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Date:   Wed, 11 May 2022 17:09:24 -0700
From:   Ricardo Neri <ricardo.neri-calderon@linux.intel.com>
To:     Thomas Gleixner <tglx@linutronix.de>
Cc:     x86@kernel.org, Tony Luck <tony.luck@intel.com>,
        Andi Kleen <ak@linux.intel.com>,
        Stephane Eranian <eranian@google.com>,
        Andrew Morton <akpm@linux-foundation.org>,
        Joerg Roedel <joro@8bytes.org>,
        Suravee Suthikulpanit <Suravee.Suthikulpanit@amd.com>,
        David Woodhouse <dwmw2@infradead.org>,
        Lu Baolu <baolu.lu@linux.intel.com>,
        Nicholas Piggin <npiggin@gmail.com>,
        "Ravi V. Shankar" <ravi.v.shankar@intel.com>,
        Ricardo Neri <ricardo.neri@intel.com>,
        iommu@lists.linux-foundation.org, linuxppc-dev@lists.ozlabs.org,
        linux-kernel@vger.kernel.org
Subject: Re: [PATCH v6 01/29] irq/matrix: Expose functions to allocate the
 best CPU for new vectors
Message-ID: <20220512000924.GA16273@ranerica-svr.sc.intel.com>
References: <20220506000008.30892-1-ricardo.neri-calderon@linux.intel.com>
 <20220506000008.30892-2-ricardo.neri-calderon@linux.intel.com>
 <878rreh27n.ffs@tglx>
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On Fri, May 06, 2022 at 09:48:28PM +0200, Thomas Gleixner wrote:
> Ricardo,

Thank you very much for your feedback Thomas! I am sorry for my late reply, I
had been out of office.

> 
> On Thu, May 05 2022 at 16:59, Ricardo Neri wrote:
> > Certain types of interrupts, such as NMI, do not have an associated vector.
> > They, however, target specific CPUs. Thus, when assigning the destination
> > CPU, it is beneficial to select the one with the lowest number of
> > vectors.
> 
> Why is that beneficial especially in the context of a NMI watchdog which
> then broadcasts the NMI to all other CPUs?

My intent was not the NMI watchdog specifically but potential use cases that do
not involve NMI broadcasts. If the NMI targets a single CPU, it is best to
select the CPU with the lowest vector allocation count.

> 
> That's wishful thinking perhaps, but I don't see any benefit at all.
> 
> > Prepend the functions matrix_find_best_cpu_managed() and
> > matrix_find_best_cpu_managed()
> 
> The same function prepended twice becomes two functions :)
> 

Sorry, I missed this.

> > with the irq_ prefix and expose them for
> > IRQ controllers to use when allocating and activating vector-less IRQs.
> 
> There is no such thing like a vectorless IRQ. NMIs have a vector. Can we
> please describe facts and not pulled out of thin air concepts which do
> not exist?

Thank you for the clarification. I see your point. I wrote this patch because
maskable interrupts and NMIs have different entry points. As you state,
however, the also have a vector.

I can drop this patch.

BR,
Ricardo

> 
> Thanks,
> 
>         tglx