From: Peter Zijlstra <peterz@infradead.org>
To: Nitesh Narayan Lal <nitesh@redhat.com>
Cc: 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, 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 13:11:37 +0200 [thread overview]
Message-ID: <20201019111137.GL2628@hirez.programming.kicks-ass.net> (raw)
In-Reply-To: <79f382a7-883d-ff42-394d-ec4ce81fed6a@redhat.com>
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.
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.
> > 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.
next prev parent reply other threads:[~2020-10-19 11:12 UTC|newest]
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 to housekeeping CPUs 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 [this message]
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
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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