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From: Thomas Gleixner <tglx@linutronix.de>
To: Bjorn Helgaas <helgaas@kernel.org>,
	Chris Friesen <chris.friesen@windriver.com>
Cc: linux-pci@vger.kernel.org, Christoph Hellwig <hch@lst.de>,
	Nitesh Narayan Lal <nitesh@redhat.com>
Subject: Re: PCI, isolcpus, and irq affinity
Date: Mon, 12 Oct 2020 19:50:04 +0200	[thread overview]
Message-ID: <87a6wrqqpf.fsf@nanos.tec.linutronix.de> (raw)
In-Reply-To: <20201012165839.GA3732859@bjorn-Precision-5520>

On Mon, Oct 12 2020 at 11:58, Bjorn Helgaas wrote:
> On Mon, Oct 12, 2020 at 09:49:37AM -0600, Chris Friesen wrote:
>> I've got a linux system running the RT kernel with threaded irqs.  On
>> startup we affine the various irq threads to the housekeeping CPUs, but I
>> recently hit a scenario where after some days of uptime we ended up with a
>> number of NVME irq threads affined to application cores instead (not good
>> when we're trying to run low-latency applications).

These threads and the associated interupt vectors are completely
harmless and fully idle as long as there is nothing on those isolated
CPUs which does disk I/O.

> pci_alloc_irq_vectors_affinity() basically just passes affinity
> information through to kernel/irq/affinity.c, and the PCI core doesn't
> change affinity after that.

Correct.

> This recent thread may be useful:
>
>   https://lore.kernel.org/linux-pci/20200928183529.471328-1-nitesh@redhat.com/
>
> It contains a patch to "Limit pci_alloc_irq_vectors() to housekeeping
> CPUs".  I'm not sure that patch summary is 100% accurate because IIUC
> that particular patch only reduces the *number* of vectors allocated
> and does not actually *limit* them to housekeeping CPUs.

That patch is a bandaid at best and for the managed interrupt scenario
not really preventing that interrupts + threads are affine to isolated
CPUs.

Thanks,

        tglx


  parent reply	other threads:[~2020-10-12 17:50 UTC|newest]

Thread overview: 14+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-10-12 15:49 Chris Friesen
2020-10-12 16:58 ` Bjorn Helgaas
2020-10-12 17:39   ` Sean V Kelley
2020-10-12 19:18     ` Chris Friesen
2020-10-12 17:42   ` Nitesh Narayan Lal
2020-10-12 17:50   ` Thomas Gleixner [this message]
2020-10-12 18:58     ` Chris Friesen
2020-10-12 19:07       ` Keith Busch
2020-10-12 19:44         ` Thomas Gleixner
2020-10-15 18:47         ` Chris Friesen
2020-10-15 19:02           ` Keith Busch
2020-10-12 19:31       ` Thomas Gleixner
2020-10-12 20:24         ` David Woodhouse
2020-10-12 22:25           ` Thomas Gleixner

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