linux-kernel - Re: [PATCH] x86: Add irq spillover warning

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Date:   Tue, 16 Jul 2019 16:39:06 -0400
From:   Neil Horman <nhorman@...driver.com>
To:     Thomas Gleixner <tglx@...utronix.de>
Cc:     linux-kernel@...r.kernel.org, djuran@...hat.com,
        Ingo Molnar <mingo@...hat.com>, Borislav Petkov <bp@...en8.de>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org
Subject: Re: [PATCH] x86: Add irq spillover warning

On Tue, Jul 16, 2019 at 09:05:30PM +0200, Thomas Gleixner wrote:
> Neil,
> 
> On Tue, 16 Jul 2019, Neil Horman wrote:
> > On Tue, Jul 16, 2019 at 05:57:31PM +0200, Thomas Gleixner wrote:
> > > On Tue, 16 Jul 2019, Neil Horman wrote:
> > > > If a cpu has more than this number of interrupts affined to it, they
> > > > will spill over to other cpus, which potentially may be outside of their
> > > > affinity mask.
> > > 
> > > Spill over?
> > > 
> > > The kernel decides to pick a vector on a CPU outside of the affinity when
> > > it runs out of vectors on the CPUs in the affinity mask.
> > > 
> > Yes.
> > 
> > > Please explain issues technically correct.
> > > 
> > I don't know what you mean by this.  I explained it above, and you clearly
> > understood it.
> 
> It took me a while to grok it. Simply because I first thought it's some
> hardware issue. And of course after confusion settled I knew what it is,
> but just because I know that code like the back of my hand.
> 
> > > > Given that this might cause unexpected behavior on
> > > > performance sensitive systems, warn the user should this condition occur
> > > > so that corrective action can be taken
> > > 
> > > > @@ -244,6 +244,14 @@ __visible unsigned int __irq_entry do_IRQ(struct pt_regs *regs)
> > > 
> > > Why on earth warn in the interrupt delivery hotpath? Just because it's the
> > > place which really needs extra instructions and extra cache lines on
> > > performance sensitive systems, right?
> > > 
> > Because theres already a check of the same variety in do_IRQ, but if the
> > information is available outside the hotpath, I was unaware, and am happy to
> > update this patch to refelct that.
> 
> Which check are you referring to?
> 
This one:
if (desc != VECTOR_RETRIGGERED) {
                        pr_emerg_ratelimited("%s: %d.%d No irq handler for vector\n",
                                             __func__, smp_processor_id(),
                                             vector);
I figured it was already checking one condition, another wouldn't hurt too much,
but no worries, I'm redoing this in activate_reserved now.

Best
Neil

> Thanks,
> 
> 	tglx
>