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Subject: Re: [Patch v1] i40e: limit the msix vectors based on housekeeping
 CPUs
To:     "Keller, Jacob E" <jacob.e.keller@intel.com>,
        "linux-kernel@vger.kernel.org" <linux-kernel@vger.kernel.org>,
        "frederic@kernel.org" <frederic@kernel.org>,
        "mtosatti@redhat.com" <mtosatti@redhat.com>,
        "sassmann@redhat.com" <sassmann@redhat.com>,
        "Kirsher, Jeffrey T" <jeffrey.t.kirsher@intel.com>,
        "jlelli@redhat.com" <jlelli@redhat.com>
References: <20200615202125.27831-1-nitesh@redhat.com>
 <20200615202125.27831-2-nitesh@redhat.com>
 <02874ECE860811409154E81DA85FBB58C25F8001@FMSMSX102.amr.corp.intel.com>
From:   Nitesh Narayan Lal <nitesh@redhat.com>
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Organization: Red Hat Inc,
Message-ID: <bef0c237-b349-9499-9cf8-ad8b10f5ff59@redhat.com>
Date:   Mon, 15 Jun 2020 16:55:25 -0400
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On 6/15/20 4:48 PM, Keller, Jacob E wrote:
>
>> -----Original Message-----
>> From: Nitesh Narayan Lal <nitesh@redhat.com>
>> Sent: Monday, June 15, 2020 1:21 PM
>> To: linux-kernel@vger.kernel.org; frederic@kernel.org; mtosatti@redhat.c=
om;
>> sassmann@redhat.com; Kirsher, Jeffrey T <jeffrey.t.kirsher@intel.com>; K=
eller,
>> Jacob E <jacob.e.keller@intel.com>; jlelli@redhat.com
>> Subject: [Patch v1] i40e: limit the msix vectors based on housekeeping C=
PUs
>>
>> In a realtime environment, it is essential to isolate
>> unwanted IRQs from isolated CPUs to prevent latency overheads.
>> Creating MSIX vectors only based on the online CPUs could lead
>> to a potential issue on an RT setup that has several isolated
>> CPUs but a very few housekeeping CPUs. This is because in these
>> kinds of setups an attempt to move the IRQs to the limited
>> housekeeping CPUs from isolated CPUs might fail due to the per
>> CPU vector limit. This could eventually result in latency spikes
>> because of the IRQ threads that we fail to move from isolated
>> CPUs. This patch prevents i40e to add vectors only based on
>> available online CPUs by using housekeeping_cpumask() to derive
>> the number of available housekeeping CPUs.
>>
>> Signed-off-by: Nitesh Narayan Lal <nitesh@redhat.com>
>> ---
> Ok, so the idea is that "housekeeping" CPUs are to be used for general pu=
rpose configuration, and thus is a subset of online CPUs. By reducing the l=
imit to just housekeeping CPUs, we ensure that we do not overload the syste=
m with more queues than can be handled by the general purpose CPUs?

Yes.
General purpose or the housekeeping CPUs or the non-isolated CPUs.

>
> Thanks,
> Jake
>
--=20
Nitesh


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