linux-kernel - Re: [PATCH v4 4/4] PCI: Limit pci_alloc_irq

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Message-ID: <875z6w4xt4.fsf@nanos.tec.linutronix.de>
Date:   Mon, 26 Oct 2020 20:00:39 +0100
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Marcelo Tosatti <mtosatti@...hat.com>
Cc:     Nitesh Narayan Lal <nitesh@...hat.com>,
        Peter Zijlstra <peterz@...radead.org>, helgaas@...nel.org,
        linux-kernel@...r.kernel.org, netdev@...r.kernel.org,
        linux-pci@...r.kernel.org, intel-wired-lan@...ts.osuosl.org,
        frederic@...nel.org, sassmann@...hat.com,
        jesse.brandeburg@...el.com, lihong.yang@...el.com,
        jeffrey.t.kirsher@...el.com, jacob.e.keller@...el.com,
        jlelli@...hat.com, hch@...radead.org, bhelgaas@...gle.com,
        mike.marciniszyn@...el.com, dennis.dalessandro@...el.com,
        thomas.lendacky@....com, jiri@...dia.com, mingo@...hat.com,
        juri.lelli@...hat.com, vincent.guittot@...aro.org,
        lgoncalv@...hat.com
Subject: Re: [PATCH v4 4/4] PCI: Limit pci_alloc_irq_vectors() to housekeeping CPUs

On Mon, Oct 26 2020 at 14:30, Marcelo Tosatti wrote:
> On Fri, Oct 23, 2020 at 11:00:52PM +0200, Thomas Gleixner wrote:
>> So without information from the driver which tells what the best number
>> of interrupts is with a reduced number of CPUs, this cutoff will cause
>> more problems than it solves. Regressions guaranteed.
>
> One might want to move from one interrupt per isolated app core
> to zero, or vice versa. It seems that "best number of interrupts 
> is with reduced number of CPUs" information, is therefore in userspace, 
> not in driver...

How does userspace know about the driver internals? Number of management
interrupts, optimal number of interrupts per queue?

>> Managed interrupts base their interrupt allocation and spreading on
>> information which is handed in by the individual driver and not on crude
>> assumptions. They are not imposing restrictions on the use case.
>> 
>> It's perfectly fine for isolated work to save a data set to disk after
>> computation has finished and that just works with the per-cpu I/O queue
>> which is otherwise completely silent. 
>
> Userspace could only change the mask of interrupts which are not 
> triggered by requests from the local CPU (admin, error, mgmt, etc),
> to avoid the vector exhaustion problem.
>
> However, there is no explicit way for userspace to know that, as far as
> i know.
>
>  130:      34845          0          0          0          0          0          0          0  IR-PCI-MSI 33554433-edge      nvme0q1
>  131:          0      27062          0          0          0          0          0          0  IR-PCI-MSI 33554434-edge      nvme0q2
>  132:          0          0      24393          0          0          0          0          0  IR-PCI-MSI 33554435-edge      nvme0q3
>  133:          0          0          0      24313          0          0          0          0  IR-PCI-MSI 33554436-edge      nvme0q4
>  134:          0          0          0          0      20608          0          0          0  IR-PCI-MSI 33554437-edge      nvme0q5
>  135:          0          0          0          0          0      22163          0          0  IR-PCI-MSI 33554438-edge      nvme0q6
>  136:          0          0          0          0          0          0      23020          0  IR-PCI-MSI 33554439-edge      nvme0q7
>  137:          0          0          0          0          0          0          0      24285  IR-PCI-MSI 33554440-edge      nvme0q8
>
> Can that be retrieved from PCI-MSI information, or drivers
> have to inform this?

The driver should use a different name for the admin queues.

Thanks,

        tglx