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From:   Thomas Gleixner <tglx@linutronix.de>
To:     Marcelo Tosatti <mtosatti@redhat.com>
Cc:     Nitesh Narayan Lal <nitesh@redhat.com>,
        Peter Zijlstra <peterz@infradead.org>, helgaas@kernel.org,
        linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
        linux-pci@vger.kernel.org, intel-wired-lan@lists.osuosl.org,
        frederic@kernel.org, sassmann@redhat.com,
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        thomas.lendacky@amd.com, jiri@nvidia.com, mingo@redhat.com,
        juri.lelli@redhat.com, vincent.guittot@linaro.org,
        lgoncalv@redhat.com
Subject: Re: [PATCH v4 4/4] PCI: Limit pci_alloc_irq_vectors() to housekeeping CPUs
In-Reply-To: <20201026173012.GA377978@fuller.cnet>
References: <20201019111137.GL2628@hirez.programming.kicks-ass.net> <20201019140005.GB17287@fuller.cnet> <20201020073055.GY2611@hirez.programming.kicks-ass.net> <078e659e-d151-5bc2-a7dd-fe0070267cb3@redhat.com> <20201020134128.GT2628@hirez.programming.kicks-ass.net> <6736e643-d4ae-9919-9ae1-a73d5f31463e@redhat.com> <260f4191-5b9f-6dc1-9f11-085533ac4f55@redhat.com> <20201023085826.GP2611@hirez.programming.kicks-ass.net> <9ee77056-ef02-8696-5b96-46007e35ab00@redhat.com> <87ft6464jf.fsf@nanos.tec.linutronix.de> <20201026173012.GA377978@fuller.cnet>
Date:   Mon, 26 Oct 2020 20:00:39 +0100
Message-ID: <875z6w4xt4.fsf@nanos.tec.linutronix.de>
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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