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From: Jakub Kicinski <kuba@kernel.org>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Nitesh Narayan Lal <nitesh@redhat.com>,
	linux-kernel@vger.kernel.org, netdev@vger.kernel.org,
	linux-pci@vger.kernel.org, intel-wired-lan@lists.osuosl.org,
	frederic@kernel.org, mtosatti@redhat.com, sassmann@redhat.com,
	jesse.brandeburg@intel.com, lihong.yang@intel.com,
	helgaas@kernel.org, jeffrey.t.kirsher@intel.com,
	jacob.e.keller@intel.com, jlelli@redhat.com, hch@infradead.org,
	bhelgaas@google.com, mike.marciniszyn@intel.com,
	dennis.dalessandro@intel.com, thomas.lendacky@amd.com,
	jiri@nvidia.com, mingo@redhat.com, peterz@infradead.org,
	juri.lelli@redhat.com, vincent.guittot@linaro.org,
	lgoncalv@redhat.com, Dave Miller <davem@davemloft.net>,
	Magnus Karlsson <magnus.karlsson@intel.com>,
	Saeed Mahameed <saeedm@nvidia.com>
Subject: Re: [PATCH v4 4/4] PCI: Limit pci_alloc_irq_vectors() to housekeeping CPUs
Date: Wed, 21 Oct 2020 17:02:24 -0700
Message-ID: <20201021170224.55aea948@kicinski-fedora-pc1c0hjn.dhcp.thefacebook.com> (raw)
In-Reply-To: <877drj72cz.fsf@nanos.tec.linutronix.de>

On Wed, 21 Oct 2020 22:25:48 +0200 Thomas Gleixner wrote:
> On Tue, Oct 20 2020 at 20:07, Thomas Gleixner wrote:
> > On Tue, Oct 20 2020 at 12:18, Nitesh Narayan Lal wrote:  
> >> However, IMHO we would still need a logic to prevent the devices from
> >> creating excess vectors.  
> >
> > Managed interrupts are preventing exactly that by pinning the interrupts
> > and queues to one or a set of CPUs, which prevents vector exhaustion on
> > CPU hotplug.
> >
> > Non-managed, yes that is and always was a problem. One of the reasons
> > why managed interrupts exist.  
> 
> But why is this only a problem for isolation? The very same problem
> exists vs. CPU hotplug and therefore hibernation.
> 
> On x86 we have at max. 204 vectors available for device interrupts per
> CPU. So assumed the only device interrupt in use is networking then any
> machine which has more than 204 network interrupts (queues, aux ...)
> active will prevent the machine from hibernation.
> 
> Aside of that it's silly to have multiple queues targeted at a single
> CPU in case of hotplug. And that's not a theoretical problem.  Some
> power management schemes shut down sockets when the utilization of a
> system is low enough, e.g. outside of working hours.
> 
> The whole point of multi-queue is to have locality so that traffic from
> a CPU goes through the CPU local queue. What's the point of having two
> or more queues on a CPU in case of hotplug?
> 
> The right answer to this is to utilize managed interrupts and have
> according logic in your network driver to handle CPU hotplug. When a CPU
> goes down, then the queue which is associated to that CPU is quiesced
> and the interrupt core shuts down the relevant interrupt instead of
> moving it to an online CPU (which causes the whole vector exhaustion
> problem on x86). When the CPU comes online again, then the interrupt is
> reenabled in the core and the driver reactivates the queue.

I think Mellanox folks made some forays into managed irqs, but I don't
remember/can't find the details now.

For networking the locality / queue per core does not always work,
since the incoming traffic is usually spread based on a hash. Many
applications perform better when network processing is done on a small
subset of CPUs, and application doesn't get interrupted every 100us. 
So we do need extra user control here.

We have a bit of a uAPI problem since people had grown to depend on
IRQ == queue == NAPI to configure their systems. "The right way" out
would be a proper API which allows associating queues with CPUs rather
than IRQs, then we can use managed IRQs and solve many other problems.

Such new API has been in the works / discussions for a while now.

