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Date: Tue, 13 Mar 2018 16:35:41 +0800
From: Ming Lei <ming.lei@redhat.com>
To: Artem Bityutskiy <dedekind1@gmail.com>
Cc: Dou Liyang <douly.fnst@cn.fujitsu.com>,
	Thomas Gleixner <tglx@linutronix.de>, Jens Axboe <axboe@kernel.dk>,
	Christoph Hellwig <hch@infradead.org>, linux-kernel@vger.kernel.org,
	linux-block@vger.kernel.org, Laurence Oberman <loberman@redhat.com>,
	"Rafael J. Wysocki" <rafael@kernel.org>
Subject: Re: [PATCH V3 0/4] genirq/affinity: irq vector spread among online
 CPUs as far as possible
Message-ID: <20180313083535.GA21612@ming.t460p>
References: <20180308105358.1506-1-ming.lei@redhat.com>
 <1520515113.20980.31.camel@gmail.com>
 <20180308133440.GA2713@ming.t460p>
 <alpine.DEB.2.21.1803090011150.1454@nanos.tec.linutronix.de>
 <20180309012458.GD5228@ming.t460p>
 <alpine.DEB.2.21.1803091107520.1364@nanos.tec.linutronix.de>
 <20180309120833.GB30257@ming.t460p>
 <alpine.DEB.2.21.1803091531000.1364@nanos.tec.linutronix.de>
 <5e5f3852-5314-c479-245e-d0a575e533a5@cn.fujitsu.com>
 <1520926721.20980.210.camel@gmail.com>
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On Tue, Mar 13, 2018 at 09:38:41AM +0200, Artem Bityutskiy wrote:
> On Tue, 2018-03-13 at 11:11 +0800, Dou Liyang wrote:
> >  I also
> > met the situation that BIOS told to ACPI that it could support
> > physical
> > CPUs hotplug, But actually, there was no hardware slots in the
> > machine.
> > the ACPI tables like user inputs which should be validated when we
> > use.
> 
> This is exactly what happens on Skylake Xeon systems. When I check
> dmesg or this file:
> 
> /sys/devices/system/cpu/possible
> 
> on 2S (two socket) and 4S (four socket) systems, I see the same number
> 432.
> 
> This number comes from ACPI MADT. I will speculate (did not see myself)
> that 8S systems will report the same number as well, because of the
> Skylake-SP (Scalable Platform) architecture.
> 
> Number 432 is good for 8S systems, but it is way too large for 2S and
> 4S systems - 4x or 2x larger than the theoretical maximum.
> 
> I do not know why BIOSes have to report unrealistically high numbers, I
> am just sharing my observation.
> 
> So yes, Linux kernel's possible CPU count knowledge may be too large.
> If we use that number to evenly spread IRQ vectors among the CPUs, we
> end up with wasted vectors, and even bugs, as I observe on a 2S
> Skylake.

Then looks this issue need to fix by making possible CPU count accurate
because there are other resources allocated according to num_possible_cpus(),
such as percpu variables.

Thanks,
Ming