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From:   Sumit Saxena <sumit.saxena@broadcom.com>
References: <eccc46e12890a1d033d9003837012502@mail.gmail.com> <20180829084618.GA24765@ming.t460p>
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Date:   Wed, 29 Aug 2018 16:16:23 +0530
Message-ID: <300d6fef733ca76ced581f8c6304bac6@mail.gmail.com>
Subject: RE: Affinity managed interrupts vs non-managed interrupts
To:     Ming Lei <ming.lei@redhat.com>
Cc:     tglx@linutronix.de, hch@lst.de, linux-kernel@vger.kernel.org,
        Kashyap Desai <kashyap.desai@broadcom.com>,
        Shivasharan Srikanteshwara 
        <shivasharan.srikanteshwara@broadcom.com>
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> -----Original Message-----
> From: Ming Lei [mailto:ming.lei@redhat.com]
> Sent: Wednesday, August 29, 2018 2:16 PM
> To: Sumit Saxena <sumit.saxena@broadcom.com>
> Cc: tglx@linutronix.de; hch@lst.de; linux-kernel@vger.kernel.org
> Subject: Re: Affinity managed interrupts vs non-managed interrupts
>
> Hello Sumit,
Hi Ming,
Thanks for response.
>
> On Tue, Aug 28, 2018 at 12:04:52PM +0530, Sumit Saxena wrote:
> >  Affinity managed interrupts vs non-managed interrupts
> >
> > Hi Thomas,
> >
> > We are working on next generation MegaRAID product where requirement
> > is- to allocate additional 16 MSI-x vectors in addition to number of
> > MSI-x vectors megaraid_sas driver usually allocates.  MegaRAID adapter
> > supports 128 MSI-x vectors.
> >
> > To explain the requirement and solution, consider that we have 2
> > socket system (each socket having 36 logical CPUs). Current driver
> > will allocate total 72 MSI-x vectors by calling API-
> > pci_alloc_irq_vectors(with flag- PCI_IRQ_AFFINITY).  All 72 MSI-x
> > vectors will have affinity across NUMA node s and interrupts are
affinity
> managed.
> >
> > If driver calls- pci_alloc_irq_vectors_affinity() with pre_vectors =
> > 16 and, driver can allocate 16 + 72 MSI-x vectors.
>
> Could you explain a bit what the specific use case the extra 16 vectors
is?
We are trying to avoid the penalty due to one interrupt per IO completion
and decided to coalesce interrupts on these extra 16 reply queues.
For regular 72 reply queues, we will not coalesce interrupts as for low IO
workload, interrupt coalescing may take more time due to less IO
completions.
In IO submission path, driver will decide which set of reply queues
(either extra 16 reply queues or regular 72 reply queues) to be picked
based on IO workload.
>
> >
> > All pre_vectors (16) will be mapped to all available online CPUs but e
> > ffective affinity of each vector is to CPU 0. Our requirement is to
> > have pre _vectors 16 reply queues to be mapped to local NUMA node with
> > effective CPU should be spread within local node cpu mask. Without
> > changing kernel code, we can
>
> If all CPUs in one NUMA node is offline, can this use case work as
expected?
> Seems we have to understand what the use case is and how it works.

Yes, if all CPUs of the NUMA node is offlined, IRQ-CPU affinity will be
broken and irqbalancer takes care of migrating affected IRQs to online
CPUs of different NUMA node.
When offline CPUs are onlined again, irqbalancer restores affinity.
>
>
> Thanks,
> Ming