(Magnus keep me honest here, if you disagree the queue API solves this.)

  parent reply index

Thread overview: 55+ messages / expand[flat|nested]  mbox.gz  Atom feed  top
2020-09-28 18:35 [PATCH v4 0/4] isolation: limit msix vectors " Nitesh Narayan Lal
2020-09-28 18:35 ` [PATCH v4 1/4] sched/isolation: API to get number of " Nitesh Narayan Lal
2020-09-28 18:35 ` [PATCH v4 2/4] sched/isolation: Extend nohz_full to isolate managed IRQs Nitesh Narayan Lal
2020-10-23 13:25   ` Peter Zijlstra
2020-10-23 13:29     ` Frederic Weisbecker
2020-10-23 13:57       ` Nitesh Narayan Lal
2020-10-23 13:45     ` Nitesh Narayan Lal
2020-09-28 18:35 ` [PATCH v4 3/4] i40e: Limit msix vectors to housekeeping CPUs Nitesh Narayan Lal
2020-09-28 18:35 ` [PATCH v4 4/4] PCI: Limit pci_alloc_irq_vectors() " Nitesh Narayan Lal
2020-09-28 21:59   ` Bjorn Helgaas
2020-09-29 17:46     ` Christoph Hellwig
2020-10-16 12:20   ` Peter Zijlstra
2020-10-18 18:14     ` Nitesh Narayan Lal
2020-10-19 11:11       ` Peter Zijlstra
2020-10-19 14:00         ` Marcelo Tosatti
2020-10-19 14:25           ` Nitesh Narayan Lal
2020-10-20  7:30           ` Peter Zijlstra
2020-10-20 13:00             ` Nitesh Narayan Lal
2020-10-20 13:41               ` Peter Zijlstra
2020-10-20 14:39                 ` Nitesh Narayan Lal
2020-10-22 17:47                   ` Nitesh Narayan Lal
2020-10-23  8:58                     ` Peter Zijlstra
2020-10-23 13:10                       ` Nitesh Narayan Lal
2020-10-23 21:00                         ` Thomas Gleixner
2020-10-26 13:35                           ` Nitesh Narayan Lal
2020-10-26 13:57                             ` Thomas Gleixner
2020-10-26 17:30                           ` Marcelo Tosatti
2020-10-26 19:00                             ` Thomas Gleixner
2020-10-26 19:11                               ` Marcelo Tosatti
2020-10-26 19:21                               ` Jacob Keller
2020-10-26 20:11                                 ` Thomas Gleixner
2020-10-26 21:11                                   ` Jacob Keller
2020-10-26 21:50                                     ` Thomas Gleixner
2020-10-26 22:13                                       ` Jakub Kicinski
2020-10-26 22:46                                         ` Thomas Gleixner
2020-10-26 22:52                                         ` Jacob Keller
2020-10-26 22:22                                       ` Nitesh Narayan Lal
2020-10-26 22:49                                         ` Thomas Gleixner
2020-10-26 23:08                                           ` Jacob Keller
2020-10-27 14:28                                             ` Thomas Gleixner
2020-10-27 11:47                                         ` Marcelo Tosatti
2020-10-27 14:43                                           ` Thomas Gleixner
2020-10-19 14:21         ` Frederic Weisbecker
2020-10-20 14:16   ` Thomas Gleixner
2020-10-20 16:18     ` Nitesh Narayan Lal
2020-10-20 18:07       ` Thomas Gleixner
2020-10-21 20:25         ` Thomas Gleixner
2020-10-21 21:04           ` Nitesh Narayan Lal
2020-10-22  0:02           ` Jakub Kicinski [this message]
2020-10-22  0:27             ` Jacob Keller
2020-10-22  8:28             ` Thomas Gleixner
2020-10-22 12:28           ` Marcelo Tosatti
2020-10-22 22:39             ` Thomas Gleixner
2020-10-01 15:49 ` [PATCH v4 0/4] isolation: limit msix vectors " Frederic Weisbecker
2020-10-08 21:40   ` Nitesh Narayan Lal

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