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Date: Mon, 19 Oct 2020 11:00:05 -0300 From: Marcelo Tosatti <mtosatti@...hat.com> To: Peter Zijlstra <peterz@...radead.org> Cc: Nitesh Narayan Lal <nitesh@...hat.com>, linux-kernel@...r.kernel.org, netdev@...r.kernel.org, linux-pci@...r.kernel.org, intel-wired-lan@...ts.osuosl.org, frederic@...nel.org, sassmann@...hat.com, jesse.brandeburg@...el.com, lihong.yang@...el.com, helgaas@...nel.org, jeffrey.t.kirsher@...el.com, jacob.e.keller@...el.com, jlelli@...hat.com, hch@...radead.org, bhelgaas@...gle.com, mike.marciniszyn@...el.com, dennis.dalessandro@...el.com, thomas.lendacky@....com, jiri@...dia.com, mingo@...hat.com, juri.lelli@...hat.com, vincent.guittot@...aro.org, lgoncalv@...hat.com Subject: Re: [PATCH v4 4/4] PCI: Limit pci_alloc_irq_vectors() to housekeeping CPUs 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: > > >> + hk_cpus = housekeeping_num_online_cpus(HK_FLAG_MANAGED_IRQ); > > >> + > > >> + /* > > >> + * If we have isolated CPUs for use by real-time tasks, to keep the > > >> + * latency overhead to a minimum, device-specific IRQ vectors are moved > > >> + * to the housekeeping CPUs from the userspace by changing their > > >> + * affinity mask. Limit the vector usage to keep housekeeping CPUs from > > >> + * running out of IRQ vectors. > > >> + */ > > >> + if (hk_cpus < num_online_cpus()) { > > >> + if (hk_cpus < min_vecs) > > >> + max_vecs = min_vecs; > > >> + else if (hk_cpus < max_vecs) > > >> + max_vecs = hk_cpus; > > > is that: > > > > > > max_vecs = clamp(hk_cpus, min_vecs, max_vecs); > > > > Yes, I think this will do. > > > > > > > > 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. > > > 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. > > > And what are the (desired) semantics vs hotplug? Using a cpumask without > > > excluding hotplug is racy. > > > > The housekeeping_mask should still remain constant, isn't? > > In any case, I can double check this. > > The goal is very much to have that dynamically configurable. Yes, but this patch is a fix for customer bug in the old, static on-boot isolation CPU configuration. --- Discussing the dynamic configuration (not this patch!) case: Would need to enable/disable interrupts for a particular device on a per-CPU basis. Such interface does not exist yet. Perhaps that is what you are looking for when writing "proposed interface is wrong" Peter?
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