linux-kernel - Re: [PATCH] x86,cpu-hotplug: clear llc_shared

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Message-ID: <53BCE540.1010306@jp.fujitsu.com>
Date:	Wed, 9 Jul 2014 15:46:24 +0900
From:	Yasuaki Ishimatsu <isimatu.yasuaki@...fujitsu.com>
To:	Borislav Petkov <bp@...en8.de>
CC:	<tglx@...utronix.de>, <mingo@...hat.com>, <hpa@...or.com>,
	<x86@...nel.org>, <toshi.kani@...com>, <imammedo@...hat.com>,
	<huawei.libin@...wei.com>, <paul.gortmaker@...driver.com>,
	<linux-kernel@...r.kernel.org>, <srivatsa.bhat@...ux.vnet.ibm.com>
Subject: Re: [PATCH] x86,cpu-hotplug: clear llc_shared_mask at CPU hotplug

Hi Borislav,

Sorry for late reply and thank you for your explanation.
I'm implementing your suggestion. After finishing test, I'll post a patch again.
So when I post a patch, please review it.

Thanks,
Yasuaki Ishimatsu

(2014/07/04 19:59), Borislav Petkov wrote:
> On Fri, Jul 04, 2014 at 09:14:57AM +0900, Yasuaki Ishimatsu wrote:
>> If so, why following maps are cleared by CPU offline?
>>    - cpu_sigling_map
>>    - cpu_core_map
>
> I'll let you figure that out on your own by doing some quality code
> staring. Hint: search for usages outside of the CPU hotplug path.
>
> In any case, let me try to explain it to you one more time: if the core
> numbers are static and don't change across physical hotplug, the cores
> which share last level cache also remain the same. This is a static
> property which doesn't simply change.
>
> For example, if cores 6-11 and 34-41 have been sharing the last level
> cache, then if the node containing them gets unplugged and replugged
> back again - then the same cores share that exact same cache and nothing
> has changed.
>
> Except the core numbers, as you've shown. Which would be not only a
> problem for the llc_shared_mask but also a big annoyance for sysadmins
> and users having to realize that the topology enumeration has changed
> and trying to make sense of what node went where and why isn't it the
> same as when the machine booted.
>
> So what I'm trying to say is, we should keep the core numbering stable
> across hotplug to avoid unnecessary confusion; the llc_shared_mask is
> just a small issue which results from not doing that.
>


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