linux-kernel - Re: [PATCH 4/4][RFC v2] x86/apic: Spread the vectors by choosing the idlest CPU

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Date:   Thu, 7 Sep 2017 08:59:34 +0200 (CEST)
From:   Thomas Gleixner <tglx@...utronix.de>
To:     Dan Williams <dan.j.williams@...el.com>
cc:     Christoph Hellwig <hch@....de>, Yu Chen <yu.c.chen@...el.com>,
        X86 ML <x86@...nel.org>, Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, Rui Zhang <rui.zhang@...el.com>,
        LKML <linux-kernel@...r.kernel.org>,
        "Rafael J. Wysocki" <rjw@...ysocki.net>,
        Len Brown <lenb@...nel.org>,
        Peter Zijlstra <peterz@...radead.org>,
        Jeff Kirsher <jeffrey.t.kirsher@...el.com>
Subject: Re: [PATCH 4/4][RFC v2] x86/apic: Spread the vectors by choosing
 the idlest CPU

On Wed, 6 Sep 2017, Dan Williams wrote:
> On Wed, Sep 6, 2017 at 10:59 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> >> commit 7c9ae7f053e9e896c24fd23595ba369a5fe322e1
> >
> > -ENOSUCHCOMMIT
> 
> Sorry, that's still pending in -next.

Ok.

> >> Author: Carolyn Wyborny <carolyn.wyborny@...el.com>
> >> Date:   Tue Jun 20 15:16:53 2017 -0700
> >>
> >>     i40e: Fix for trace found with S4 state
> >>
> >>     This patch fixes a problem found in systems when entering
> >>     S4 state.  This patch fixes the problem by ensuring that
> >>     the misc vector's IRQ is disabled as well.  Without this
> >>     patch a stack trace can be seen upon entering S4 state.

Btw. This changelog is pretty useless.....

> >> However this seems like something that should be handled generically
> >> in the irq-core especially since commit c5cb83bb337c
> >> "genirq/cpuhotplug: Handle managed IRQs on CPU hotplug" was headed in
> >> that direction. It's otherwise non-obvious when a driver needs to
> >> release and re-acquire interrupts or be reworked to use managed
> >> interrupts.
> >
> > There are two problems here:
> >
> > 1) The driver allocates 300 interrupts and uses exactly 8 randomly chosen
> >    ones.
> >
> > 2) It's not using the managed affinity mechanics, so the interrupts cannot
> >    be sanely handled by the kernel, neither affinity wise nor at hotplug
> >    time.
> 
> Ok, this driver is an obvious candidate, but is there a general
> guideline of when a driver must use affinity management? Should we be
> emitting a message when a driver exceeds a certain threshold of
> unmanaged interrupts to flag this in the future?

I guess everything which uses multi queues and therefor allocates 8+
vectors is something which falls into that category.

Aside of that, drivers should be sane in terms of allocations. Allocating
metric tons of interrupts for nothing is not really a sign of a proper
thought out resource management.

Thanks,

	tglx