Re: [PATCH] PCI: iproc-msi: Bound events handled per IRQ

[Florian Fainelli <florian.fainelli@broadcom.com>]

On 10/16/26 05:18, 'Eugen Hristev' via BCM-KERNEL-FEEDBACK-LIST,PDL wrote:
> Limit each iProc MSI parent IRQ to 16 event-queue entries.
> 
> A continuously fed queue could otherwise keep the chained handler running
> and starve other work. The budget still drains pending events and lets the
> next parent interrupt continue processing, improving fairness and bounding
> interrupt-handler latency.
> 
> Without this starvation avoidance, one CPU can stall indefinitely
> in the loop and not serve other interrupts, timers, etc.
> 
> While testing it with an Ethernet NIC,  when too many MSI interrupts
> come in, CPU might get starved:
> 
> Connecting to host 192.168.30.1, port 5201
> [  5] local 192.168.30.2 port 52586 connected to 192.168.30.1 port 5201
> [ ID] Interval           Transfer     Bitrate         Retr  Cwnd
> [  5]   0.00-1.00   sec   240 MBytes  2.01 Gbits/sec    2    334 KBytes
> [  5]   1.00-2.00   sec  63.2 MBytes   531 Mbits/sec    1   1.41 KBytes
> [  5]   2.00-3.00   sec  0.00 Bytes  0.00 bits/sec    0   1.41 KBytes
> [  5]   3.00-4.00   sec  0.00 Bytes  0.00 bits/sec    0   1.41 KBytes
> [  5]   4.00-5.00   sec  0.00 Bytes  0.00 bits/sec    0   1.41 KBytes
> [  5]   5.00-6.00   sec  0.00 Bytes  0.00 bits/sec    0   1.41 KBytes
> [  5]   6.00-7.00   sec  0.00 Bytes  0.00 bits/sec    0   1.41 KBytes
> [  5]   7.00-8.00   sec  0.00 Bytes  0.00 bits/sec    0   1.41 KBytes
> [  5]   8.00-9.00   sec  0.00 Bytes  0.00 bits/sec    0   1.41 KBytes
> [  177.568497] rcu: INFO: rcu_preempt detected stalls on CPUs/tasks:
> [  177.568913] rcu:     0-...!: (1 GPs behind) idle=28ac/1/0x4000000000000002 softirq=28922/28930 fqs=1
> [  177.569457] rcu:     (detected by 1, t=5256 jiffies, g=21629, q=819 ncpus=4)
> [  177.569870] Sending NMI from CPU 1 to CPUs 0:
> 
> With this bounded approach, IRQ handler will complete and continue
> to serve MSI events on the next handler invocation.
> 
> Assisted-by: Github Copilot
> Signed-off-by: Eugen Hristev <ehristev@kernel.org>

Reviewed-by: Florian Fainelli <florian.fainelli@broadcom.com>
-- 
Florian