with IP addresses on private subnet for IPoIB performance testing. Regression identified in this bug is in TCP latency in this stack as reported by qperf tcp_lat metric:
We have one system listen as a qperf server: [root@yourQperfServer ~]# qperf
Have the other system connect to qperf server as a client (in this case, it’s X7 server with Mellanox card): [root@yourQperfClient ~]# numactl -m0 -N0 qperf 20.20.20.101 -v -uu -ub --time 60 --wait_server 20 -
oo msg_size:4K:1024K:*2 tcp_lat
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, deferrably so. This also batches things a bit more -- once per jiffy -- which is probably okay now that mix_interrupt_randomness() can credit multiple bits at once. It still puts a bit of pressure on fast_mix(), but hopefully that's acceptable.
I though NOHZ systems didn't take a timer interrupt every 'jiffy'. If that is true what actually happens?
The TIMER_DEFERRABLE part of this patch is a mistake; I'm going to make that 0. However, since expires==jiffies, there's no difference. It's still undesirable though.
Jason