linux-kernel - Re: [PATCH v6 01/29] irq/matrix: Expose functions to allocate the best CPU for new vectors

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Message-ID: <20220512000924.GA16273@ranerica-svr.sc.intel.com>
Date:   Wed, 11 May 2022 17:09:24 -0700
From:   Ricardo Neri <ricardo.neri-calderon@...ux.intel.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     x86@...nel.org, Tony Luck <tony.luck@...el.com>,
        Andi Kleen <ak@...ux.intel.com>,
        Stephane Eranian <eranian@...gle.com>,
        Andrew Morton <akpm@...ux-foundation.org>,
        Joerg Roedel <joro@...tes.org>,
        Suravee Suthikulpanit <Suravee.Suthikulpanit@....com>,
        David Woodhouse <dwmw2@...radead.org>,
        Lu Baolu <baolu.lu@...ux.intel.com>,
        Nicholas Piggin <npiggin@...il.com>,
        "Ravi V. Shankar" <ravi.v.shankar@...el.com>,
        Ricardo Neri <ricardo.neri@...el.com>,
        iommu@...ts.linux-foundation.org, linuxppc-dev@...ts.ozlabs.org,
        linux-kernel@...r.kernel.org
Subject: Re: [PATCH v6 01/29] irq/matrix: Expose functions to allocate the
 best CPU for new vectors

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