linux-kernel - Re: [PATCH v3 2/2] irqchip/gic-v3-its: Balance initial LPI affinity across CPUs

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Date:   Tue, 17 Mar 2020 18:43:01 +0000
From:   John Garry <john.garry@...wei.com>
To:     Marc Zyngier <maz@...nel.org>
CC:     <linux-kernel@...r.kernel.org>,
        <linux-arm-kernel@...ts.infradead.org>,
        chenxiang <chenxiang66@...ilicon.com>,
        Zhou Wang <wangzhou1@...ilicon.com>,
        Ming Lei <ming.lei@...hat.com>,
        Jason Cooper <jason@...edaemon.net>,
        "Thomas Gleixner" <tglx@...utronix.de>, <luojiaxing@...wei.com>
Subject: Re: [PATCH v3 2/2] irqchip/gic-v3-its: Balance initial LPI affinity
 across CPUs

>>
>>> +        int this_count = its_read_lpi_count(d, tmp);
>>
>> Not sure if it's intentional, but now there seems to be a subtle
>> difference to what Thomas described for non-managed interrupts - for
>> non-managed interrupts, x86 selects the CPU based on the total
>> interrupt load per CPU (or, more specifically, lowest vector
>> allocation count), and not just the non-managed load. Or maybe I
>> misread it.
> 
> So far, I'm trying to keep the two allocation paths separate, as the
> two systems I have access to have very different behaviours: D05 has
> no managed interrupts to speak of, and my top-secret work machine
> has almost no unmanaged interrupts, so the two sets are almost
> completely disjoint.

Sure, but I'd say that it would be a more common scenario to have a 
mixture of both.

> 
> Also, it all depends on the interrupt allocation order, and whether
> something will rebalance the non-managed interrupts at a later time.
> At least, these two patches make it easy to alter the placement policy
> (the behaviour you describe above is a 2 line change).
> 
>> Anyway, we can test this now for NVMe with its managed interrupts.
> 
> Looking forward to hearing from you!
>

On my D06CS board (128 core), there seems to be something wrong, as the 
q0 affinity mask looks incorrect:

PCI name is 81:00.0: nvme0n1 
 

         irq 322, cpu list 69, effective list 69 
 

         irq 325, cpu list 32-38, effective list 32 
 

         irq 326, cpu list 39-45, effective list 40 
 

         irq 327, cpu list 46-51, effective list 47 
 

         irq 328, cpu list 52-57, effective list 53 
 

         irq 329, cpu list 58-63, effective list 59


And something stranger for my colleague Luo Jiaxing, specifically the 
effective affinity:

PCI name is 85:00.0: nvme2n1
irq 196, cpu list 0-31, effective list 82
irq 377, cpu list 32-38, effective list 32
irq 378, cpu list 39-45, effective list 39
irq 379, cpu list 46-51, effective list 46

But then v5.6-rc5 vanilla also looks to have this issue when I tested on 
my board:

john@...ntu:~$ more /proc/irq/322/smp_affinity_list 
 

69

My D06ES (96 core) board looks sensible for the affinity in this regard 
(I did not try vanilla v5.6-rc5, but only with your patches on top). 
I'll need to debug this.

Cheers,
John