Re: [PATCH tip:irq/core v1] genirq: remove auto-set of the mask when setting the hint

From: Nitesh Lal <nilal@redhat.com>
To: Thomas Gleixner <tglx@linutronix.de>
Cc: Jesse Brandeburg <jesse.brandeburg@intel.com>,
	Robin Murphy <robin.murphy@arm.com>,
	Ingo Molnar <mingo@kernel.org>,
	linux-kernel@vger.kernel.org, intel-wired-lan@lists.osuosl.org,
	jbrandeb@kernel.org, "frederic@kernel.org" <frederic@kernel.org>,
	"juri.lelli@redhat.com" <juri.lelli@redhat.com>,
	Marcelo Tosatti <mtosatti@redhat.com>,
	Alex Belits <abelits@marvell.com>,
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	Marc Zyngier <maz@kernel.org>
Subject: Re: [PATCH tip:irq/core v1] genirq: remove auto-set of the mask when setting the hint
Date: Mon, 17 May 2021 18:44:29 -0400	[thread overview]
Message-ID: <CAFki+Lkjn2VCBcLSAfQZ2PEkx-TR0Ts_jPnK9b-5ne3PUX37TQ@mail.gmail.com> (raw)
In-Reply-To: <87pmxpdr32.ffs@nanos.tec.linutronix.de>

On Mon, May 17, 2021 at 4:48 PM Thomas Gleixner <tglx@linutronix.de> wrote:
>
> On Tue, May 04 2021 at 09:23, Jesse Brandeburg wrote:
> > I'd add in addition that irqbalance daemon *stopped* paying attention
> > to hints quite a while ago, so I'm not quite sure what purpose they
> > serve.
>
> The hint was added so that userspace has a better understanding where it
> should place the interrupt. So if irqbalanced ignores it anyway, then
> what's the point of the hint? IOW, why is it still used drivers?
>

Took a quick look at the irqbalance repo and saw the following commit:

dcc411e7bf    remove affinity_hint infrastructure

The commit message mentions that "PJ is redesiging how affinity hinting
works in the kernel, the future model will just tell us to ignore an IRQ,
and the kernel will handle placement for us.  As such we can remove the
affinity_hint recognition entirely".

This does indicate that apparently, irqbalance moved away from the usage of
affinity_hint. However, the next question is what was this future model?
I don't know but I can surely look into it if that helps or maybe someone
here already knows about it?

> Now there is another aspect to that. What happens if irqbalanced does
> not run at all and a driver relies on the side effect of the hint
> setting the initial affinity. Bah...
>

Right, but if they only rely on this API so that the IRQs are spread across
all the CPUs then that issue is already resolved and these other drivers
should not regress because of changing this behavior. Isn't it?

> While none of the drivers (except the perf muck) actually prevents
> userspace from fiddling with the affinity (via IRQF_NOBALANCING) a
> deeper inspection shows that they actually might rely on the current
> behaviour if irqbalanced is disabled. Of course every driver has its own
> convoluted way to do that and all of those functions are well
> documented. What a mess.
>
> If the hint still serves a purpose then we can provide a variant which
> solely applies the hint and does not fiddle with the actual affinity,
> but if the hint is useless anyway then we have a way better option to
> clean that up.
>

+1

-- 
Thanks
Nitesh