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From: Nitesh Narayan Lal <nitesh@redhat.com>
To: 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, 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: Sun, 18 Oct 2020 14:14:46 -0400
Message-ID: <79f382a7-883d-ff42-394d-ec4ce81fed6a@redhat.com> (raw)
In-Reply-To: <20201016122046.GP2611@hirez.programming.kicks-ass.net>

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On 10/16/20 8:20 AM, Peter Zijlstra wrote:
> On Mon, Sep 28, 2020 at 02:35:29PM -0400, Nitesh Narayan Lal wrote:
>> If we have isolated CPUs dedicated for use by real-time tasks, we try to
>> move IRQs to housekeeping CPUs from the userspace to reduce latency
>> overhead on the isolated CPUs.
>>
>> If we allocate too many IRQ vectors, moving them all to housekeeping CPUs
>> may exceed per-CPU vector limits.
>>
>> When we have isolated CPUs, limit the number of vectors allocated by
>> pci_alloc_irq_vectors() to the minimum number required by the driver, or
>> to one per housekeeping CPU if that is larger.
>>
>> Signed-off-by: Nitesh Narayan Lal <nitesh@redhat.com>
>> ---
>>  drivers/pci/msi.c | 18 ++++++++++++++++++
>>  1 file changed, 18 insertions(+)
>>
>> diff --git a/drivers/pci/msi.c b/drivers/pci/msi.c
>> index 30ae4ffda5c1..8c156867803c 100644
>> --- a/drivers/pci/msi.c
>> +++ b/drivers/pci/msi.c
>> @@ -23,6 +23,7 @@
>>  #include <linux/slab.h>
>>  #include <linux/irqdomain.h>
>>  #include <linux/of_irq.h>
>> +#include <linux/sched/isolation.h>
>>  
>>  #include "pci.h"
>>  
>> @@ -1191,8 +1192,25 @@ int pci_alloc_irq_vectors_affinity(struct pci_dev *dev, unsigned int min_vecs,
>>  				   struct irq_affinity *affd)
>>  {
>>  	struct irq_affinity msi_default_affd = {0};
>> +	unsigned int hk_cpus;
>>  	int nvecs = -ENOSPC;
>>  
>> +	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.

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?

>
> 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.

>
>> +	}
>> +
>>  	if (flags & PCI_IRQ_AFFINITY) {
>>  		if (!affd)
>>  			affd = &msi_default_affd;
>> -- 
>> 2.18.2
>>
-- 
Thanks
Nitesh


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  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 [this message]
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
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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