linux-kernel - Re: [PATCH 0/2 v3] cpu hotplug: Preserve topology directory after soft remove event

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Date:   Wed, 28 Sep 2016 06:06:45 -0400
From:   Prarit Bhargava <prarit@...hat.com>
To:     Peter Zijlstra <peterz@...radead.org>
CC:     Greg Kroah-Hartman <gregkh@...uxfoundation.org>,
        Borislav Petkov <bp@...e.de>, linux-kernel@...r.kernel.org,
        Thomas Gleixner <tglx@...utronix.de>,
        Ingo Molnar <mingo@...hat.com>,
        "H. Peter Anvin" <hpa@...or.com>, x86@...nel.org,
        Len Brown <len.brown@...el.com>,
        Andi Kleen <ak@...ux.intel.com>, Jiri Olsa <jolsa@...hat.com>,
        Juergen Gross <jgross@...e.com>
Subject: Re: [PATCH 0/2 v3] cpu hotplug: Preserve topology directory after
 soft remove event



On 09/28/2016 02:48 AM, Peter Zijlstra wrote:
> On Tue, Sep 27, 2016 at 11:26:14AM -0400, Prarit Bhargava wrote:
>> I see now that the issue is not understanding the difference between physical
>> and soft thread removal.  I will write that up and get back to everyone.
> 
> You don't seem to understand that from the kernels POV there is no such
> distinction.
> 

And I'm saying you're wrong.  Unlike PCI, CPU sysfs unplug does not completely
remove the device from the kernel's knowledge.  If that were the case then

/sys/devices/system/cpu/cpuX/online (and other files left after the unplug)
wouldn't exist.

If you do a physical removal (signaled via ACPI or interrupt) the entire device
is removed from the kernel's POV.

Yes, the sysfs removal is a subset of the physical removal.  But the end result
is very different.

> There is only one unplug operation that caters to both cases. Therefore
> unplug needs to assume the most stringent (ie. physical) for things to
> work right.

See last comment above.

P.