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From: Bjorn Helgaas <helgaas@kernel.org>
To: Chris Friesen <chris.friesen@windriver.com>
Cc: linux-pci@vger.kernel.org, Christoph Hellwig <hch@lst.de>,
	Thomas Gleixner <tglx@linutronix.de>,
	Nitesh Narayan Lal <nitesh@redhat.com>
Subject: Re: PCI, isolcpus, and irq affinity
Date: Mon, 12 Oct 2020 11:58:39 -0500	[thread overview]
Message-ID: <20201012165839.GA3732859@bjorn-Precision-5520> (raw)
In-Reply-To: <83d03f27-d92d-1b28-ad28-cbe5d011fe33@windriver.com>

[+cc Christoph, Thomas, Nitesh]

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).

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.

> Looking at the code, it appears that the NVME driver can in some scenarios
> end up calling pci_alloc_irq_vectors_affinity() after initial system
> startup, which seems to determine CPU affinity without any regard for things
> like "isolcpus" or "cset shield".
> 
> There seem to be other reports of similar issues:
> 
> https://bugs.launchpad.net/ubuntu/+source/linux/+bug/1831566
> 
> It looks like some SCSI drivers and virtio_pci_common.c will also call
> pci_alloc_irq_vectors_affinity(), though I'm not sure if they would ever do
> it after system startup.
> 
> How does it make sense for the PCI subsystem to affine interrupts to CPUs
> which have explicitly been designated as "isolated"?

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.

Bjorn

  reply	other threads:[~2020-10-12 16:58 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 [this message]
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
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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