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

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Date:   Wed, 6 Sep 2017 23:23:41 -0700
From:   Dan Williams <dan.j.williams@...el.com>
To:     Thomas Gleixner <tglx@...utronix.de>
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, Sep 6, 2017 at 10:59 PM, Thomas Gleixner <tglx@...utronix.de> wrote:
> On Wed, 6 Sep 2017, Dan Williams wrote:
>
>> On Tue, Sep 5, 2017 at 11:15 PM, Christoph Hellwig <hch@....de> wrote:
>> > On Wed, Sep 06, 2017 at 12:13:38PM +0800, Yu Chen wrote:
>> >> I agree, the driver could be rewritten, but it might take some time, so
>> >> meanwhile I'm looking at also other possible optimization.
>> >
>> > Which driver are we talking about anyway?  Let's start looking at it
>> > and fix the issue there.
>>
>> As far as I understand, it's already fixed there:
>>
>> commit 7c9ae7f053e9e896c24fd23595ba369a5fe322e1
>
> -ENOSUCHCOMMIT

Sorry, that's still pending in -next.

>> 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.
>>
>> 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?