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From: "Sean V Kelley" <sean.v.kelley@intel.com>
To: "Bjorn Helgaas" <helgaas@kernel.org>
Cc: "Chris Friesen" <chris.friesen@windriver.com>,
	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 10:39:16 -0700	[thread overview]
Message-ID: <3FF15DD8-72AF-4C1D-BF66-248FC87FD903@intel.com> (raw)
In-Reply-To: <20201012165839.GA3732859@bjorn-Precision-5520>

On 12 Oct 2020, at 9:58, Bjorn Helgaas wrote:

> [+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


Chris,

Are you attempting a tick-less run?  I’ve seen the NO_HZ_FULL (full 
dynticks) feature behave somewhat inconsistently when PREEMPT_RT is 
enabled.  The timer ticks suppression feature can at times appear to be 
not functioning. I’m curious about how you are attempting to isolate 
the cores.

Thanks,

Sean



  reply	other threads:[~2020-10-12 17:39 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 [this message]
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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