Re: [PATCH v4 4/4] PCI: Limit pci_alloc_irq_vectors() to housekeeping CPUs

From: Nitesh Narayan Lal <nitesh@redhat.com>
To: Marcelo Tosatti <mtosatti@redhat.com>,
	Peter Zijlstra <peterz@infradead.org>
Cc: 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,
	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, 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
Date: Mon, 19 Oct 2020 10:25:14 -0400	[thread overview]
Message-ID: <48a2a9a1-d664-6770-e088-27a7786e0f7b@redhat.com> (raw)
In-Reply-To: <20201019140005.GB17287@fuller.cnet>

[-- Attachment #1.1: Type: text/plain, Size: 1958 bytes --]

On 10/19/20 10:00 AM, Marcelo Tosatti wrote:
> On Mon, Oct 19, 2020 at 01:11:37PM +0200, Peter Zijlstra wrote:
>> On Sun, Oct 18, 2020 at 02:14:46PM -0400, Nitesh Narayan Lal wrote:

[...]

>>>> Also, do we really need to have that conditional on hk_cpus <
>>>> num_online_cpus()? That is, why can't we do this unconditionally?
>>> FWIU most of the drivers using this API already restricts the number of
>>> vectors based on the num_online_cpus, if we do it unconditionally we can
>>> unnecessary duplicate the restriction for cases where we don't have any
>>> isolated CPUs.
>> unnecessary isn't really a concern here, this is a slow path. What's
>> important is code clarity.

Right, I can skip that check then.

>>
>>> Also, different driver seems to take different factors into consideration
>>> along with num_online_cpus while finding the max_vecs to request, for
>>> example in the case of mlx5:
>>> MLX5_CAP_GEN(dev, num_ports) * num_online_cpus() +
>>>                MLX5_EQ_VEC_COMP_BASE
>>>
>>> Having hk_cpus < num_online_cpus() helps us ensure that we are only
>>> changing the behavior when we have isolated CPUs.
>>>
>>> Does that make sense?
>> That seems to want to allocate N interrupts per cpu (plus some random
>> static amount, which seems weird, but whatever). This patch breaks that.
> On purpose. For the isolated CPUs we don't want network device 
> interrupts (in this context).
>
>> So I think it is important to figure out what that driver really wants
>> in the nohz_full case. If it wants to retain N interrupts per CPU, and
>> only reduce the number of CPUs, the proposed interface is wrong.
> It wants N interrupts per non-isolated (AKA housekeeping) CPU.
> Zero interrupts for isolated interrupts.

Right, otherwise we may end up in a situation where we run out of per CPU
vectors while we move the IRQs from isolated CPUs to housekeeping.

-- 
Thanks
Nitesh

[-- Attachment #2: OpenPGP digital signature --]
[-- Type: application/pgp-signature, Size: 833 bytes --]