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
